Neutron-Diffraction Study of Antiferromagnetic Ba2CoWO6 and Ba2NiWO6

Abstract

A number of ordered perovskite‐type compounds A₂BB′O₆, in which B and B′ are, respectively, a 3d transition metal cation and a diamagnetic cation, undergo antiferromagnetic transitions. The latter have been interpreted in terms of antiferromagnetic ordering of the second kind arising from appreciable superexchange interactions over quite long distances involving two anions and one diamagnetic cation. A neutron diffraction study has been made of polycrystalline samples of two such compounds, Ba₂CoWO₆ and Ba₂NiWO₆. The crystal structures are cubic, and there is a slight displacement of the oxygen ions from the ideal perovskite positions towards the more highly charged W ions of about 0.1 Å. At 4.2°K the magnetic unit cell edge is double that of the crystallographic cell in both compounds and the observed magnetic peaks are consistent with ordering of the second kind. The spin direction in Ba₂NiWO₆ is perpendicular to the [111] axis, but in Ba₂CoWO₆ it appears to lie roughly 20° from the [111] axis.