Coexistence of spin ordering on ladders and spin dimer formation in a new-structure-type compound Sr2Co3S2O3.

We report on the syntheses and characterizations of single crystalline and polycrystalline Sr2Co3S2O3 with a novel crystal structure type. It contains Co-O 2-leg rectangular ladders and necklace ladders. The two ladders share common legs and construct a hybrid spin ladder. A rare meridional heteroleptic octahedral coordination is found for the Co2+ ions in the 2-leg ladder. Within the necklace ladders, the Co2+ ions are in trans-octahedral coordination. An antiferromagnetic order is observed at TN ~ 267 K, while a broad maximum in magnetic susceptibility is found below TN. This relatively high ordering temperature among Co-based ladder compounds is related to the highly anisotropic mer-coordination of the Co2+ ions. The trans-octahedrally coordinated Co2+ ions, on the other hand, corresponds to the possible short-range magnetic correlations through dimers with an effective . This results in a rare situation that spin ordering and spin dimers coexist down to 2 K.

High-spin Co2+ (3d7) has a spin angular momentum and an orbital angular momentum L = 3 according to Hund’s rules. With the cooperation of octahedral crystal field and spin-orbital coupling, the lowest-lying orbital level of Co2+ splits into a Kramers doublet, a quartet and a sextet. The Kramers doublet ground state has an effective with large Ising-type anisotropy and is separated with the first excited quartet (effective ) by a energy gap of about 102 K order of magnitude1234. Hence, low dimensionality in octahedral Co2+ compounds can yield novel properties due to the strong quantum fluctuations for systems. For instance, the quasi one-dimensional (1D) screw chain antiferromagnets ACo2V2O8 (A = Ba, Sr), which have distorted CoO6 octahedra, can be described in terms of a highly anisotropic effective 1D XXZ model in longitudinal fields56789. At high magnetic fields, a field-induced order-to-disorder transition above 1.8 K is observed. The quasi-2D ladder compound Na2Co2(C2O4)3(H2O)2 also contains distorted Co2+ octahedra. Its magnetic properties can be realized by a spin-ladder model and show spin-glass behavior1011.

Regarding spin-ladder structures, Co-based compounds are relatively rare compared to Fe- and Cu-based compounds. To our knowledge, besides Na2Co2(C2O4)3(H2O)2, the available examples are Co(C8H8O4), Co3(2,5-pydc)2(μ3-OH)2(OH2)2 (pydc = pyridinedicarboxylate), Co7V4O16(OH)2(H2O) and Na2−Co6(OH)3[HPO4][HPO4]3111213. The properties of Co(C8H8O4) have not been measured, while the rest of them exhibit an antiferromagnetic ordering far below room temperature. Apart from 2-leg ladders, Co(H2O){C5H5N–CH2CH(OH)(PO3)(PO3H)} contains zigzag ladders (see the schematic drawing in Fig. 1a), having frustration within the ladders14. According to magnetic susceptibility measurements, it shows no magnetic ordering down to 1.8 K, while a field-induced phase transition is observed at about 1.5 T. Necklace ladders (see Fig. 1b), which can be regarded as 3-leg zigzag ladders, are so far not found in Co-based compounds but in some Cu-based materials like ferrimagnets A3Cu3(PO4)4 (A = Ca, Sr, Pb)15.

Figure 1

The schematic drawings of (a) a zigzag lattice and (b) a necklace ladder. The dots represent magnetic ions.

In this report, the novel ladder-type compound Sr2Co3S2O3 is investigated. It demonstrates a new orthorhombic crystal structure type. In Co–O layers, the unique combination of 2-leg rectangular ladders and necklace ladders constructs a hybrid spin ladder, a new type of spin ladder. Further, a rare local symmetry of Co2+, meridional heteroleptic octahedral coordination by three O2− and three S2− ions, is revealed. With the measurements of magnetic properties and specific heat, an antiferromagnetic transition is found close to room temperature (T ~ 267 K) along with a broad maximum in magnetic susceptibility below T. The broad maximum hints at the possible coexistence of spin ordering and short-range ordering below T, where the short-range ordering may be formed from dimers with an effective .

Results

Crystal structure

Based on single crystal and powder x-ray diffraction, Sr2Co3S2O3 is determined as a new-type orthorhombic structure with a space group of Pbam, which is illustrated in Fig. 2a. The single crystal refinement details are presented in Table 1, while the corresponding atomic parameters can be found in Tables 2 and 3. The powder x-ray diffraction data and simulated Rietveld pattern are shown in Fig. 3. The Bragg peaks according to the crystal structure model obtained from the single crystal x-ray diffraction can be well assigned in the powder x-ray diffraction pattern except the peaks from a small amount of CoO impurity (~1%). This is suggestive of a high-quality powder sample. Note that the broad low-angle scattering comes from the sample holder. The elemental analysis from the energy-dispersive x-ray spectroscopy (see Supplementary information) also supports that the sample’s composition is consistent with the nominal composition.

Figure 2

The schematic drawings of crystal structure of Sr2Co3S2O3.

(a) The unit cell. (b) The mer-CoS3O3 octahedral coordination in the Co2 site. (c) The trans-CoS4O2 octahedral coordination in the Co1 site. The interatomic distances are in Å. (d) The Co-O hybrid spin ladder composed of rectangular two-leg ladders (SL) and necklace ladders (NL) alternatively along ac-plane.

Table 1

Details on the single crystal refinement of Sr2Co3S2O3 at room temperature.

Empirical formula	Sr2Co3S2O3
Crystal system	Orthorhombic
Space group	Pbam (No. 55)
Z	2
a (Å)	7.50285(4)
b (Å)	9.79549(5)
c (Å)	3.99006(2)
V (Å3)	293.246(3)
Calculated density (g/cm3)	5.2567
Range of data collection	3.42 ≤ 2θ ≤ 65.84
No. of measured reflections	2785
No. of independent reflections	1552
No. of refined parameters	33
h, k, l ranges	0 ≤ h ≤ 19, 0 ≤ k ≤ 25, 0 ≤ l ≤ 10
μ (mm−1)	27.024
R(obs), Rw(obs)	0.033, 0.090
R(all), Rw(all)	0.050, 0.14
Goodness of fit	0.70
Final difference density	+0.90/−2.78
Data base - CSD numbera	431714

aFurther details of the crystal structure investigations may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, on quoting the CSD numbers).

Table 2

Atomic positions of Sr2Co3S2O3.

Atom	Wyckoff	x	y	z	Ueq
Sr1	4h	0.09375(4)	0.15762(3)	0.5	0.00617(4)
Co1	2d	0	0.5	0.5	0.00822(9)
Co2	4g	0.25962(5)	0.41255(4)	0	0.00716(6)
S1	4g	0.4051(1)	0.15825(7)	0	0.0066(1)
O1	2a	0	0	0	0.0074(5)
O2	4h	0.2341(3)	0.4085(2)	0.5	0.0069(3)

Table 3

Anisotropic displacement parameters of Sr2Co3S2O3.

Atom	U11	U22	U33	U12	U13	U23
Sr1	0.00657(8)	0.00687(8)	0.00507(8)	−0.00011(6)	0	0
Co1	0.0055(1)	0.0100(2)	0.0092(2)	0.0022(1)	0	0
Co2	0.0069(1)	0.0110(1)	0.00356(9)	−0.00216(9)	0	0
S1	0.0068(2)	0.0075(2)	0.0056(2)	−0.0003(2)	0	0
O1	0.0054(7)	0.0102(9)	0.0067(8)	0.0011(7)	0	0
O2	0.0068(5)	0.0098(6)	0.0042(5)	0.0002(5)	0	0

Figure 3

The powder x-ray diffraction pattern of Sr2Co3S2O3 at room temperature.

The different aspects of the crystal structure of Sr2Co3S2O3 are illustrated in Fig. 2. Due to anion ordering of O and S, a classical structural description starting from a close packing is inappropriate. In a local description, Co2+ is exclusively octahedrally coordinated, either by in Co2 sites (Fig. 2b) or by in Co1 sites (Fig. 2c). The former (Co2) constitutes the 2-leg ladder while the latter (Co1) contributes the central spin chain in the necklace ladder (Fig. 2d). The 2-leg ladder and the necklace ladder share the common legs. This unique combination can be referred to as a hybrid spin ladder. The Co octahedra build up a three dimensional network by sharing faces and vertices. The interatomic distances are on average: Co–O = 2.0 Å and Co–S = 2.7 Å. These are different from comparable distances in ionic compounds like NaCl-type CoO (Co–O = 2.13 Å)16 and NiAs-type CoS (Co–S = 2.34 Å)17. The relatively short Co–O and long Co–S distances indicate anomalous bonding behavior, which is accompanied with low local symmetry at the mer-coordinated Co site. The shortest Co–Co distance, across face-sharing octahedra, is about 2.9 Å, which is too long for any direct magnetic interactions. Nine-fold coordinated Sr2+ ions act as space fillers.

The Co–O–Co angles within the 2-leg ladder are ∠180° for the rungs and ∠~169° for the legs, forming almost ideal rectangular ladders. Meanwhile, the Co–O–Co angles within the necklace ladders are ∠~94°. Between the 2-leg ladders there is no geometrical frustration, i.e. the rungs have the same periodicity in the ac-plane, but the hybrid spin ladder is frustrated due to intrinsic frustration within the necklace ladders. Three layers of the hybrid spin ladders, as displayed in Fig. 4a, reveal the connection perpendicular to the serrated hybrid layers. The interlayer couplings are possible via Co–S–Co with ∠~130°.

Figure 4

(a) The crystal structure of Sr2Co3S2O3 with 3 layers of the Co-O hybrid spin ladders. The interlayer bondings with darker color represent the Co1-Co2 interlayer coupling, while that with light color represent the Co2-Co2 interlayer coupling. (b) The comparison of ladder lattices among (left) the spin ladder in SrCu2O329, (middle) the 2-leg rectangular ladder in Sr2Co3S2O3 and (right) the necklace ladder in Sr2Co3S2O3.

Before leaving this section, we would like to remark that the oxidation state and spin state of Co ions in Sr2Co3S2O3 are expected to be +2 (d7) and high spin in virtue to the charge balance deduced from the composition and the relatively large Co-octahedra.

Magnetic properties and specific heat of polycrystals

Figure 5 shows the temperature dependence of magnetic susceptibility χ(T) and specific heat C(T) of polycrystalline Sr2Co3S2O3, respectively. It is obvious in the result from C(T) (Fig. 5b) that there is a λ-type peak at T ~ 267 K (denoted as T). Due to the absence of hysteresis comparing the measurements between increasing and decreasing temperature as shown in the inset of Fig. 5b, T indicates a second order phase transition. T can be also visible in χ(T) (Fig. 5a) that a small hump is observed around T. A clearer picture can be seen in the plot of the first derivative of magnetic susceptibility χ′(T) in the inset in Fig. 5a, which indicates a significant change in χ(T) at T. These data suggest that T corresponds to a magnetic phase transition. The magnetic entropy released from T, ΔS, is calculated to be about 2.26 J mol−1 K−1 by using the integral , where the magnetic contribution of specific heat C is obtained by subtracting the phononic background in the total specific heat. Since there is lack of non-magnetic isostructural compounds as a reference for the phononic contribution, the background is roughly defined by the specific heat below the dashed line shown in the inset in Fig. 5b. The obtained value is much smaller than the theoretical value R ln(2S + 1) ~ 11.5 J mol−1 K−1, where R = 8.31 J mol−1 K−1 is the gas constant. However, considering the purity of the sample, it is safe to assume that this magnetic entropy belongs to the title compound. The small magnetic entropy is typical for low-dimensional systems that ΔS is released in a wide temperature range around the peak. In addition to the rough approximation in our analysis, the phonon contribution is hence easily overestimated and results in the small value of ΔS. Therefore, it is not possible to determine from ΔS whether all the Co spins order at T.

Figure 5

(a) The temperature dependence of magnetic susceptibility χ(T) of polycrystalline Sr2Co3S2O3 at 3 T. The dots represent the observed data, while the solid line and the dashed line are the fitted curves according to the dimer model (Eq. 2) and the dimer model (Eq. 3), respectively. The inset shows the first derivative of magnetic susceptibility χ′(T) under zero field cooling. (b) The temperature dependence of specific heat C(T) of polycrystalline Sr2Co3S2O3 at zero field. The dotted line indicates the Dulong-Petit limit. The inset displays the comparison of the measurements of C(T) with increasing and decreasing temperatures around the transition. The dashed line is an estimate of the phononic contribution.

At T < T, χ(T) demonstrates a broad maximum around 70 K and approaches to the lowest value but non-zero around 2 K. Meanwhile, there are no anomalies in C(T) ranged from 2 K ≤ T < T, disproving the presence of any further obvious phase transitions below T. At T > T, C(T) saturates at 3NR ~ 250 J mol−1 K−1 (except the entropy release from the transition), which agrees with the Dulong-Petit limit. Here N = 10 is the number of independent atoms in the unit cell. The inverse magnetic susceptibility χ−1(T) at the range T < T < 750 K, as illustrated in Fig. 6, is not linear, which can be explained by the fact that the first excited orbital levels are thermally populated in this temperature range18.

Figure 6

The temperature dependence of inverse magnetic susceptibility χ−1(T) at T > 300 K under field cooling.

We would like to remark that Sr2Co3S2O3 is highly insulating (>5 kΩm) at room temperature, which is expected for a high-spin Co2+ oxide.

Discussion

The uniqueness of the crystal structure of Sr2Co3S2O3 are twofold: the meridional (mer) heteroleptical octahedral coordination in a magnetic ion and the hybrid spin ladder. Mer-octahedral coordinations can be found in some metal organic complexes, with either homoleptic coordination by N in e.g. mer-[Co(dien)(NO2)3] (dien = diethylenetriamine)19, [Co(dien)2]X3 · 2H2O (X = Cl, Br)20 and mer-[Ni(dien)2][SCN]221 or heteroleptic coordination by N and O in [Cr(HP2O7)(NH3)3(H2O)] · 2H2O22. For non-complex inorganic materials, the mer-heteroleptic coordination exists in M2[Nb3O5X7] (M = NH4, K, Rb, Cs; X = Cl, Br) and in La6Ti2S8O5, where the octahedra are composed by a Nb5+(Ti4+) ion surrounding by three O2− and three X−(S2−) ions232425. According to powder x-ray diffraction data and theoretical calculations, mer-TaN3O3 octahedera are also reported in diamagnetic γ- and δ-TaON2627. However, due to the d0 configuration of the Nb5+, Ta5+ and Ti4+ ions, those compounds should be diamagnetic. Hence, Sr2Co3S2O3 is, to our knowledge, the first case where a magnetic ion, here high-spin Co2+, is heteroleptically mer-octahedrally coordinated in an extended lattice. Mer-coordination for d0, d5 or d10 systems is within expectations due to spherical symmetry of these ions. However, it is exceptional to discover such coordination for a d7 system, because of the uneven electronic occupancy of the d-orbitals. Hence, the situation in Sr2Co3S2O3 offers the possibility to investigate the effect of a rare crystal field on a magnetic ion.

The hybrid spin ladder is the combination of a 2-leg ladder and a necklace ladder. The former is reminiscent of those in cuprates like La2CuO4, SrCu2O3 and Sr2Cu3O52829 (Fig. 4b). The necklace ladder can be regarded as the inverse version of the 2-leg ladder (Fig. 4b) and is related to the Cu lattice in A3Cu3(PO4)4 (A = Ca, Sr, Pb)15. However, the combination of the 2-leg ladders and the necklace ladders by sharing legs constructs a hybrid spin ladder, which is, to our knowledge, a new type of N-leg spin ladders. Its uniqueness in competing exchange interactions, including frustration, between magnetic ions can initiate further investigations of novel behaviors through the reproduction of the hybrid spin ladder.

In Sr2Co3S2O3, T is at relatively high temperature compared to other Co-based ladder compounds. This origin can be realized in the mer-coordination of the Co2 sites in the 2-leg ladders. Regarding the 2-leg ladders, there are three crucial superexchange interactions: J and J connect Co2+ ions via Co–O–Co with about ∠180° along the rungs and the legs in the 2-leg ladders, respectively (see Fig. 2d). J transforms the quasi 1D 2-leg ladders into a 3D network via long Co–S–Co with ∠103–130° (see Figs 2d and 4a). According to the Kanamori-Goodenough rules3031, J and J are expected to be antiferromagnetic interactions while J can be antiferromagnetic or ferromagnetic but relatively weak. In addition to the frustration brought from the neighbor necklace ladders, the 2-leg ladders should be able to order antiferromagnetically but not at very high temperatures due to quantum fluctuations. However, as shown in Fig. 2b, the mer-CoS3O3 octahedron has low symmetry with respect to the Co ion. This gives a strongly anisotropic crystal field to the Co ion and thus a preferred orientation for the Co spin, which is referred to the phenomenon called single ion anisotropy. Therefore, although the 2-leg ladders are expected to suppress 3D magnetic orderings because of their low dimensionality, the easy axis for the Co spins in the mer-coordination is strong enough to favor the magnetic ordering at higher temperatures.

After the discussion of the spin ordering in the Co2 sites, it naturally comes to the question about the spin ordering of the remaining Co1 sites within the necklace ladders. The observation of the broad maximum in χ(T) below T provides a hint at a rare situation. If all the Co sites order at T, the broad maximum can stem from the spin canting at the two Co sites. However, this is unlikely due to the following reasons: First, the centrosymmetric space group does not allow for the residual ferromagnetic spin component by spin canting to result in the broad maximum. Second, the Co1 sites are geometrically frustrated. They should be less likely to order at such high T unless the spin ordering is somewhat much more energetically favorable. Furthermore, the Co1 sites have trans-octahedral coordination with compressed Jahn-Teller distortion (see Fig. 2c). In a rough approximation, the different crystal fields between the two Co sites suggest that they should have different ordering temperatures. This disagrees with the absence of further phase transitions below T as seen in C(T). Alternatively, we propose that the Co1 sites could have no long-range ordering down to 2 K. In this framework, the broad maximum corresponds to short-range antiferromagentic correlations for low-dimensional materials. Owing to the mirror plane on the c-axis for the centrosymmetric Pbam space group, spin dimer formation along the c-axis is a possible candidate for the short-range ordering on the Co1 sites.

To prove this argument, the data of χ(T) below T is fitted using the following relation:

where, the first term χ0 corresponds to temperature-independent Van-Vleck paramagnetism, diamagnetism and impurity contributions. The second term χ is the magnetic susceptibility arising from dimers at all the Co1 sites, which correspond to one magnetic Co2+ ion per formula. It is modelled by the following equations32:

for and

for , where N is Avogadro constant, μ is Bohr magneton, k is Boltzmann constant, g is the Landé g-factor and x = J/kT with J being the nearest-neighbor intradimer exchange constant. The third term χ represents the magnetic susceptibility contributed from the antiferromagnetic ordering at all the Co2 sites, which correspond to two magnetic Co2+ ions per formula. Its temperature dependence is originally presumed according to the mean-field theory that χ gradually decreases with temperature and reaches of the value of at T = 0 K for powder samples. However, it was found during the analysis that the temperature dependence of χ is much weaker than that of χ, leading to the difficulty of handling multiple free parameters. Hence, to reduce the number of free parameters, this term is thus regarded as temperature independent and refined together with χ0. Note that the Curie-Weiss contribution arising from the breakdown of dimers due to crystal defects is ignored since no upturn is observed at low temperatures (<10 K).

The fitted curves for and are plotted in Fig. 5a. The curve for fails to fit the observed curve, rejecting the possibility for dimers. In contrast, the curve for agrees well with the broad maximum of χ(T). This is suggestive of the existence of quasi-1D dimers with an effective for octahedrally coordinated Co2+ ions. The slight discrepancy between the observed and fitted curves may stem from the complex dynamics between the antiferromagnetic ordering and the dimers as well as the oversimplified temperature dependence of χ. The parameters of the fitted curve for dimers are obtained as g = 3.768(8), J/k = −49.7(1) K and χ0 + χ = 0.00878(6) emu mol−1. g ≫ 2 supports the anisotropic feature of the effective in dimers. The relatively large J/k suggests that the antiferromagnetic interactions between dimers are strong. The value of is close to the expected value (~0.008) of the magnetic susceptibility for the antiferromagnetically ordered Co sites (i.e. Co2 sites) at 0 K, agreeing with the assumption made in the fitting analysis.

In order to find out the experimental evidences for the existence of dimers, we have attempted to measure Sr2Co3S2O3 by nuclear magnetic resonance (NMR) and electron spin resonance (ESR) techniques. Unfortunately, there is no resolvable signal for further analysis, because the lifetime of nuclear and electronic spin excited states are too short and thus broaden the signal. Note that the existence of spin chains is excluded since no reasonable fits are obtained by using neither the model by Bonner and Fisher33 nor the model from Padé approximations by Law et al.34.

In conclusion, single crystalline and polycrystalline samples of Sr2Co3S2O3 were successfully synthesized. The compound is identified as a new-type structure with two interesting features. One is the unique hybrid spin ladder consisted of 2-leg ladders and necklace ladders. The other is the rare mer-heteroleptical octahedral coordination in magnetic Co2+ ions, providing a novel crystal field. Through magnetic property and specific heat measurements of the polycrystalline samples, an antiferromagnetic order forms at T ~ 267 K. Such high temperature for T is correlated to the highly anisotropic mer-coordination of the Co2 sites in the 2-leg ladders, which gives an easy axis for the Co spins to order at higher temperatures than other Co-based spin ladders. Below T, a broad maximum in χ(T) is observed along with the absence of further phase transitions. It is suggestive of the short-range magnetic correlations of the Co1 sites within the necklace ladders. The data analysis for the broad maximum proposes the possible coexistence of spin ordering and spin dimers with an effective below T. However, this has to be further confirmed by additional experimental investigations.

Methods

Synthesis

Polycrystalline Sr2Co3S2O3 was synthesized by solid state reaction using SrO, Co (Alfa Aesar 99.8%), Co3O4 (Alfa Aesar 99.7%) and S (Alfa Aesar 99.95%) as starting materials. SrO was obtained by heating SrCO3 (Aldrich 99.9+%) at 1080 °C overnight at dymanic vacuum (<10−4 mbar). The starting materials were mixed to homogeneity inside a dry argon filled glovebox (O2, H2O < 1 ppm). The mixture was then pressed into pellets and inserted into an alumina crucible. The crucible was inserted into an silica tube which was immediately evacuated to high vacuum (~10−4 mbar) and sealed. The sample was annealed at 1050 °C for 20 h. The reaction process was repeated 3 times with intermediate grinding but the annealing time was set to 10 h. Small plate-like single crystals were able to obtain by the similar procedures but were annealed at 1300 °C for 12 h following with cooling to 1050 °C in 96 h. The samples are black and stable in air.

Sample characterization

Single crystal x-ray diffraction of Sr2Co3S2O3 was performed in a Bruker Apex D8 Venture with a Mo-Kα (λ = 0.71073 Å) radiation at room temperature. The numerical absorption correction was completed by using XRED (v. 1.07, STOE & Cie GmbH) and X-shape (v. 1.01, STOE & Cie GmbH). The crystal structure was determined and refined by treating the single crystal x-ray diffraction data with the JANA2006 software35. The powder x-ray diffraction was carried out in a focusing camera with a Co (λ = 1.78892 Å) radiation. The corresponding Rietveld refinement was also performed in JANA2006. Elemental analysis was conducted in an energy-dispersive x-ray spectroscopy (EDX) inside a scanning electron microscope (Philips SEM XL30).

Measurements of physical properties

The magnetic properties of the polycrystalline samples were measured by a magnetic property measurement system (Quantum Design MPMS XL). For T > 350 K, a furnace was inserted for additional heating. The specific heat measurements were performed in a physical property measurement system (Quantum Design PPMS) with the standard non-adiabatic thermal relaxation technique.

Additional Information

How to cite this article: Lai, K. T. and Valldor, M. Coexistence of spin ordering on ladders and spin dimer formation in a new-structure-type compound Sr2Co3S2O3. Sci. Rep. 7, 43767; doi: 10.1038/srep43767 (2017).

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.