Crystal Growth, Structural Transitions, and Magnetic Properties of the Fluorite-Related Osmates:  Sm₃OsO₇, Eu₃OsO₇, and Gd₃OsO₇

The Ln₃OsO₇ (Ln = Sm, Eu, Gd) compounds were grown as single crystals from molten hydroxide fluxes. At temperatures above 235, 330, and 430 K, respectively, the Ln₃OsO₇ (Ln = Sm, Eu, Gd) compounds exist in the orthorhombic space group Cmcm. When they are cooled below these temperatures, the compounds undergo a structural phase transition from space group Cmcm to P2₁nb. The structure transition results in a loss of lattice centering, a doubling of the b axis, a distortion of the vertex-shared Os−O chains, and a reduction in the coordination of one of the rare earth cations from 8-fold to 7-fold. Sm₃OsO₇ and Eu₃OsO₇ exhibit complex magnetic behavior below about 50 K, and Gd₃OsO₇ shows a ferromagnetic-like order at 34 K in applied fields of less than 10 kG.