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

The structures and magnetic properties of the Os (5d) halides KOsCl, KOsBr, NaOsBr, and NaOsBr·6HO are described. KOsCl and KOsBr 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 NaOsBr·6HO 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. NaOsBr·6HO and NaOsBr are isostructural with the analogous iridium chlorides, NaIrCl·6HO and NaIrCl, and dehydration proceeds via different intermediate phases. The magnetic moments of four compounds display a Kotani-like behavior consistent with a = 0 ground state; however, the magnetic susceptibility measurements reveal unusual low temperature properties indicative of a weak magnetic ground state.