Ligand dependent magnetism of theJeff= 3/2 Mott insulator Cs(= Ta, Nb,= Br, Cl).

Magnetic and structural properties of double perovskite type bromides CsBr(= Ta, Nb), where Ta(5) and Nb(4) ions form the face centered cubic lattice, are investigated and compared with chlorides CsCl. CsTaBrexhibits the effective magnetic moment of 0.24, which is much smaller than the spin only value of aion, 1.73, and comparable to 0.25in CsTaCl. On the other hand, the effective magnetic moment of CsNbBris 0.7and is substantially smaller than 1.0in CsNbCl. On cooling, successive structural and magnetic phase transitions accompanying the release of electronic entropy approximatelyln 4 in total as expected for the= 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= 3/2 Mott insulators are discussed in the light of metal-ligand hybridization and the electron-lattice coupling.