A New Cation-Ordered Structure Type with Multiple Thermal Redistributions in Co2 InSbO6.

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO3 ) and LiNbO3 type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO3 derivative with R32 symmetry has never been observed. Here we show that Co2 InSbO6 recovered from high pressure has a new, ordered-R32 A2 BCO6 variant of the corundum structure. Co2 InSbO6 is also remarkable for showing two cation redistributions, to (Co0.5 In0.5 )2 CoSbO6 and then Co2 InSbO6 variants of the ordered-LiNbO3 A2 BCO6 structure on heating. The cation distributions change magnetic properties as the final ordered-LiNbO3 product has a sharp ferrimagnetic transition unlike the initial ordered-R32 phase. Future syntheses of metastable corundum derivatives at pressure are likely to reveal other cation-redistribution pathways, and may enable ABO3 materials with the R32 structure to be discovered.

Cation ordering within extended oxide structures is an important way to control physical properties such as introduction of ferroelectricity and multiferroism from arrangements that break inversion symmetry.[ , ] This is notably illustrated by the corundum (α‐Al2O3) type A2O3 structure which has a simple centrosymmetric arrangement with rhombohedral space group symmetry. A2O9 dimer units of two octahedra sharing a common face are separated by single vacant octahedra to form AA_AA chains (Figure 1). Two cation‐ordered ABO3 derivatives are known—the ilmenite (FeTiO3) and LiNbO3 types with centric and acentric R3c symmetry respectively. Both have AB cation pairs in the dimer units with antiparallel AB_BA alignment in ilmenite but parallel AB_AB order leading to polarity in the LiNbO3 type. It is intriguing to note that a third ABO3 cation ordering type is also possible within the corundum unit cell as shown in Figure 1. This structure has R32 symmetry with AA_BB chains of dimer pairs, and no examples have been reported. Further cation ordering within the ABO3 structures leads to A2BCO6 derivatives, referred to in the literature as “ordered‐ABO3” types. These were first found in Li2MTeO6 phases which adopt the ordered‐LiNbO3 structure for M=Zr and Hf, and the ordered‐ilmenite type for M=Ge. The “ordered‐R32” A2BCO6 derivative of the R32‐type has not been reported. These three A2BCO6 arrangements all have R3 symmetry with four symmetry‐independent octahedral cation sites. This structure is known as the Ni3TeO6 (NTO) type and represents a special case of all three A2BCO6 types (Figure 1) where A=B=Ni and C=Te.

Figure 1

Crystal structures of the corundum type A2O3 structure (top), and ABO3 (middle row) and A2BCO6 (“ordered‐ABO3”, bottom row) derivatives obtained through cation ordering. Symmetry descents are indicated by the arrows and space groups are shown. Colours indicate the cation occupancies of octahedra in each structure (A/B/C=green/red/blue). The four site labels shown by the A2O3 structure are used throughout this paper, where M4=Sb in Co2InSbO6.

The non‐centrosymmetric cation ordered corundum derivatives offer a rich variety of properties[ , , ] according to their point group symmetry. R3c LiNbO3‐types (C 3v point group) have allowed polar and piezoelectric activity, and LiNbO3 itself is an important ferroelectric, piezoelectric and non‐linear optical material. The R32 ABO3 structure (D 3) is notable as belonging to the class of space groups that are non‐polar but allow enantiomorphic and piezoelectric activity, so discovered examples would be of great interest. The ordered A2BCO6 (NTO‐type) structures (C 3) have allowed polar, chiral and piezoelectric properties arising from their R3 symmetry. Further coupling of these structural orders to magnetism (multiferroism) can be introduced by use of magnetic cations that adopt long‐range spin orders at low temperatures. High pressure is often used to stabilise these acentric cation‐ordered corundum derivatives, for example, MnTiO3 changes from a centric ilmenite type at ambient pressure to an acentric LiNbO3‐type high‐pressure polymorph where weak ferromagnetism offers a mechanism for multiferroic switching. Mn(Fe0.5M0.5)O3 (M=Nb, Ta) are further examples of ABO3 LiNbO3‐types, with Fe/M disorder. Within the R3 A2BCO6 types, β‐Mn2InSbO6 has the ordered‐ilmenite arrangement while M2ScSbO6 (M=Mn, Co, Ni ) and Mn2FeWO6 are ordered‐LiNbO3 types. Mn2FeMoO6 recovered from high pressure synthesis has an ordered‐LiNbO3 structure but this changes to an ordered‐ilmenite type on heating and the stabilisation of these two types was rationalised from band structure calculations. Magnetoelectric effects are reported in Ni3TeO6[ , , ] and ternary NTO‐type analogues have recently been discovered for A3TeO6 (A=Mn, Co) and Mn3WO6 at high pressure. In this communication, we report the synthesis of the new double‐corundum material Co2InSbO6 and thermal cation redistributions of unprecedented complexity from the previously unobserved ordered‐R32 type to two different ordered‐LiNbO3 types.

A mixture of CoO, In2O3 and Sb2O5 in stoichiometric proportions for product Co2InSbO6 was treated under high pressure and temperature conditions using a multi‐anvil apparatus. Further details are in Supporting Information. A sample recovered from 6 GPa and 1373 K was found to contain a CaCl2‐type product with an orthorhombic structure that is unrelated to the corundum types and characterisation of this phase is described in Supporting Information. Synthesis under 8 GPa and 1373 K led to a recovered Co2InSbO6 product with R3 symmetry (lattice parameters a=5.2882(3) Å and c=14.029(1) Å) consistent with A2BCO6 structures shown in Figure 1.

Synchrotron powder X‐ray diffraction data from the Co2InSbO6 sample were collected in situ while heating from 300 to 1073 K to determine the structure and any thermal changes. Refinement of the recovered Co2InSbO6 product structure at 300 K (fit and results in Supporting Information) gave cation site occupancies M1=Co0.3In0.7, M2=Co0.7In0.3, M3=Co and M4=Sb. Remarkably, the two Co‐rich sites M2 and M3 are present in the same dimer units so that the cation distribution is close to the ordered‐R32 type rather than the ordered‐ ilmenite or LiNbO3 types. This is an important structural discovery given that no ordered‐R32 A2BCO6 or R32 ABO3 structures have been reported amongst many known corundum‐derived phases. The present compound thus represents a new structural type within the corundum group (Figure 1).

Comparison of the variable temperature patterns in Figure 2 shows that Co2InSbO6 persists as a R3 corundum‐derived material up to 1073 K but changes in peak positions and intensities near 900 K reveal structural rearrangement. Initial unconstrained fits (summarised in Supporting Information) demonstrated that while one cation site (M4) remains occupied by Sb throughout, Co/In occupancies at the other three sites change with temperature. Thermal variations of the cell parameters, M1–M3 site occupancies and M−O bond lengths from final refinements constrained to the overall Co2InSbO6 stoichiometry are shown in Figure 3. Further results are tabulated in Supporting Information.

Figure 2

a) Selected powder X‐ray diffraction data from the high pressure Co2InSbO6 sample collected in situ while heating from 300 to 1073 K. b) Diffraction intensity map for low‐angle (003), (101) and (012) peaks. Changes between 850 and 950 K reflect the evolving cation distributions.

Figure 3

Refined X‐ray structure parameters from Co2InSbO6 while heating from 300 to 1073 K. a) Lattice parameters and cell volume showing the structural anomaly between 850 and 950 K. b) Co occupancies at M1, M2 and M3 sites revealing Co/In intersite rearrangements. c) Average M−O bond lengths for each MO6 octahedron where M1–M3 are occupied by Co/In and Sb is at the M4 site.

Refined cell parameters for Co2InSbO6 in Figure 3a show a change in curvature on heating above 650 K and a dramatic lattice anomaly between 850 and 950 K. A negative expansion in the a parameter, a small excess positive expansion in c, and almost zero volume expansion over this interval are observed. Corresponding changes in the Co/In occupancies at the M1–M3 sites in Figure 3b reveal that two successive cation rearrangements occur on heating. Cation populations remain constant from 300 up to 650 K, reflecting the kinetic sluggishness of migration on the timescale of the X‐ray diffraction experiment. Above 650 K the M3 site remains fully occupied by Co, but the Co/In occupancies of the M1 and M2 sites gradually converge and are estimated to become equal at T c1=880 K from a mean field fit to the occupancy difference (shown in Supporting Information). This transition corresponds to a change between different A2BCO6 types in Co2InSbO6: from the ordered R32‐type in the recovered sample to an ordered‐LiNbO3 arrangement upon heating. In the latter structure where the two A sites (M1 and M2) have identical disordered Co0.5In0.5 compositions at T c1. This transition highlights the instability of the R32 cation distribution and ordered‐R32 derivative at ambient conditions. Furthermore, the ordered‐LiNbO3 phase (Co0.5In0.5)2CoSbO6 shows a thermal instability immediately above T c1 as In from the M1 and M2 sites rapidly exchanges with Co from the M3 site between T c1 and T c2≈950 K. Above T c2, the cation distribution is close to another A2BCO6 ordered‐LiNbO3 type, with A sites having M1≈80 % and M2=100 % Co. Hence the discovered sequence of structural changes (showing ideal A2BCO6 cation site occupancies as displayed in the Table of Contents graphic) is;

These observations demonstrate that the overall transformation of Co2InSbO6 from an ordered‐R32 to an ordered‐LiNbO3 polymorph occurs via a cation‐disordered (Co0.5In0.5)2CoSbO6 ordered‐LiNbO3 intermediate phase. The second transition, from a cation‐disordered to a cation‐ordered structure on heating, is unusual given the loss of configurational entropy and evidences likely metastability of the intermediate (Co0.5In0.5)2CoSbO6 phase.

Average metal–oxygen bond lengths (Figure 3c) are consistent with the Co/In occupancy rearrangements, given the cation sizes (6‐coordinate ionic radii are Co2+=0.745, In3+=0.80, and Sb5+=0.60 Å). M1−O and M2−O distances both decrease on heating from 850 to 950 K as their Co‐populations increase, while an increase in the M3−O distance reflects the almost complete replacement of Co2+ by In3+. The similar sizes and charges of Co2+ and In3+ cations allow the changing cation distributions at M1‐M3 sites while the smaller and more highly charged Sb5+ occupies only the M4 site throughout.

Interplay between cation sizes and charges provides a likely explanation for the observed sequence of structures for Co2InSbO6. Efficient cation packing is favoured under the high pressure (8 GPa) at which the initial sample was synthesised. The ordered‐R32 structure with Co2+Co2+_In3+Sb5+ chains of cation pairs is thus stabilised as Co2+ (0.745 Å) is similar in size to the average (0.70 Å) of the larger In3+ and smaller Sb5+ cations. Thermal relaxation at ambient pressure leads to (Co0.5In0.5)2.5+Co2+_(Co0.5In0.5)2.5+Sb5+ and then to Co2+In3+_Co2+Sb5+ sequences of cation pairs in the successive ordered‐LiNbO3 type products. This reduces electrostatic repulsions between cations in the dimer pairs which becomes more significant at ambient pressure where packing constraints are less important. Repulsions between cation charges q in the dimer pairs can be quantified in a simple nearest‐neighbour approximation as E=q A q B+q C q D for AB_CD cation order in the chains assuming fixed cation‐cation separations. The sequence of structures shown above as (1) have E=19→17.5→16 and the Co2+In3+_Co2+Sb5+sequence in the final ordered‐LiNbO3 product has the lowest possible electrostatic repulsion energy within the family of A2BCO6 structures (Figure 1), as the ordered‐ilmenite alternative would have greater repulsion across Co2+In3+_Sb5+Co2+ pairs. This lowering of cation‐cation repulsion is consistent with the decrease in thermal expansion of a and V cell parameters on heating across the two transitions seen in Figure 3a.

The effects of the cation rearrangement on the magnetic properties of Co2InSbO6 were explored by comparing the original sample recovered from high pressure with ordered‐R32 structure type (OR32 sample), with a sample subsequently heated to 1073 K having the final ordered‐LiNbO3 cation arrangement (OLN sample). Magnetic susceptibility measurements show that both samples are Curie–Weiss paramagnets at high temperatures (Figure 4), with effective paramagnetic moment μ eff=5.19 μB per Co2+ and Weiss temperature θ=−43 K for the OR32 sample, and μ eff=5.30 μB and θ=−106 K for OLN. The moments are in excess of spin‐only values showing that strong orbital contributions are present, and similar values up to ≈5.20 μB have been reported for other Co2+ oxides such as Co2ScSbO6. Negative values of θ indicate that dominant spin‐spin interactions are antiferromagnetic. Both samples show deviation of the susceptibility above the Curie–Weiss variation at temperatures below ≈65 K suggesting antiparallel but ferrimagnetic spin alignments, given the negative values of θ. The OR32 sample shows no discontinuity or divergence of zero‐field cooled and field cooled (ZFC and FC) susceptibilities, which suggest short range ferrimagnetism. However, the OLN sample has a sharp Curie transition at T C=65 K, similar to T C=59 K for isostructural ferrimagnetic Co2ScSbO6. This contrasting behaviour reflects a key difference in cobalt spin distributions in the two structures. In the R32 structure, the Co spins are located in dimers which results in frustration between successive dimer layers, but in the ordered‐LiNbO3 structure the spins are distributed in a less frustrated, three‐dimensional network. Both samples have susceptibility peaks at 15 K indicative of a possible antiferromagnetic or a spin‐glass transition. The latter could result from the 20–25 % Co/In disorder between two sites observed for both samples. Neutron diffraction will be needed to confirm the spin orders or their absence. Magnetization‐field loops show substantial magnetization for the two samples at low temperatures (Figure 4). The moments at 5 K and 5 T approach 2 and 1 μB per Co2InSbO6 formula unit for OR32 and OLN samples, respectively. The OR32 sample exhibits a small hysteresis at 5 K (remnant magnetization M r=0.04 μB and coercive field B c=0.03 T) but hysteresis for the OLN sample is more substantial (M r=0.12 μB and B c=0.16 T), consistent with the well‐defined ferrimagnetic transition for this phase.

Figure 4

Magnetic measurements for a) and b) the OR32, and c) and d) the OLN, samples of Co2InSbO6. a) and c) ZFC and FC susceptibilities and inverse ZFC susceptibilities with high‐temperature Curie–Weiss fits. b) and d) Magnetisation‐field loops with insets showing low field regions.

These results demonstrate that a new A2BCO6 ordered‐R32 variant of the corundum structure is stabilised in Co2InSbO6 prepared at high pressure. This discovery of dimer units containing the same cations is unprecented in ABO3 or A2BCO6 corundum derivatives and likely results from similar average cation sizes in the Co2O9 and InSbO9 dimer units minimising volume at pressure. High pressure may thus enable discovery of ABO3 phases with the as‐yet unreported R32 structure. These would be of interest for the stabilization of spin‐dimer materials with A/B=magnetic/non‐magnetic cation combinations. Co2InSbO6 is also remarkable for showing an unprecedented sequence of two cation rearrangements on heating: first to (Co0.5In0.5)2CoSbO6 and then to Co2InSbO6 phases with the ordered‐LiNbO3 structure. These rearrangements reduce cation‐cation repulsions and favour the ordered‐LiNbO3 structure that is often observed in corundum‐derived A2BCO6 materials. The cation distributions change magnetic properties as the final ordered‐LiNbO3 product has a sharp ferrimagnetic transition at 65 K whereas the initial ordered‐R32 phase has a broader feature indicative of short‐range spin ordering. Both samples have susceptibility peaks at 15 K indicative of an antiferromagnetic or spin‐glass transition. All of the Co2InSbO6 phases have acentric R3 symmetry, and so are of interest for future exploration of acentric and multiferroic properties.

In conclusion, Co2InSbO6 and previously‐reported Mn2FeMoO6 demonstrate that high pressure may be used to recover metastable cation arrangements within the corundum family that can be thermally relaxed to new structures. This enables dependence of physical properties on the cation‐ordering patterns to be explored. Further syntheses of high‐pressure corundum derivatives are likely to reveal other cation‐redistribution pathways, and may enable ABO3 materials with the as‐yet unreported R32 structure type to be discovered.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

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