Structural and Magnetic Properties of Some Vacancy-Ordered Osmium Halide Perovskites

Abstract

The structures and magnetic properties of the Os⁴⁺ (5d⁴) halides K₂OsCl₆, K₂OsBr₆, Na₂OsBr₆, and Na₂OsBr₆·6H₂O are described. K₂OsCl₆ and K₂OsBr₆ have a cubic vacancy-ordered double perovskite structure but undergo different symmetry-lowering structural phase transitions upon cooling associated with a combination of the relative size of the ions and differences in their chemical bonding. The structure of Na₂OsBr₆·6H₂O has been determined for the first time and the thermal stability of this has been established using a combination of in situ diffraction and TGA. Na₂OsBr₆·6H₂O and Na₂OsBr₆ are isostructural with the analogous iridium chlorides, Na₂IrCl₆·6H₂O and Na₂IrCl₆, and dehydration proceeds via different intermediate phases. The magnetic moments of four compounds display a Kotani-like behavior consistent with a J_eff = 0 ground state; however, the magnetic susceptibility measurements reveal unusual low temperature properties indicative of a weak magnetic ground state.