Spin-orbit coupling in a half-filled t2g shell: The case of 5d3 K2ReCl6

The half-filled t 2 g shell of the t 3 2 g configuration usually, in LS coupling, hosts a S = 3 / 2 ground state with quenched orbital moment. This state is not Jahn-Teller active. Sufficiently large spin-orbit coupling zeta has been predicted to change this picture by mixing in orbital moment, giving rise to a sizable Jahn-Teller distortion. In 5 d 3 K 2 ReCl 6 we study the electronic excitations using resonant inelastic x-ray scattering and optical spectroscopy. We observe on-site intra- t 2 g excitations below 2 eV and corresponding overtones with two intra- t 2 g excitations on adjacent sites, the Mott gap at 2.7 eV, t 2 g -to- e g excitations above 3 eV, and charge-transfer excitations at still higher energy. The intra- t 2 g excitation energies are a sensitive measure of zeta and Hund's coupling J H . The sizable value of zeta approximate to 0 . 29 eV places K 2 ReCl 6 into the intermediate coupling regime, but zeta/ J H approximate to 0 . 6 is not sufficiently large to drive a pronounced Jahn-Teller effect. We discuss the ground state wave function in a Kanamori picture and find that the S = 3 / 2 multiplet still carries about 97% of the weight. However, the finite admixture of orbital moment allows for subtle effects. We discuss small temperature-induced changes of the optical data and find evidence for a lowering of the ground state by about 3 meV below the structural phase transitions.