Double Double to Double Perovskite Transformations in Quaternary Manganese Oxides.

Control of cation ordering in ABX perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA'BB'O double double perovskites, where both A and B sites have 1:1 cation order, to (A A' ) BB'O double perovskites with fully disordered A/A' cations can be achieved under pressure in CaMnMnWO and SmMnMnTaO , 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 CaMnMnWO to spin glass behaviour in the highly frustrated double perovskite polymorph.