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

The structures and magnetic properties of the Os 4+ (5d 4 ) halides K 2 OsCl 6 , K 2 OsBr 6 , Na 2 OsBr 6 , and Na 2 OsBr 6 ·6H 2 O are described. K 2 OsCl 6 and K 2 OsBr 6 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 Na 2 OsBr 6 ·6H 2 O 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. Na 2 OsBr 6 ·6H 2 O and Na 2 OsBr 6 are isostructural with the analogous iridium chlorides, Na 2 IrCl 6 ·6H 2 O and Na 2 IrCl 6 , 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.