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 (A0.5 A'0.5 )2 BB'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.  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.
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