Ligand dependent magnetism of the = 3/2 Mott insulator Cs2 MX 6 (M = Ta, Nb, X = Br, Cl)

Abstract

Magnetic and structural properties of double perovskite type bromides Cs2 MBr6 (M = Ta, Nb), where Ta4+ (5d 1) and Nb4+ (4d 1) ions form the face centered cubic lattice, are investigated and compared with chlorides Cs2 MCl6. Cs2TaBr6 exhibits the effective magnetic moment of 0.24 μ B, which is much smaller than the spin only value of a d 1 ion, 1.73 μ B, and comparable to 0.25 μ B in Cs2TaCl6. On the other hand, the effective magnetic moment of Cs2NbBr6 is 0.7 μ B and is substantially smaller than 1.0 μ B in Cs2NbCl6. On cooling, successive structural and magnetic phase transitions accompanying the release of electronic entropy approximately Rln 4 in total as expected for the J eff = 3/2 state are observed. The type of the ligand changes the temperature dependence of magnetic susceptibility at low temperature as well as its magnitude. The role of the ligands on the magnetism of J eff = 3/2 Mott insulators are discussed in the light of metal–ligand hybridization and the electron–lattice coupling.