Sharp-Line Luminescence from ${\mathrm{Os}}^{4+}$ and ${\mathrm{Mo}}^{3+}$ in ${\mathrm{Cs}}_2$Hf${\mathrm{Cl}}_6$

Sharp-line luminescence spectra have been obtained for ${\mathrm{Os}}^{4+}$ ($5d^4$) and ${\mathrm{Mo}}^{3+}$ ($4d^3$) in single cubic crystals of ${\mathrm{Cs}}_2$Hf${\mathrm{Cl}}_6$. For ${\mathrm{Os}}^{4+}$, luminescence from both the ${}_{}{}^1{}_{}{}^{}E\left({\Gamma }_3\right)$ and ${}_{}{}^1{}_{}{}^{}T\left({\Gamma }_5\right)$ levels of the $t^4$ configuration to all the spin-orbit split levels of the ${}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}$ ground state are observed. Luminescence spectra from the ${}_{}{}^2{}_{}{}^{}E$ ("$R$" lines) of ${\mathrm{Mo}}^{3+}$ are reported for the first time. Sharp vibronic lines are observed in the vicinity of 1. 1 μ due to coupling with odd-mode vibrations of the nearest-neighbor complex. Excitation spectra reveal pumping due to absorption into the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{}$ bands at 23.0 × ${10}^3$ ${\mathrm{cm}}^{-1\mathrm{}}$ and 19.5 × ${10}^3$ ${\mathrm{cm}}^{-1\mathrm{}}$, respectively, and to the ${}_{}{}^2{}_{}{}^{}T$ level at 14.0 × ${10}^3$ ${\mathrm{cm}}^{-1\mathrm{}}$. The excitation bands do not correlate with the major absorption bands observed in these crystals, suggesting that other charge states of molybdenum are simultaneously present in the crystals.