CaMnFeTaO6 has been synthesised at 1200 °C under 10 GPa pressure. Powder neutron diffraction shows that CaMnFeTaO6 adopts a double double perovskite structure (tetragonal space group P42/n, lattice parameters a = 7.683(3) and c = 7.685(7) Å) with cation disorder at all transition metal sites. Magnetization measurements reveal an apparent ferro-or ferri-magnetic transition at Tm1 = 51 K and a susceptibility peak at Tm2 = 20 K, but no long-range magnetic order is observed by neutron diffraction down to 1.5 K. This is attributed to formation of superparamagnetic clusters of ferrimagnetically ordered spins below Tm1 that freeze into a cluster spin glass at Tf =Tm2. AC magnetisation measurements confirm the cluster spin glass ground state. Disorder from substitution of Fe 2+ for Mn 2+ and Fe 3+ /Ta 5+ inversion disrupts the network of superexchange interactions leading to the cluster spin glass ground state, in contrast to other P42/n double double perovskites where long range magnetic order is stabilised.
Control of cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA′BB′O6 double double perovskites, where both A and B sites have 1:1 cation order, to (A0.5A′0.5)2BB′O6 double perovskites with fully disordered A/A′ cations can be achieved under pressure in CaMnMnWO6 and SmMnMnTaO6, enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.
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 Co2InSbO6 recovered from high pressure has a new, ordered‐R32 A2BCO6 variant of the corundum structure. Co2InSbO6 is also remarkable for showing two cation redistributions, to (Co0.5In0.5)2CoSbO6 and then Co2InSbO6 variants of the ordered‐LiNbO3 A2BCO6 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.
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