Electronic excitations in 5d4 J=0 Os4+ 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 ζ=0.34eV, Hund's coupling JH=0.43eV, the onset of excitations across the Mott gap at Δ=2.2 eV, the cubic crystal-field splitting 10Dq=3.3 eV, and the charge-transfer energy ΔCT=4.6 eV. With JH/ζ=1.3,K2OsCl6 is in the intermediate-coupling regime. In a t2g-only Kanamori picture, the above values correspond to ζeff=0.41eV and JHeff=0.28eV, 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 ΔCT, while the spin-orbit entangled intra-t2g excitations below 2 eV and hence ζ and JH are reduced by less than 4%. Furthermore, the Mott gap in K2OsBr6 is reduced to about 1.8 eV.