Sharp-Line Luminescence of ${\mathrm{Re}}^{4+}$ in Cubic Single Crystals of ${\mathrm{Cs}}_2$Zr${\mathrm{Cl}}_6$ and ${\mathrm{Cs}}_2$Hf${\mathrm{Cl}}_6$

Sharp-line luminescence has been observed for ${\mathrm{Re}}^{4+}$ ($5d^3$) in single crystals of ${\mathrm{Cs}}_2$Zr${\mathrm{Cl}}_6$ and ${\mathrm{Cs}}_2$Hf${\mathrm{Cl}}_6$ at low temperature. Well-resolved vibronic structure is seen, forming nearly perfect mirror symmetry between emission and absorption about a zero-phonon line at 13 879 ${\mathrm{cm}}^{-1\mathrm{}}$. Using the assignments of Dorain and Wheeler, these are identified as being due to transitions within the ${t_{2g}}^3$ configuration from the ${\Gamma }_7\left({}_{}{}^2{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\right)$ level to the ground state ${\Gamma }_8\left({}_{}{}^4{}_{}{}^{}A_{2g}^{}{}_{}{}^{}\right)$. Vibrational energies of the odd modes of the Re${\mathrm{Cl}}_6^{2-}$ complex are found to be the same for both the ground and excited states. The flourescence lifetime of 100 μsec at 2 K suggests the presence of other modes of decay than the observed luminescence. The luminescence can be excited either by transitions into the electric dipole allowed levels arising from $t_{2}e$ configuration or to the ${\Gamma }_8\left({}_{}{}^2{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\right)$, (${t_2}^3$) levels.