Phase transitions of the magnetoelectric A(2)NiMoO(6) (A = Ba, Sr) and Ca2NiWO6 by neutron diffraction

The magneticferroelectric double perovskites A(2)NiMoO(6) (A = Ba, Sr), and Ca2NiWO6 have been characterised using neutron powder diffraction (NPD) data. The Ba2NiMoO6 compound has previously been reported in cubic or hexagonal symmetry which led to investigation of polymorphism. High temperature NPD data of the Sr2NiMoO6 compound indicates a continuous structural phase transition from tetragonal to cubic symmetry at approximately 550 K. The reduction in symmetry is linked to spontaneous polarization allowing the structural perturbation to be described as an improper ferroelectric phase transition. The Ca2NiWO6 compound shows similar behaviour as Sr2NiMoO6 above 700 K, possibly P2(I)/n -> (I2/m) -> 14/m -> Fm-3m