Probing Oxide-Ion Mobility in the Mixed Ionic-Electronic Conductor La2NiO4+δ by Solid-State (17)O MAS NMR Spectroscopy.

While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic-electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution (17)O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic (17)O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.

Introduction

Mixed ionic and electronic conducting (MIEC) ceramics have shown promise in recent years as oxygen-transport membranes in solid oxide fuel cells (SOFCs) and for chemical looping applications.[1−7] The use of mixed conductors, primarily perovskite-type oxides, as SOFC cathodes has been shown to improve oxygen reduction kinetics and thus enable device operation at lower temperatures.[8,9] Typically, the advanced functionality of these mixed-conducting systems derives from the mutual influence of metal cation mixed valency and oxygen nonstoichiometry.[10] While the latter property, manifesting as oxygen vacancies or interstitials, has been directly implicated in the bulk performance of MIECs, the mechanistic origins of oxide-ion conductivity often remain unclear at the atomic level. Atomistic simulations have provided insight into underlying interstitial and vacancy mediated contributions to ionic conductivity,[11] but direct experimental confirmation of these details of ionic motion remains a challenge.

Unlike conventional diffraction-based methods sensitive to long-range order, solid-state NMR spectroscopy can provide insight into local, atomic-scale distortions in solids, with direct relevance to ionic conduction.[12−15] Moreover, exchange rates and activation energies for thermally activated transport processes can be derived from NMR spectra acquired at variable temperature (VT-NMR). Our group[16−21] and others[22−30] have shown that solid-state 17O VT-NMR in particular can enable detailed mechanistic studies of fast oxide-ion conductors, subject to successful enrichment[31,32] of samples with 17O (natural abundance 0.037%). Moreover, as a spin-5/2 nucleus with moderate electric quadrupole moment, 17O is sensitive to electric field gradients (EFGs) generated by local charge and bonding asymmetry, as quantified through site-specific quadrupole coupling constants (CQ). This quadrupolar interaction can further discriminate among different coordination environments in diamagnetic oxides.[33]

Similar 17O NMR studies of paramagnetic oxides such as MIECs, however, have met with comparatively limited success.[34] In paramagnetic materials, electron–nuclear spin interactions lead to large Fermi contact (hyperfine) shifts and anisotropic dipolar broadening that complicate spectral detection, resolution and assignment. Previous reports have generally been confined to single crystal samples, with spectra often recorded at cryogenic temperatures.[35−37] In a significant advance by Kong et al., the first 17O magic-angle spinning (MAS) NMR spectra of paramagnetic transition metal complexes have been recorded and assigned.[38] In this work, we extend paramagnetic 17O MAS NMR to perform advanced pulse sequence techniques and variable-temperature measurements on a model MIEC, La2NiO4+δ, to explore the structural and mechanistic details of the oxide-ion transport in this material.

La2NiO4+δ, a perovskite-derived K2NiF4-type mixed ionic–electronic conductor, shows high oxygen transport across a large temperature range and is an important candidate SOFC cathode material.[39] With both paramagnetic (axial Oax, equatorial Oeq) and diamagnetic (interstitial Oi) oxygen sites, the system is an elegant model for initial 17O MAS NMR studies of MIECs. (By a “paramagnetic” or “diamagnetic” site, we here refer to the presence or absence, respectively, of electron spin-bearing cations such as Ni2+ in the local (first shell) oxygen coordination environment.)

Structurally, La2NiO4+δ is a member of the Ruddlesden–Popper family and consists of alternating LaNiO3 perovskite-like layers and “La2O2” rocksalt-like layers in an offset ABA′B′ arrangement (Figure a). Equatorial oxygen sites lie within the perovskite plane; axial sites bridge the layers. Incorporation of interstitial oxygen within the rocksalt layers is remarkably facile, and affords a considerable range of oxygen hyperstoichiometry (δ), reported from δ = 0 to ∼0.3, with δ > 0.15 for SOFC applications.[40,41] Structural and magnetic properties are very sensitive to oxygen excess.[42,43] In this work, therefore, we consider only the highly hyperstoichiometric phases most relevant for fast oxide-ion conduction in SOFC cathodes. We first show that incorporation of interstitial oxygen leads to distinct and well-resolved displacement of axial oxygen sites, and paramagnetic 17O NMR is uniquely sensitive to these local structural distortions. Results from periodic density functional theory (DFT) calculations are integral in the interpretation of the 17O NMR spectra; the axial distortion leads to a splitting of calculated hyperfine shifts, as observed experimentally, and calculated CQ values corroborate assignment of the Oeq and Oax sites.

Figure 1

Room-temperature 17O MAS NMR spectrum of La2NiO4+δ with proposed assignments. (a) Crystal structure of the high-temperature tetragonal (space group I4/mmm) phase of La2NiO4.17 as reported by Skinner et al.[41] Partially occupied sites (Oax, Oi) are depicted as partially filled spheres. (b) Individual subspectra collected at different offset frequencies (colored) summed to give the broadband spin–echo mapping spectrum (black). Proposed assignments depict the local geometry about each oxygen environment (equatorial Oeq, axial Oax, and interstitial Oi). A rotor-synchronized Hahn echo pulse sequence (π/6−τ–π/3−τ–acquire) was used for each subspectrum. Spectra were collected at 7.05 T at a MAS rate of 12.5 kHz, with 120 000 scans per subspectrum and a recycle delay of 0.5 s. Asterisks denote spinning sidebands. (c) Inset showing the “diamagnetic region” of the summed spin–echo mapping spectrum in (b). Features at 532 and 170 ppm are assigned to interstitial oxygen (Oi) in La2NiO4+δ, and a LaAlO3 impurity phase, respectively. Asterisks denote spinning sidebands.

We next perform variable-temperature NMR to probe oxide-ion dynamics associated with the orthorhombic-to-tetragonal phase transition of La2NiO4+δ reported near 150 °C. At the composition δ = 0.3, Aguadero et al. have reported the phase transition temperature Tp = 132 °C.[44] We argue that this transition is directly correlated with the onset of rapid interstitial motion between 110 and 130 °C, a process for which we extract an activation energy Ea = 0.59 ± 0.07 eV. In particular, fast exchange between interstitial and axial oxygen sites, as in the hypothesized interstitialcy mechanism of Chroneos et al.,[45] occurs simultaneously with the abrupt disappearance of the aforementioned local distortion. This in turn induces the disruption of the cooperative tilting of perovskite layers responsible for the long-range orthorhombic (Fmmm) distortion, incoherently averaging the structure to the tetragonal (I4/mmm) phase. Our proposed model of oxide-ion dynamics at these temperatures involves two coupled motional processes: (1) exchange between interstitial and axial sites and (2) “rocking” of NiO6 octahedra dynamically altering the displacement of axial and equatorial sites, that together determine the observed 17O VT-NMR lineshapes. In summary, we report the first 17O MAS solid-state VT-NMR spectra of a paramagnetic oxide-ion conductor, with DFT-aided assignment of the local structure and oxide-ion dynamics, which should ultimately enable future studies of functionally relevant paramagnetic oxides by similar methods.

Experimental and Theoretical Methods

Synthesis, 17O-Enrichment, and Characterization

Samples of La2NiO4+δ were prepared by a solid-state reaction route as described previously.[46−48] Stoichiometric amounts of La2O3 (Alfa Aesar, REacton, 99.999%; predried) and NiO (Aldrich, 99.999%) were mixed in a mortar and pestle, pressed isostatically, sintered in air at 1300 °C for 6–12 h, and ground into powder. Multiple intermediate sintering and grinding steps were repeated until phase purity was achieved, as determined by laboratory powder X-ray diffraction (Figure S1).

Samples were also prepared via a modified sol–gel (Pechini) method similar to that previously reported.[49] Stoichiometric amounts of La(NO3)3·6H2O (Alfa Aesar, REacton, 99.999%) and Ni(NO3)2·6H2O (Aldrich, 99.999%) were dissolved in an aqueous solution of poly(vinyl alcohol) (15% w/v; Merck, Mw = 60 000), with a mole ratio of metal cations to hydroxyl groups in poly(vinyl alcohol) of approximately 1:3. Continuous heating at 100 °C produced a viscous green xerogel which was subsequently heated to autoignition at 400 °C. The resulting off-black powder was pressed isostatically and sintered in air at 1300 °C for 12 h. Sol–gel-synthesized samples did not differ appreciably in phase purity or excess oxygen content from samples obtained via the solid-state reaction route.

Samples of 17O-enriched La2NiO4+δ were obtained by heating the as-synthesized powder (0.1–0.3 g) to 1000 °C under an atmosphere of 70% 17O2 (Cambridge Isotope Laboratories, used as received) in a sealed quartz tube for 24 h. Samples were slowly cooled (1 °C min) from the enrichment temperature to maximize uptake of 17O.

Phase purity of all samples was determined with powder X-ray diffraction (XRD) using a Panalytical Empyrean X-ray diffractometer equipped with a Cu Kα source (λ = 1.5406 and 1.5418 Å) and X’celerator CCD detector. Scans were performed on a spinning sample stage in reflection mode over the range 2θ = 5° to 80° (step size 2θ = 0.0167°). Diffraction patterns were analyzed with the X’Pert HighScore Plus software package and PDF pattern database, and Rietveld refinements were performed with the GSAS and EXPGUI software packages.[50,51]

Oxygen excess (δ) in La2NiO4+δ was determined via thermogravimetric analysis (TGA) performed with a Mettler Toledo TGA/SDTA 851 thermobalance. Powder samples of 20–40 mg were placed in a 100 μL Al2O3 crucible and heated to 900 °C (at 3 °C min–1) under a reducing atmosphere of 5% H2 in N2 (50 mL min–1). Raw mass data collected during the heating profile were corrected from blank experiments and smoothed subject to a local regression (LOESS) algorithm. Sample nonstoichiometry was calculated as the ratio of mass losses during the two discrete reduction steps (Figure S2), where we assumed these mass losses correspond to the reactionswhich were driven to completion given the gas flow conditions.[52]

Solid-State NMR Spectroscopy

Solid-state 17O MAS NMR experiments were carried out on 7.05 T Bruker Avance II and Avance III 300 MHz spectrometers using a Bruker 4 mm HX probe (Figures and 5); a 4.7 T Bruker Avance III 200 MHz using a Bruker 1.9 mm HX probe (Figure ); and a 16.4 T Bruker Avance III 700 MHz spectrometer using a Bruker 4 mm X probe (Figure ). Experimental parameters for all NMR data are summarized below.

Figure 5

Broadband variable-temperature NMR spectra of La2NiO4+δ. Spectra were acquired at 7.05 T at a MAS rate of 12.5 kHz. Spectra were normalized to number of scans (between ∼300 000 and ∼6 700 000 per spectrum) and then scaled as shown to obtain similar intensity for the Oax feature (∼3500 ppm) present in all spectra. #Indicates the feature at 2400 ppm assigned to the La3Ni2O7/La4Ni3O10 impurity phase (see SI, section 5). Asterisks denote visible spinning sidebands, for clarity only indicated for Oi at 35 °C and for Oeq at 140 °C. (A close-up view of the spectrum at 140 °C depicting the weakly resolved peaks in the diamagnetic region is shown in Figure S9.)

Figure 3

Comparison of Hahn echo and MATPASS NMR spectra of La2NiO4+δ, with quadrupolar filtering. Spectra were acquired at 4.7 T with a MAS rate of 40 kHz. Splitting of Oax is partially resolved in the Hahn echo and fully resolved in the MATPASS data. Both experiments show pulse length dependence (π/6 vs π/2) consistent with a single highly quadrupolar environment (Oeq). Asterisks denote spinning sidebands where apparent.

Figure 4

Variable-temperature NMR spectra of La2NiO4+δ, focusing on the interstitial oxygen site. (a, top to bottom) 17O MAS NMR spectra of La2NiO4+δ acquired at the indicated temperatures, and at 35 °C after cooling from high temperature (red). The difference between normal room temperature and the lowest sample temperature (35 °C) is due to frictional heating by MAS. Spectra were acquired at 16.4 T under a MAS rate of 12.5 kHz. Spectra shown are normalized to the number of scans. (b) Detail of (a) with spectra scaled to highlight broadening and shift of interstitial site. Asterisks denote spinning sidebands (for clarity shown only at 35 °C).

Spin–echo mapping experiments at 7.05 T were performed under a MAS frequency of 12.5 kHz using a rotor-synchronized Hahn echo pulse sequence of the form (π/6)–τ–(π/3)–τ–acquire with a pulse length of 1.67 μs (π/6 for liquid H2O) at an inherent rf field strength of ∼50 kHz and a quantitative recycle delay of 0.5 s. Similar experiments at 4.7 T were carried out at a MAS frequency of 40 kHz using a pulse length of 0.75 μs (π/6 for liquid H2O) at an inherent rf field strength of ∼111 kHz, and a recycle delay of 20 ms. The pulse carrier frequency step size was 2500 ppm (∼102 kHz at 7.05 T, ∼68 kHz at 4.7 T), i.e., smaller than the rf field strength, and a total of six subspectra were acquired at 500, 3000, 5500, 8000, 10 500, and 13 000 ppm. Further spin–echo mapping experiments at 4.7 T employed longer pulse lengths of 2.2 μs (π/2 for liquid H2O) at an inherent rf field strength of ∼114 kHz, with identical pulse carrier frequency offsets. Finally, all other experiments not employing spin–echo mapping for broadband excitation (i.e., MATPASS or variable temperature measurements) fixed the pulse carrier frequency offset at 3000 ppm (at 7.05 and 4.7 T) or 500 ppm (at 16.4 T). Standard saturation-recovery experiments (at 7.05 T) were used to obtain T1 values.

Projection magic angle turning and phase adjusted sideband separation (MATPASS) NMR experiments[53] were performed at 4.7 T and were rotor-synchronized at a MAS rate of 40 kHz. A series of five π/6 (or π/2) pulses with pulse lengths of 0.73 μs (or 2.2 μs) were employed. A total of eight t1 increments were recorded in each experiment. The recycle delay was 50 ms.

Temperature calibration of the probes was performed in separate one-pulse MAS experiments at 7.05 and 16.4 T using the 207Pb resonance of Pb(NO3)2, with an accuracy of ±5 °C.[54,55] All 17O NMR spectra were collected on recently 17O-enriched samples packed in ZrO2 rotors with Kel-F or ZrO2 caps (for room temperature and variable temperature experiments, respectively). 17O chemical shifts were externally referenced to H2O at 0.0 ppm at room temperature. NMR spectra were processed and deconvoluted with the Bruker Topspin 3.2[56] and dmfit[57] software packages.

First-Principles Calculations

Calculations were performed with the CRYSTAL09 linear combinations of atomic orbitals (LCAO) code[58] using the B3LYP spin-polarized hybrid exchange-correlation functional. In a two-step approach, initial experimental structures were first geometry optimized with respect to lattice parameters and atomic positions using a more limited basis set (denoted BS-I). Next, single-point energy calculations were performed with an extended basis set (BS-II) to model the core region more accurately. Relevant NMR parameters (spin density at the nuclear positions, electron–nuclear dipolar tensors, and quadrupolar coupling constants) were computed after convergence of the wave function in the second step. Full details of the BS-I and BS-II basis sets are presented in the Supporting Information (SI). For all calculations, truncation thresholds of 10–7, 10–7, 10–7, 10–7, and 10–14 were applied to the integral series for Coulomb overlap, Coulomb penetration, exchange overlap, g- and n-exchange penetration, respectively.

The experimental room-temperature orthorhombic (Fmmm) structure of La2NiO4+δ (δ = 0.17)[41] was used to construct a 2 × 2 × 2 57-atom supercell (La16Ni8O33) corresponding to δ = 0.125. The supercell was tetragonal due to expansion of the orthorhombic structure by √2 along new axes equivalent to [110] and [11̅0]. Different initial Ni3+/Ni2+ configurations were explored with only minimal changes in the final optimized geometry, electronic structure, and computed properties. Ni3+ ions (d7) were initialized in the low-spin configuration (t2g6eg1, S = 1/2), as suggested by experimental evidence;[59] calculations failed to converge in the high-spin configuration. Full structural optimizations of atomic positions and lattice parameters were performed without symmetry constraints, with convergence tolerances on the SCF cycle total energy, root-mean-square (rms) gradient, and rms displacement of 10–7 au, 0.0003 au Å–1, and 0.0012 Å, respectively. As a consequence of lattice anisotropy, reciprocal space sampling employed a compressed 3 × 3 × 2 Monkhorst–Pack k-point mesh.

Two types of NMR parameters were extracted from the calculations: (1) quadrupolar coupling constants (and associated asymmetry parameters) and (2) hyperfine shifts for all oxygen sites. Quadrupolar coupling constants CQ = eQV/h and asymmetry parameters were determined from the principal components of the calculated electric field gradient (EFG) tensor, ordered such that |V| ≥ |V| ≥ |V|, where Q is the nuclear quadrupole moment (−25.58 mbarn for 17O, as experimentally determined[60]). Values of the electron spin density at the oxygen nuclear positions were converted to hyperfine (Fermi contact) 17O NMR shifts using a theoretical methodology outlined previously.[61,62] In brief, calculations were first performed in the ferromagnetic state by “locking” the alignment of the Ni spins, and then the system was allowed to relax in the absence of spin constraints to a ferromagnetic local minimum. Spin density values obtained from the relaxed system were then scaled to the paramagnetic regime at the temperature of the NMR experiment assuming ideal Curie–Weiss behavior. Experimental values of μeff = 2.56 μB and Θ ≈ −400 K were used, as previously reported for La2NiO4+δ.[63,64]

Results

Characterization of La2NiO4+δ by XRD and TGA

Owing to the wide range of oxygen nonstoichiometry reported for this system, samples have been carefully characterized by XRD and TGA. Calculated values of δ from TGA measurements were found to range from δ = 0.12 to 0.17 (Figure S2). The oxygen content is significantly affected by the 17O-enrichment procedure, increasing from δ = 0.12–0.14 for as-synthesized batches to δ = 0.15–0.17 following 17O-enrichment. Our work concurs with previous findings: treatment of La2NiO4+δ under high oxygen pressure leads to even more highly nonstoichiometric samples with δ as large as 0.3.[65]

As-synthesized samples are found to be phase-pure by XRD (Figure S1). Laboratory XRD data are not sufficiently sensitive to the lighter O atoms to permit refinement of low-occupancy interstitial sites. Nonetheless, changes in the lattice parameters (as refined to the room-temperature Fmmm structure) mirror differences in oxygen content, as previously shown.[40,47,66,67] Here, incorporation of additional interstitial oxygen in the 17O-enriched samples leads to expansion of the lattice along c, with concomitant decrease of the a and b lattice parameters (Figure S3). The refined lattice parameters for the 17O-enriched samples are in good agreement with Skinner[41] and also Aguadero et al.[44,65] at similar levels of oxygen excess (that is, similar values of δ) determined by TGA.

Following 17O-enrichment, a weak, broad feature is observed in the XRD pattern, which is consistent with the (117) reflection of La4Ni3O10 (Figure S1), suggesting that the La4Ni3O10 impurity phase (estimated at ∼3 wt %) forms during the enrichment step. Aguadero et al. and Sayers et al. also note the decomposition of La2NiO4+δ into the higher-order Ruddlesden–Popper phases La3Ni2O7 and La4Ni3O10 at high temperature and under highly oxidizing conditions.[65,68] Formation of these deleterious secondary phases may impair device longevity in functional SOFCs. In our case, these phases are difficult to distinguish from La2NiO4+δ (and from each other) due to the small phase fractions and considerable overlap of XRD reflections (Figure S1c). We turn to NMR as a (potentially) more sensitive probe of the minor impurity phases as well as the local structure of the low-occupancy interstitial sites of La2NiO4+δ.

Room-Temperature NMR

Acquisition of Broadband Spectra

Initial 17O MAS NMR spectra of the enriched samples (Figure b) reveal an extremely broad set of features spanning more than 0.5 MHz at 7.05 T, exceeding the excitation bandwidth of a single radio frequency (rf) pulse, a direct consequence of the paramagnetism of La2NiO4+δ. Acquiring the complete broadband spectrum necessitates the use of “spin–echo mapping” or “variable offset cumulative spectroscopy”, VOCS,[69] here performed by collecting and summing six subspectra (colored traces, Figure b) with progressively larger rf carrier frequency offsets (step size equal to 2500 ppm or ∼102 kHz). Pell et al. have shown that, for nonquadrupolar nuclei, spin–echo mapping under MAS achieves nearly uniform broadband excitation,[70] but no work to date has demonstrated the validity of the technique for quadrupolar nuclei such as 17O. Given the significant width of the major features in our spectra, however, any line shape distortions are likely insignificant, and the use of a short, nonselective rf pulse ensures quantitative excitation that is independent of CQ. As seen in Figure b, the spin–echo mapped spectrum (black) comprises two very broad components centered at ∼6500 ppm and ∼3500 ppm, and a narrow peak at 532 ppm with associated spinning sideband (ssb) manifold. A minor component appears at 170 ppm (Figure c), but its intensity is sample-dependent (Figure S7).

Peak Assignments

The resonances at 532 and 170 ppm fall within the 0–1000 ppm region occupied by shifts of diamagnetic oxides. Yang et al.[71] have reported a 17O shift of the tetrahedral oxygen site in hexagonal La2O3 of 584 ppm. Given the known[72] pseudotetrahedral coordination of interstitial oxygen (Oi) in La2NiO4+δ (Figure a, bottom right), we, therefore, assign the 532 ppm feature to the interstitial oxygen environment in La2NiO4+δ. We also assign the sample-dependent 170 ppm resonance to a very minor LaAlO3 impurity phase (as previously reported by Bastow et al.[73]) formed during synthesis (in an alumina crucible) but not immediately apparent in the XRD data. (Conversely, the La4Ni3O10 impurity seen by XRD is not observed in our initial NMR experiments, but is later resolved in later high-temperature spectra as a minor feature at ∼2400 ppm (see below).)

The highly shifted and broadened features at ∼6500 ppm and ∼3500 ppm are assigned to equatorial Oeq and axial Oax sites, respectively (Figure a, top and middle right). These large hyperfine shifts are attributable to delocalization of unpaired electron spin density from the 3d orbitals of the Ni2/3+ cations to the s orbitals of proximate 17O nuclei: equatorial Oeq sites lying in the perovskite layer, with two nearby Ni2/3+ cations at a short distance (∼1.9 Å), are expected to experience a stronger hyperfine coupling than Oax sites with only one directly bonded Ni2/3+ further away (∼2.2 Å). Additional support for this assignment comes from T1 relaxation measurements, being sensitive to proximity to paramagnetic centers.[74] As expected, the T1 value for the assigned Oeq site (<500 μs) is noticeably shorter than that for Oax (∼1 ms). Finally, experimental evidence presented in subsection of a much larger quadrupolar coupling constant (CQ) for Oeq is in good agreement with DFT-calculated values (subsection ), confirming the assignment.

We note that our assignment of the paramagnetic sites Oeq and Oax in La2NiO4+δ agrees with previous static 17O NMR results of the isostructural cuprate La1.85Sr0.15CuO4, with the higher frequency resonance assigned to equatorial oxygen in CuO2 planes, and the lower frequency feature assigned to axial sites.[75,76] However, the magnitude of the Fermi contact shifts is significantly smaller in the cuprate (equatorial: 1800 ppm; axial: 500 ppm), presumably as a consequence of reduced spin delocalization from Cu2+, a cation with smaller magnetic moment than Ni2+.

Quantification

All subspectra in Figure b have been recorded using a quantitative recycle delay (500 ms, at least 5 times T1 for the Oi site) so as to compare intensity across different environments. To this end, we have fitted the broadband spectrum to a sum of two Lorentzian functions (justified simply because it provided the best fit) with associated satellite transition intensity for Oeq and Oax and a CSA-only spinning sideband manifold for Oi (Figure S4). The integrated intensity ratio of the model, Oeq:Oax:Oi = 48:47:5 ≈ 2:2:0.2, agrees with the sample stoichiometry and is suggestive of a fully stochastic (i.e., not selective) 17O-enrichment. A stochastic enrichment indirectly confirms fast ionic conduction involving all distinct oxygen sites in La2NiO4+δ at the enrichment temperature (1000 °C), as expected of a fast oxide-ion conductor. We also note that the relative integrated intensity of the Oi site (∼0.2) concurs reasonably well with the oxygen excess calculated by TGA (0.15–0.17) given that we have not accounted for the signal lost from the paramagnetically relaxed Oeq and Oax sites during the refocusing period prior to acquisition, i.e., we have not performed a spin–spin (T2) relaxation correction. (We note that the short rf pulse length of π/6 is quantitative[77,78] given the size of the CQ values as determined later.) Finally, in comparing the weak LaAlO3 signal to the Oi centerband intensity, we calculate the proportion of this minor impurity for this sample to be 0.7–0.8 mol %.

DFT Calculations

First-principles periodic DFT calculations were performed to corroborate the argument for the given peak assignment, as well as to gain additional structural insights. Our La16Ni8O33 supercell, corresponding to La2NiO4.125, contains a single interstitial oxygen defect (Oi) within one of the two rocksalt layers (Figure S5b). Following geometry optimization, we observe the rotation of the NiO6 octahedra away from the neighboring interstitial defect. Frayret et al. have also reported interstitial-induced octahedral tilting in a compositionally identical supercell, corresponding to rotation along the [100] or, equivalently, [010] axes.[79] Our tilt axes could be described as being intermediate between the [100] and [110] directions, with the former dominant. Unlike the previous work, we observe minor tilting among octahedra not directly adjacent to the interstitial defect, possibly due to cooperative contraction of the unit cell along the stacking direction (c-axis).

Importantly, as also shown by Frayret et al., Oax sites directly adjacent to Oi are displaced away from the interstitial defect and slightly toward their respective Ni centers.[79] This differential displacement, correlated with proximity to the interstitial site, amounts to a splitting of the Oax sites into different types. On the basis of Ni–O bond lengths, four Oax types are identified, as depicted in Figure : (1) Oax,1, which are immediately adjacent to Oi; (2) Oax,2, which are located within the same rocksalt layer as Oi but not adjacent to Oi; (3) Oax,3, which are the oppositely positioned Oax sites in the NiO6 octahedra containing Oax,1; and (4) Oax,4, which are the oppositely positioned Oax sites in NiO6 octahedra containing Oax,2. The Oax,3 and Oax,4 sites are located within the “empty” (i.e., interstitial-free) rocksalt layer and hence not directly displaced by the interstitial defect. Average Ni–Oax bond lengths for the split sites (Oax,1 through Oax,4) are 2.13, 2.16, 2.24, and 2.26 Å, respectively, as compared to the experimental value of 2.25–2.26 Å for the stoichiometric (δ = 0) compound.[72,80]

Figure 2

Local structural distortion induced by nearby interstitial defect (Oi), from part of the DFT-optimized La16Ni8O33 supercell. Axial sites (in orange) closest to the interstitial undergo the largest displacement toward the Ni center, with concomitant tilting of the NiO6 octahedra. The four types of axial oxygen sites, ordered by increasing Ni–Oax bond length, are depicted in orange (Oax,1), green (Oax,2), cyan (Oax,3) and purple (Oax,4). Nickel atoms are depicted in gray and nonaxial (equatorial) oxygen atoms in red. (For clarity only part of the structure is shown, omitting La.)

Although this axial distortion depends on subtle, long-range interstitial and charge ordering effects (SI, section 2), it strongly affects the calculated NMR parameters. Table provides the average hyperfine shifts for equatorial, axial and interstitial sites. For each of the four Oax types (labeled Oax,1 through Oax,4 in Table and depicted in Figure ) we calculate a distinct hyperfine shift, ranging from ∼3200 to ∼3900 ppm. Shifts are inversely correlated with the Ni–Oax distance. Although distinct resonances cannot be resolved in our initial spin–echo mapped room-temperature spectrum, where all Oax sites appear within a single broad feature, the calculated hyperfine shift averaged among all axial sites (3539 ppm) is in good agreement with the experimental Oax shift of ∼3500 ppm.

Table 1

Calculated and Experimental Structural and 17O NMR Parameters (Computed Ni–O Bond Lengths, Experimental Isotropic Chemical Shifts δiso,exptl, Calculated Fermi Contact Shifts δFC,calcd, and Quadrupolar Coupling Constants CQ) for La2NiO4+δ at Room Temperaturea

				CQ (MHz)
	Ni–O dist. (Å)	δiso,exptl (ppm)	δFC,calcd (ppm)	exptl	calcd
Oeq	∼1.9 (avg)	∼6500; 6860 (6) (MATPASS)	10 322	≥4.6	4.73
Oax	∼2.2 (avg)	∼3500	3539 (avg)	<4.6	1.14 (avg)
Oax,0	–b	5590 (5)	–b	<4.6	–b
Oax,1	2.13	4775 (4)	3914	<4.6	2.35
Oax,2	2.16	4315 (3)	3821	<4.6	1.09
Oax,3	2.24	3960 (3)	3234	<4.6	0.67
Oax,4	2.26	3640 (2)	3189	<4.6	0.46
Oi	–c	532 (1)	19	<4.6	0.38

Standard errors in the fitted experimental shifts are given in parentheses.

Not observed in DFT-optimized supercell.

Not bonded to Ni.

The calculated shifts of the more distant equatorial oxygen sites are less influenced by the interstitial defect. In this case competing effects are at work: (1) a distribution of Ni–Oeq bond lengths due to lattice distortion of the −Ni–Oeq–Ni– chains, and (2) differences in spin density transfer to Oeq via the Ni d orbital due to Ni2/3+ charge ordering. Moreover, the range of calculated hyperfine shifts across all Oeq sites is small relative to the average shift of ∼10 000 ppm. In short, Oeq shifts do not cluster in distinct groups and, unlike the axial environments, cannot be easily classified by structural type. Nonetheless, the calculated hyperfine shift is much larger for Oeq than Oax, as expected, confirming the spectral assignment. The theoretical and experimental shifts for Oeq, however, differ by nearly 4000 ppm. This large discrepancy is not entirely unreasonable; theoretical calculations of 17O hyperfine shifts in the solid state remain rudimentary, particularly at the level of hybrid DFT. Only Kong et al. have reported attempts for various paramagnetic coordination complexes, with results highly functional-dependent and with errors as large as 2500 ppm.[38] We believe the error is partly attributable to a nonoptimal choice of functional (Kong et al. describe errors of nearly 6000 ppm before selection of an appropriate functional), but ultimately derives from the self-interaction error in DFT that enables excessive spin density delocalization onto the nearby Oeq sites.[81] We also note that the higher concentration of Ni2+ in the theoretical supercell relative to experiment would increase the calculated Oeq (but not Oax) shift. Additionally, residual antiferromagnetic couplings within the perovskite layers could lead to lower-than-predicted experimental Oeq shifts.

The interstitial site has a very small calculated hyperfine shift (of only 19 ppm), as expected, and hence the experimentally observed shift (of 532 ppm) is dominated by the chemical shift, not considered in these calculations.

Calculated quadrupolar coupling constants (Table ) reflect the diversity of local charge and bonding asymmetry among oxygen environments. Typical values of CQ for 17O nuclei in inorganic oxides vary from hundreds of kHz in highly symmetric alkaline earth oxides to 5–10 MHz in phosphates and chlorates;[82] here, calculated CQ’s for La16Ni8O33 span the known experimental range. The single interstitial oxygen site has a very nearly tetrahedral coordination to La3+, and a correspondingly small CQ (0.38 MHz). By contrast, Oax sites possess an octahedrally coordinated geometry with five La3+ and one Ni2/3+. In this case the charge asymmetry (tri- vs bivalent neighboring cations), distorted bond angles, and the short Ni–Oax distance yield larger CQ values. Furthermore, the axial displacement generating a distribution of Ni–Oax bond lengths also yields distinct CQ’s for each split Oax site, ranging from ∼0.5 to 2.4 MHz. Lastly, equatorial sites (Oeq) experience a locally octahedral environment as well, but with different neighbors: four La3+ and two Ni2/3+ in an axially compressed arrangement. Much shorter Ni–Oeq distances (1.9 Å) relative to La–Oeq (2.5–2.7 Å), combined with the effect of NiO6 tilting distortions, give rise to a much larger CQ (4.73 MHz).

Room-Temperature MATPASS NMR with Spectral Editing

Suspecting from DFT calculations that additional spectral features could appear at higher resolution, we have performed further NMR experiments at faster magic-angle spinning (40 kHz). Spin–echo mapped spectra acquired at this spinning frequency now show evidence for overlapping spinning sidebands from additional sites (Figure , upper black trace). However, even with fast spinning, it is difficult to clarify details of the underlying fine structure due to spinning sideband overlap. The use of higher spinning speeds (e.g., 60 kHz) is unfortunately not feasible here, as the reduced sample volume would prohibitively increase acquisition time. Alternatively, experiments at lower magnetic field would increase the effective spectral distance between sidebands, improving resolution at a given spinning speed, but second-order quadrupolar effects would likely worsen resolution.

We therefore turn to a method of spinning sideband separation, (projection) magic angle turning and phase-adjusted sideband separation, or MATPASS. This two-dimensional pulse sequence has been used by Hu et al. to obtain broadband “infinite”-MAS spectra in the case of large (>1 MHz) shift anisotropy.[83] The MATPASS experiment also succeeds in the familiar case of moderately quadrupolar nuclei (6Li, 7Li) in paramagnetic environments.[53] Applying the technique to 17O-enriched La2NiO4+δ and extracting the isotropic slice (Figure , bottom black trace and Figure S6, inset) reveals six distinct paramagnetic features from ∼3500–7000 ppm, in addition to the usual peak at 532 ppm previously assigned to Oi.

Since the calculated hyperfine shifts from DFT (subsection ) are not necessarily sufficiently accurate to discriminate among the different sites, we employ a form of spectral editing, quadrupolar filtering, exploiting the quadrupolar interaction to selectively suppress environments with large CQ to aid in the assignments. This approach hinges on CQ-dependent differences in quadrupolar nutation behavior, wherein sites with quadrupolar frequencies υQ much larger than the rf field strength υ1 experience more efficient excitation by short rf pulses, on account of selective excitation of the central transition.[84] (For spin-5/2 nuclei, the quadrupolar frequency υQ is equal to 3CQ/20.) In practice, rf pulses with short flip angles will resolve all sites regardless of CQ, whereas application of longer rf pulses (e.g., π/2 for a liquid reference) will preferentially select small-CQ environments. Kentgens has shown that for spin-5/2 nuclei, a value of υQ ≥ 6υ1 is a reasonable threshold for full attenuation of signal using a longer, π/2 pulse (calibrated on a liquid reference).[77] In our case, where υ1 = 114 kHz, this corresponds to a threshold CQ of 4.6 MHz.

Repeating the MATPASS experiment using a longer π/2 pulse (bottom blue trace, Figure ), we observe the loss of the paramagnetic feature centered at 6860 ppm; we conclude that the CQ of this site must equal or exceed 4.6 MHz. Among all DFT-calculated CQ’s, only that of Oeq (4.73 MHz) does so. On this basis, the furthest shifted feature is again assigned to Oeq. None of the other environments are fully attenuated by the longer rf pulse. Thus, the remaining five paramagnetic sites are all assigned to (distorted) Oax sites, for which CQ < 4.6 MHz (Table ).

The five distinct Oax features resolved by MATPASS experiments are extremely suggestive of the split Oax sites obtained from DFT. Here, we do not necessarily imply that the experimental axial environments correspond to the geometries depicted in Figure , but rather that the presence of some form of axial displacement is consistent with the splitting of Oax features in the spectra. For completeness we “assign” four of the five experimental Oax features to the Oax,1–Oax,4 sites. The range of experimental and calculated hyperfine shifts is in reasonable agreement, though results from DFT underestimate the experimental values. The otherwise unassigned, highly shifted Oax feature at 5590 ppm (labeled Oax,0 in Table ) could correspond to a structural motif not considered in the DFT calculations, such as an axial oxygen with two nearby interstitials, which would experience a substantial displacement and much larger hyperfine shift. This assertion seems plausible given the higher experimental concentration of interstitials (δ = 0.15–0.17) compared to the DFT-optimized supercell (δ = 0.125). It is interesting that the Oax features cluster in well-defined peaks rather than display a broad continuum, suggesting a discrete set of Ni–Oax bond lengths, which may imply a degree of two-dimensional ordering of the interstitial defects. We can compare these results to the neutron diffraction study of Demourgues et al. of “La8Ni4O17” (δ = 0.25), wherein eight Oax sites are identified, with five distinct Ni–Oax distances.[85]

Relative intensities of split Oax sites in the MATPASS spectra cannot be considered quantitative, as the apparent intensities are inversely weighted by dipolar broadening. That is, sites with large anisotropy have significant intensity distributed across the spinning sideband manifold and so appear smaller in the isotropic slice. The presence of residual spinning sidebands in the isotropic slice, as well as T2 relaxation effects and the use of a very short recycle delay (50 ms) further complicate quantification. However, the presence of several split features in the spectra implies that most axial oxygens reside in slightly displaced environments.

As a conclusive check, the isotropic shifts obtained from MATPASS were used to model spinning sideband patterns that approximately reproduce the Hahn echo spectra recorded at 40 kHz, with both short (π/6) and long (π/2) rf pulse lengths (upper traces, Figure ) as described in more detail in the SI (Figure S6).

Variable-Temperature NMR (≤150 °C)

We hypothesize that the orthorhombic-to-tetragonal phase transition near 150 °C may be associated with changes in the local distortion of the Oax environments induced by motion of nearby Oi. To test this conjecture, we employ 17O VT-NMR as a probe of thermally activated oxygen motion in La2NiO4+δ. As technical restrictions limit the concurrent use of fast MAS and sample heating, the following spectra have been acquired under slower spinning (12.5 kHz).

Focus on Interstitial Site

We first study the variable-temperature behavior of the interstitial oxide site at ∼535 ppm, choosing a field strength of 16.4 T. (This feature moves by ∼ +3 ppm at high field.) At such a large field, though the paramagnetic features (Oax, Oeq) broaden and become difficult to separate, more spinning sidebands arise for the Oi feature, potentially providing detailed information about the local geometry of this site.

Figure shows the 17O MAS NMR spectra of the interstitial oxide site in La2NiO4+δ from room temperature to 134 °C. The most salient change in the spectra is a slight broadening and a loss of signal at and above 80 °C, especially between 94 and 107 °C (Figure a). At the highest temperature measured (134 °C), at most, 3% of the initial intensity remains. Concurrent with the loss of Oi signal at 107 and 134 °C, a broad, asymmetric shoulder appears at a higher frequency of approximately 30 ppm, at 565 ppm (Figure b). The resonance falls between the pseudotetrahedral interstitial environment of La2NiO4+δ at 535 ppm and the tetrahedrally coordinated oxygen environment of La2O3 previously reported at 584 ppm.[71] Among OLa4 sites with known 17O shifts (only La2NiO4+δ in this work, LaO(OH), and La2O3), the shift moves to higher frequency with reduction of the average O–La bond length and an increase in local tetrahedral symmetry.[86] A shift of 565 ppm is therefore suggestive of OLa4 in a less stretched and distorted environment as compared to Oi in La2NiO4+δ, although not as symmetric as in La2O3. On this basis, we tentatively suggest that the feature at 565 ppm arises from OLa4 sites in a slightly distorted La2O3 phase at the surface of the La2NiO4+δ particles. (This feature is not due to a separate bulk La2O3 phase, as the 584 ppm shift of OLa4 in bulk La2O3 remains at this shift with increase in temperature; see Figure S15.) The existence of a La-enriched surface layer is consistent with reports on the unexpectedly strong preference for AO termination in ABO3 perovskites, and has moreover been observed experimentally in La2NiO4+δ via SIMS-LEIS.[87,88] Lastly, on cooling to room temperature (red trace, Figure a), the Oi signal returns at 535 ppm with, remarkably, nearly quantitative (98%) recovery of the original integrated intensity. We argue in subsection that the intensity changes are consistent with a motional process involving the interstitial defects.

Broadband Spectra

To correlate the onset of interstitial motion with mechanistic details by probing temperature-dependent changes in the paramagnetic sites, we have recorded broadband spectra at similar temperatures. Here, a lower field strength (7.05 T), and thus narrower spectral width, ensures that some signal is recorded from all sites (Oeq, Oax, and Oi) when acquiring from a single rf carrier frequency. This central-carrier approach obviates a time-consuming spin–echo mapping experiment, though also preventing quantitative comparison of intensities between different sites.

The broadband VT spectra (Figure ) display a complex temperature-dependent behavior that, for convenience, we sequentially describe in terms of the diamagnetic region (near Oi), the near paramagnetic region (near Oax), and the far paramagnetic region (near Oeq). A significant loss of intensity occurs across all sites with increasing temperature, such that the spectra in Figure are scaled arbitrarily to present similar overall signal strength.

In the diamagnetic region, signal from Oi at 532 ppm decreases with increasing temperature. The absolute integrated intensity of Oi shows a qualitatively similar loss as a function of temperature as seen at high field. As before, the Oi feature vanishes in the high-temperature spectra (130 and 140 °C), revealing the 565 ppm shoulder near Oi. Weak features at 380 ppm (ZrO2 sample holder[89]) and 170 ppm (LaAlO3 impurity) are also rendered more obvious at these temperatures in the absence of overlap by spinning sidebands of Oi (see Figure S9).

The near paramagnetic region at modest temperatures (to 110 °C) shows a moderate loss of signal at Oax, which is slightly more pronounced than that for Oi, concurrent with the appearance of spinning sidebands on top of the broad underlying paramagnetic feature. The positions of these spinning sidebands are highly temperature-dependent, and they are not associated with the fixed-position Oi feature. Above 110 °C, a major narrowing of the Oax site occurs, centering the shift at 3650 ppm, and revealing a minor feature at 2400 ppm. Moreover, small spinning sidebands associated with this latter feature appear at lower frequencies (between 2200 and 800 ppm).

Finally, the far paramagnetic region is characterized by a decrease in intensity but very little change in the line shape until above 110 °C, where the Oeq site suddenly sharpens and grows in intensity relative to Oax. This narrowing is so profound that, at 140 °C, spinning sidebands flank either side of the isotropic shift, and the apparent isotropic shift also moves to lower frequency (6000 ppm). Higher-frequency features are observed out to 8000 ppm and are approximately spaced at the MAS rate relative to the Oeq isotropic shift indicating that they correspond to the satellite transitions of Oeq.

Discussion

Kinetics of Interstitial Motion

One can postulate several reasons for the dramatic loss of the interstitial feature on heating (Figure ), such as changes in the Boltzmann distribution of spin states or the physical removal (outgassing) of 17O as O2. The first case proves unlikely, as a decrease in the population difference of the central transition spin states on heating from 35 to 134 °C (308 to 407 K) can only diminish the signal by at most about 25% (as the spin population difference varies essentially linearly with temperature). Notably, the previously assigned LaAlO3 impurity feature at 170 ppm retains the majority of its intensity with increase in temperature (Figure S8), which is consistent with this. We also discount the gaseous elimination of 17O at elevated temperature, because the recovery of the original spectrum on cooling suggests that the post-VT sample remains comparably 17O-enriched.

Instead, we propose that the spectral changes in Figure are consistent with the onset of oxide-ion dynamics on the NMR time scale, namely, exchange between the interstitial site and a paramagnetic oxygen environment. In an ideal, thermally activated two-site exchange, as the exchange rate increases, the two spectral features broaden and eventually coalesce. Furthermore, rapid exchange enhances spin dephasing and leads to greatly reduced T2 relaxation times. This in turn induces significant loss of spectral intensity in multiple-pulse NMR sequences such as the Hahn echo experiments performed here.

Assuming thermally activated (Arrhenius-type) exchange, in the so-called slow motion or “visit-limited” regime of the Meiboom chemical exchange model,[90,91]where I is the integrated signal intensity as observed experimentally, T is the sample temperature, Ea is the activation energy, and the other variables are experimental constants or proportionality constants: I0 is the integrated signal intensity from a one-pulse experiment, τ is the (fixed) rotor period, c = kexT2 (where kex is the exchange rate), A0 is the Arrhenius pre-exponential factor, and R is the gas constant. (For derivation and further details see SI, section 3.) Analyzing the loss of integrated Oi intensity in this way (after first subtracting the intensity belonging to the surface OLa4 sites at 565 ppm), we extract an Ea for interstitial motion equal to 0.59 ± 0.07 eV (see SI, section 3 and Figure S11). This result agrees well with the MD–simulated value of 0.51 eV given by Chroneos et al. for exchange between axial and interstitial sites,[45] as well as a value of 0.54 eV for oxygen self-diffusion in polycrystalline La2NiO4+δ as determined by TOF-SIMS.[92] We thus assign this motional process to interstitial–axial exchange. We note, however, that our activation energy is calculated over a much lower temperature range (<134 °C) than in the previous literature (350–700 °C).

Asymmetric two-site exchange simulations[93] (Figure S12) were performed to explore the effect of exchange between the interstitial and axial anions on the observed lineshapes. We estimate a conservative upper bound kex < 320 kHz for this process, with the correlation time for interstitial jumps no shorter than 3.2 μs, at 134 °C.

Analysis of Broadband Variable-Temperature Spectra

The complex line shape changes in the paramagnetic features (Figure ) allow us to explore the onset of oxygen motion in the context of the reported orthorhombic–tetragonal phase transition near 150 °C. The most prominent change in the spectra is the sudden narrowing of the Oeq feature at 130 °C, which suggests a much smaller distribution of isotropic, time-averaged hyperfine shifts. The further increase in the Oeq centerband intensity at 140 °C also indicates a significant lessening of sources of spectral anisotropy such as electron–nuclear dipolar broadening. We assign the Oeq line shape changes to the rocking motion of NiO6 octahedra entering the fast motion regime at, or nearly at, the phase transition temperature. On the basis of the maximum frequency separation between Oeq sites of ∼2000 ppm ≈ 82 kHz seen at room temperature, we calculate that the motional rate of rocking exceeds 180 kHz (= π/√2 × 82 kHz)[94] at 130 °C.

The previous neutron diffraction study by Skinner has also resolved a significant loss of anisotropy of the equatorial oxygen site near the phase transition temperature.[41] In that work, the Oeq thermal factors at 25 °C show a significant c-axis displacement (U33/U11 ≈ 2, U33/U22 ≈ 5), similar to the out-of-plane equatorial distortion depicted in Figure . At and above 150 °C, however, the Oeq thermal ellipsoid appears isotropic. Here, the VT-NMR spectra corroborate the collapse of the Oeq environment to an isotropic (in-plane) environment at these temperatures, consistent with fast rocking of the NiO6 octahedra.

The Oax site similarly narrows at and above 130 °C (Figure ) to a feature approximately at the average of the distorted Oax environments resolved by MATPASS experiments (shown in Figure ), which suggests a time-averaging of the mean Oax distortion. We attribute this change to a similar motional mechanism, i.e. local rocking of NiO6 octahedra, while noting that simultaneous exchange with interstitial sites competes by contributing to broadening of this site (relative to Oeq, which does not exchange at these temperatures). Furthermore, the Oax dynamics are likely subject to a distribution of motional correlation times. The presence of spinning sidebands at lower temperatures (∼70 °C), for example, suggests a partial averaging of a subset of the distorted Oax environments by motion already in the fast exchange limit. In previous VT-NMR studies of anionic conductors, the coexistence of multiple correlation times has been attributed to vacancy–dopant ordering or to the presence of mobile and rigid domains with differing defect concentrations.[18,95] In this system, the apparent variation in Oax motional rates likely results from the larger population of axial sites relative to that of the interstitial defects with which they exchange.[96]

The influence of interstitial motion on the dynamics of the Oax and Oeq sites can be inferred in several ways. We note that the calculated motional rate of NiO6 rocking (in excess of 180 kHz) is near the coalescence regime for interstitial–axial exchange (Figure S12), suggesting that near the phase transition temperature, the exchange motion is coupled to the octahedral rocking (or vice versa). Second, clearly shown in the DFT calculations, and experimentally from the distribution of shifts, the Oax distortion arises from proximity to interstitial defects, and therefore the long-range motion of interstitials necessarily contributes to averaging of the Oax distortion. We conclude that the loss, or significant reduction of, the local Oi-induced distortion occurs at or near the phase transition and is moreover correlated with exchange between interstitial and axial sites.

Extension to Other Systems and Higher Temperatures

The VT-NMR results clarify that the previously observed orthorhombic–tetragonal phase transition arises from the loss of a local structural distortion that is correlated with rapid oxide-ion dynamics. It remains unclear how this distortion leads to the cooperative tilting of perovskite layers in the bulk and thus the low-temperature orthorhombic structure, but we note that even for very subtle phase transitions, VT-NMR spectra should resolve the relevant motional changes that drive the transition. An example system is the related phase La2NiO4.11–4.13, which undergoes a subtle transition at ∼300 K with both low- and high-temperature phases nominally tetragonal, but with long-range 3D interstitial ordering only apparent below 300 K.[97] The sensitivity of the NMR spectra could in this case provide a convenient check on the interplay of oxygen motion and interstitial ordering, even where the latter property is not readily apparent by diffraction or other techniques.

Work is in progress to acquire and assign high-temperature (150 °C–800 °C) 17O NMR spectra of La2NiO4+δ to explore motion involving exchange between all of the oxygen sites and to examine the effect of temperature on oxygen interstitial content.

Conclusion

A combined experimental (NMR spectroscopy) and computational (DFT) methodology has been employed to clarify the local structure and dynamics of the mixed ionic–electronic conductor La2NiO4+δ by obtaining 17O MAS NMR spectra of this paramagnetic oxide in the solid state. Our main conclusions are as follows:

(1) Small compositional changes in La2NiO4+δ occur as a result of 17O-enrichment; we observe an increase in the oxygen excess (before: δ = 0.12–0.14; after: δ = 0.15–0.17 by XRD, δ = ∼0.2 by quantitative NMR) and the formation of small amounts (∼3 wt %) of an impurity assigned to the higher-order La4Ni3O10 phase.

(2) Room-temperature 17O MAS NMR spectra of 17O-enriched La2NiO4+δ acquired by spin–echo mapping reveal three distinct oxygen environments assigned to interstitial (Oi), axial (Oax) and equatorial (Oeq) sites, with quantitative measurements suggesting fully stochastic 17O-enrichment.

(3) DFT calculations of La2NiO4+δ provide local structural insight and are used to obtain Fermi contact (hyperfine) shifts and quadrupolar coupling constants that corroborate the assignment of the experimental spectra. In particular, paramagnetic Oeq and Oax features can be distinguished on the basis of the much larger CQ of the former (4.7 MHz vs ∼1.1 MHz).

(4) High-resolution MATPASS NMR spectra, in combination with quadrupolar filtering techniques, reveal the splitting of the Oax site into five discrete axial environments (3640–5590 ppm). Our DFT calculations also show a similar clustering of four distinct Oax shifts, which can be rationalized on the basis of Ni–Oax distances. We demonstrate that this axial splitting arises from a local structural distortion directly induced by the interstitial defect.

(5) Variable-temperature NMR spectra at high field resolve the (reversible) loss of the interstitial oxide feature due to a thermally activated motional process with Ea = 0.59 ± 0.07 eV; we assign this motion to exchange with axial sites.

(6) Analysis of the entire broadband spectrum as a function of temperature allows us to elucidate the types of motion and exchange affecting the interstitial, axial, and equatorial oxygen sites. In brief, exchange between interstitial and axial sites dominates the intensity loss of the Oi resonance. Local rocking motion of NiO6 octahedra, with associated averaging effects on axial and equatorial displacement, is dominant for the Oax and Oeq lineshapes, with a motional rate larger than 180 kHz at 130 °C. Abrupt changes in the VT-NMR spectra are associated with approach to the previously reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ, these changes occurring here between 110 and 130 °C. We observe a significant reduction in the magnitude of local structural distortions due to NiO6 octahedral motion, correlated with interstitial–axial exchange, at the phase transition temperature; this illustrates how oxide-ion motion at the atomic level directly influences the phase transition in the bulk.

The design of next-generation MIECs with improved oxide-ion conductivity relies on a fundamental understanding of the underlying anion dynamics across a wide temperature range, which 17O VT-NMR spectroscopy is uniquely poised to resolve. Work is ongoing to obtain high-temperature spectra (>150 °C) of La2NiO4+δ to provide insight into oxygen conduction mechanisms in the conditions most relevant for IT-SOFC and sensor operation. We also anticipate that cation doping of perovskite MIECs, a common strategy in tuning material properties such as the electronic and ionic conductivity, will have significant effects on the paramagnetic 17O spectra and allow for comprehensive depictions of the local structure and dynamics, as well as the fundamental defect chemistry, of these functionally relevant oxides.