Nitride Spinel: An Ultraincompressible High-Pressure Form of BeP2 N4.

Owing to its outstanding elastic properties, the nitride spinel γ-Si3 N4 is of considered interest for materials scientists and chemists. DFT calculations suggest that Si3 N4 -analog beryllium phosphorus nitride BeP2 N4 adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite-type BeP2 N4 by single-crystal synchrotron X-ray diffraction and report the phase transition into the spinel-type phase at 47 GPa and 1800 K in a laser-heated diamond anvil cell. The structure of spinel-type BeP2 N4 was refined from pressure-dependent in situ synchrotron powder X-ray diffraction measurements down to ambient pressure, which proves spinel-type BeP2 N4 a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel-type BeP2 N4 is an ultraincompressible material.

Due to a broad range of materials properties and applications, oxide spinels with the general formula AB2O4 (A, B=metal ions) are an extensively investigated field of research and numerous compounds have been reported.1 In contrast, only few representatives of nitride spinels (AB2N4) have been prepared, as yet.2 However, they have already been proven to compete with oxide materials for outstanding materials properties, especially with regard to mechanical resilience.3

The synthesis of the group 14 nitrides γ‐Si3N4,3, 4 γ‐Ge3N4,5 and Sn3N4 6 heralded a new era of nitride chemistry, as these compounds represent the first nitride spinels.2 γ‐Si3N4 has been prepared in diamond anvil cells (DAC),3 multianvil presses,4 as well as in shockwave experiments,7 and recently even the preparation of macroscopic transparent polycrystalline γ‐Si3N4 windows has been achieved.8 The isothermal bulk modulus K 0 and the Vickers hardness H V of γ‐Si3N4 have been determined to K 0=290–317 GPa3, 8, 9, 10 and H V=30–43 GPa,8, 10, 11, 12 which makes it one of the most incompressible and hardest low‐density materials.

Due to the topological rigidity of spinels, they are considered to intrinsically feature outstanding elastic properties, which might be further enhanced by strong covalent A−N and B−N bonds in the case of nitride spinels.13, 14, 15, 16 The elemental diversity of nitride spinels, however, is comparatively small, as experimental and theoretical research on nitride spinels has most widely been limited to the binary and ternary nitrides of C, Si, Ge, Sn, Pb, Ti, and Zr,16, 17, 18, 19 and only nitride spinels of tetravalent cations (AIV, BIV=Si, Ge, Sn) have been prepared, as yet.3, 5, 6, 20

Besides tetravalent cations, a nitride spinel with the general formula AB2N4 may also be composed of AVI and BIII (AVIBIII 2N4) or AII and BV cations (AIIBV 2N4), when electrostatic neutrality is stipulated. The II–V combination has been reported for several phosphorus(V) nitride materials with the general formula M IIP2N4 (M II=Be, Ca, Sr, Ba, Mn, Cd), which form PN4 tetrahedra based networks.21, 22 To the best of our knowledge, a spinel‐type phase, however, has not been reported for any AVIBIII 2N4 or AIIBV 2N4, as yet.

Theoretical investigations have predicted spinel‐type (sp) BeP2N4 as a stable polymorph at elevated pressures, which makes it a promising candidate for the first AIIBV 2N4‐type nitride spinel.21, 23, 24, 25 Hitherto, only phenakite‐type (phe) BeP2N4 has been reported, which is isoelectronic and homeotypic with β‐Si3N4 and features BeN4 and PN4 tetrahedra.21, 26, 27 By analogy with the Si3N4 polymorphism, phe‐BeP2N4 is considered to undergo a phase transition into the regular spinel structure with Be and P occupying tetrahedral and octahedral voids of the cubic close‐packing of N, respectively.21 BeN4 tetrahedra are a common motif in crystal chemistry of beryllium nitrides,28 whereas PN6 octahedra have only been reported in the high‐pressure polymorph β‐BP3N6, recently.29

The phenakite‐ to spinel‐type transition pressure of BeP2N4 has been predicted to 14–24 GPa from DFT calculations and due to its covalent character, sp‐BeP2N4 30 is suggested to be quenchable to ambient pressure as a metastable phase.21, 23, 24 Its isothermal bulk modulus has been calculated to be in the range of 263–291 GPa, which emphasizes the kinship with γ‐Si3N4.21, 23, 24, 25 Moreover, the Vickers hardness H V of sp‐BeP2N4 has been estimated to approximately H V=45 GPa using (semi‐)empirical approaches, which would make it a promising candidate for a superhard low‐density material.24, 31

Herein, we report on the phe‐BeP2N4→sp‐BeP2N4 phase transition at 47 GPa, which was investigated in a laser‐heated DAC employing in situ synchrotron X‐ray diffraction (XRD) measurements. The structure of sp‐BeP2N4 was refined using the Rietveld method and its elastic properties have been investigated upon cold decompression to ambient pressure.

phe‐BeP2N4 was initially synthesized from Be3N2 and P3N5 in a large volume press at 7 GPa and 1500 °C, employing the multianvil technique [Eq. (1), more details are provided in the Supporting Information].21

To select a suitable particle for in situ high‐pressure investigations and to verify the phenakite‐type structure of BeP2N4, several polycrystalline grains were screened by synchrotron XRD measurements at ambient conditions (Supporting Information, Figure S2). Integration of the most intense domain of a multi‐domain crystalline grain yielded a suitable single‐crystal data set (Figure S3), from which the structure of phe‐BeP2N4 was elucidated (R (no. 148), a=12.6979(15), c=8.3595(10) Å, V=1167.3(5) Å3, Z=18). All atoms were refined with anisotropic displacement parameters and the mean interatomic Be−N and P−N distances are 1.734(15) and 1.636(8) Å, respectively, which is in line with values that have been reported for the binary nitrides.28, 32 The here obtained structural model verifies the model previously reported by Pucher et al. that has been solved and refined from powder XRD data (Tables S4 and S7).21, 33 Figure 1 illustrates the single‐crystal structure of phe‐BeP2N4 as well as the constituting BeN4 and PN4 tetrahedra. More detailed information on the synchrotron XRD measurement and the structure refinement of phe‐BeP2N4 is provided in the Supporting Information (Tables S4–S7).

Figure 1

Crystal structure of phe‐BeP2N4 as obtained from single‐crystal synchrotron XRD. Be (gray) and P (black) are in fourfold N‐coordination and ellipsoids are displayed at 99 % probability level.

To investigate the predicted phe‐BeP2N4→sp‐BeP2N4 phase transition, the pre‐selected particle of phe‐BeP2N4 was loaded in a DAC with Ne serving as a pressure transmitting medium and ruby as an internal pressure standard. The sample was cold‐compressed in two steps to a maximum pressure of 47.3(9) GPa. At both steps a XRD step scan was collected that could be indexed with the metrics of phe‐BeP2N4 (Figure S8, Table S9). Owing to very low intensities, a refinement of the integrated data, however, was not feasible at those pressures. At 47.3(9) GPa the unit cell of phe‐BeP2N4 has contracted by approximately 16 vol % in comparison to the ambient pressure model (Figure S11).

To induce the phase transition into the spinel‐type structure, the phe‐BeP2N4 particle was laser‐heated from both sides to an average temperature of 1800(200) K at 47.3(9) GPa (NIR fiber laser (λ=1070 nm). This pressure was deemed sufficient for the formation of PN6 octahedra, as this motif was recently proven at 42 GPa in β‐BP3N6.29 The sample was monitored with in situ synchrotron XRD scans for the course of the heating period. After a few seconds, unidentified Bragg reflections appeared and heating for another minute led to an almost full conversion of phe‐BeP2N4 (Figure 2 and S10). Subsequently, the sample was allowed to cool down to ambient temperature, after which a XRD wide scan was collected. The powder XRD pattern of the new phase matched the Bragg reflections of the predicted spinel‐type phase (Figures 2 and S10).21 Therefore, the experimental pressure of about 47 GPa is proven sufficiently high for the phe‐BeP2N4→sp‐BeP2N4 phase transition, but the minimum transition pressure may be most likely significantly lower, considering theoretical investigations on sp‐BeP2N4 (p trans=14–24 GPa)21, 24 and experimental examinations of isoelectronic γ‐Si3N4 (p trans ≈13 GPa).3, 4, 9

Figure 2

(a) XRD scans of the BeP2N4 sample before (left) and after laser heating at 47.3(9) GPa (right), corresponding to phe‐ and sp‐BeP2N4, respectively. (b) Rietveld refinement of sp‐BeP2N4 and Ne at 47.3(9) GPa from in situ X‐ray measurements using synchrotron radiation (λ=0.2894 Å). Observed and calculated XRD intensities: black circles, gray line; difference plot: dotted gray line; positions of Bragg reflections of sp‐BeP2N4 and Ne: black and gray vertical bars. Reflections of minor residues of phe‐BeP2N4 are marked by asterisks and weak scattering of the Re gasket is labelled.21

Single‐crystal XRD measurements of sp‐BeP2N4 were not feasible, as the title compound did not form any adequate domains (Figure 2 a). Therefore, the sp‐BeP2N4 structure was refined on PXRD data obtained at 47.3(9) GPa employing the Rietveld method (Table S12).34 The DFT‐based model was used as a starting point for the refinement and was subsequently corroborated by the experimental data.21 sp‐BeP2N4 crystallizes in the regular spinel structure (Fd m, no. 227, a=7.1948(2) Å, V=372.44(3) Å3, Z=8) with site symmetries Be(8b, 3m), P(16c, . m), and N(32e, .3m).21, 35 More detailed information on the structure refinement is provided in the Supporting Information (Tables S12 and S14–S16, Figure S13). No experimental evidence for Be/P disorder or an inverse spinel structure was observed. This is in agreement with the calculations presented by Pucher et al. that characterized the inverse spinel‐type BeP2N4 to be unfavored towards phe‐ and sp‐BeP2N4.21 The refined crystal structure of sp‐BeP2N4 and the respective coordination polyhedra of Be and P are illustrated in Figure 3. The interatomic Be−N and P−N distances at 47.3(9) GPa are 1.635(2) and 1.755(2) Å, respectively, corresponding to fourfold coordinated Be and sixfold coordinated P.

Figure 3

The crystal structure of sp‐BeP2N4 as refined from PXRD data collected at 47.3(9) GPa. Be (gray) occupies tetrahedral and P (black) octahedral voids in a cubic close‐packing of N (white), corresponding to the regular spinel structure.

To verify the sp‐BeP2N4 structure in terms of electrostatics, the Madelung part of lattice energy (MAPLE) was analyzed.36 The calculated MAPLE value of sp‐BeP2N4 is 58 140 kJ mol−1, which is in very good agreement with the values calculated for phe‐BeP2N4 (58 542 kJ mol−1, ΔE=0.7 %) and the weighted sum of the binary nitrides P3N5 and Be3N2 (58 992 kJ mol−1, ΔE=1.4 %). More detailed information on MAPLE calculations are provided in Table S19.

Incremental cold decompression of sp‐BeP2N4 to ambient pressure was monitored by in situ PXRD measurements at 17 pressure points (Figure S11, Table S16). The pressure‐dependent Rietveld refinements show that sp‐BeP2N4 is quenchable to ambient conditions. The expansion of the unit cell upon decompression from 47.3(9) GPa to ambient pressure was 14 vol %, while the interatomic Be−N and P−N distances at ambient pressure expanded to 1.752(2) and 1.808(2) Å, respectively (Table S16, Figure S17). These values are in good agreement with values from the DFT model and with those reported for BeN4 and PN6 polyhedra in Be3N2 and β‐BP3N6.21, 28, 29

According to DFT calculations, sp‐BeP2N4 is considered to show a very low compressibility (263hardness and thermal stability, however, have not been investigated experimentally, as yet. As the title compound is quenchable to ambient conditions and may form at significantly lower pressures, its synthesis may be reproduced in large volume presses, providing sample amounts suitable for future investigations in terms of its elastic, physical, and optical properties. More detailed information on the BM EoS fits and the elastic properties of sp‐BeP2N4 is provided in the Supporting Information.

Figure 4

The pressure‐volume data from pressure‐dependent Rietveld refinements were fitted with a 2nd and 3rd order Birch‐Murnaghan equation of state (BM EoS,), with fitting parameters provided in the main text. The isothermal bulk modulus of >300 GPa renders sp‐BeP2N4 an ultraincompressible material.13

Recapitulating, phe‐BeP2N4 was synthesized in a high‐pressure high‐temperature reaction and the literature‐known structure model was confirmed by single‐crystal synchrotron XRD measurements. As predicted from theoretical studies, phe‐BeP2N4 was transformed into the spinel‐type phase at 47 GPa and 1800 K using a laser‐heated DAC. sp‐BeP2N4 was proven to be quenchable to ambient pressure and it is rendered an ultraincompressible material from equation of states. Therefore, the title compound is the first AIIBV 2N4‐type nitride spinel and a pioneer compound that extends the still narrow field of nitride spinels by introducing ions with oxidation states +II and +V. This should encourage further experimental investigations on mixed nitride spinels, as they appear as promising compounds for next‐generation materials. Thus, future investigations may deal with the incorporation of divalent (e.g. Mg, Zn, Cu, Ni) and pentavalent cations (e.g. V, Nb, or Ta) into II‐V nitride spinels, which might introduce intriguing optical and magnetic properties to this emerging field of research.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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