A new family of perovskites RMnMnTaO6 has been synthesised at high pressure for representative R ?= ?La, Nd, Sm, Eu, Gd, Y and Lu that span the whole rare earth series. The structure type changes from tetragonal P42/n double perovskite (DDPv) with columnar R3+/Mn2+ cation order (R Sm) to monoclinic P21/n (DPv) (R0.5Mn0.5)2MnTaO6 ?Sm, Y, Lu) as R3+ size decreases. No antisite disorder or nonstoi...
