Electronic excitations in 5d4J=0Os4+ halides studied by resonant inelastic x-ray scattering and optical spectroscopy

We demonstrate that the cubic antifluorite-type halides K2OsCl6, K2OsBr6, and Rb2OsBr6 are excellent realizations of nonmagnetic J = 0 compounds. The magnetic susceptibility shows the corresponding Van Vleck type of behavior and no sign of defects. We investigate the electronic excitations with two complementary techniques, resonant inelastic x-ray scattering and optical spectroscopy. This powerful combination allows us to thoroughly study, e.g., on-site intra-t2g excitations and t2g-to-eg excitations as well as intersite excitations across the Mott gap and an exciton below the gap. In this way, we determine the electronic parameters with high accuracy, altogether yielding a comprehensive picture. In K2OsCl6, we find the spin-orbit coupling constant zeta = 0.34 eV, Hund's coupling JH = 0.43 eV, the onset of excitations across the Mott gap at A = 2.2 eV, the cubic crystal-field splitting 10Dq = 3.3 eV, and the charge-transfer energy ACT = 4.6 eV. With JH/zeta = 1.3, K2OsCl6 is in the intermediate-coupling regime. In a t2g-only Kanamori picture, the above values correspond to zeta eff = 0.41 eV and Jeff H = 0.28 eV, which is very close to results reported for related 5d4 iridates. In the tetragonal phase at 5 K, the noncubic crystal field causes a peak splitting of the J = 1 state as small as 4 meV. Compared to K2OsCl6, the bromides K2OsBr6 and Rb2OsBr6 show about 12-14% smaller values of 10Dq and ACT, while the spin-orbit entangled intra-t2g excitations below 2 eV and hence zeta and JH are reduced by less than 4%. Furthermore, the Mott gap in K2OsBr6 is reduced to about 1.8 eV.