Structural determination of geometrically frustrated Ba₂BMoO₆ double perovskites

Double perovskites A₂BB’O₆ with “rock-salt” ordering of magnetic B’ cations have attracted substantial interest in recent years as their tetrahedral coordination of magnetic moments gives rise to geometric magnetic frustration in the presence of antiferromagnetic correlations. The double perovskite structure exhibits rich chemical versatility through a variety of structural distortions, making them ideal laboratories for systematic studies of frustration physics. In particular, systems with 4d¹ and 5d¹ electronic configurations exhibit extreme quantum effects as well as spin-orbit coupling, leading to an exotic array of ground states both proposed in theory and realized in experiment. We have synthesized and structurally characterized a number of double perovskites Ba₂BMoO₆, based on the 4d¹ Mo⁵⁺ ion and a number of trivalent B cations, using x-ray diffraction. We have determined the dependence of the lattice parameter a on the ionic radius of the B cation, and localized the cubic/tetragonal phase transition to occur at a Goldschmidt tolerance factor of 0.975-0.976.