High temperature phase transition of the magnetoelectric double perovskite Sr 2NiMoO 6 by neutron diffraction

Abstract

The magnetic ferroelectric double perovskite Sr 2NiMoO 6 has been characterised using neutron powder diffraction (NPD) data and magnetic susceptibility measurements. High temperature NPD data indicate that the material undergoes a continuous structural phase transition from tetragonal ( I4 /m) to cubic ( Fm 3 ¯ m) at approximately 550 K. The transition is linked to the level of rotation of the NiO 6 and MoO 6 octahedra, which form perfectly segregated sub-lattices in the material. The reduction in symmetry at 550 K is linked to the onset of a spontaneous dipole in the material allowing the structural perturbation to be described as an improper ferroelectric phase transition. The appearance of low angle reflections in the 20 K dataset indicates the presence of an antiferromagnetic ordered ground state. The magnetic moment was refined to 1.88(8) μ B confirming that the magnetism originates from the Ni 2+ (S = 1) ions.