High Resolution Magnetic Circular Dichroism and Absorption Spectra of Cs2ZrBr6:Ir4+

The high resolution absorption and MCD spectra of Ir⁴⁺ doped into the cubic host Cs₂ZrBr₆ are reported over the range $\text{∼ 11\hspace{0.17em}000–20\hspace{0.17em}000\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ at liquid helium temperature and as a function of temperature. This host has not been used previously for optical studies and yields an extraordinarily well resolved Ir⁴⁺ spectrum with individual vibronic linewidths in the range $\text{∼ 3–15\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . A study of the detailed vibronic structure of three of the bands including the associated ``hot'' absorption proves very helpful in making definitive assignments. It is shown clearly that the strong bands at $\text{∼ 17\hspace{0.17em}000}$ and 19 000 cm⁻¹ correspond to the allowed ligand‐to‐metal charge‐transfer transitions, $E_g^{\mathrm{\prime \prime }}{(}^2{T}_{\text{2g}}{\mathrm{)\; \to \; E}}_u^{\mathrm{\prime \prime }}{(}^2{T}_{\text{2u}})$ and $E_g^{\mathrm{\prime \prime }}{(}^2{T}_{\text{2g}}{\mathrm{)\; \to \; U}}_u^{\prime }{(}^2{T}_{\text{2u}})$ , and that in each case the extensive fine structure is associated with a single allowed electronic transition. The third band which shows sharp vibronic structure, at $\text{∼ 14\hspace{0.17em}500\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , is very likely the parity‐forbidden charge‐transfer transition $E_g^{\mathrm{\prime \prime }}{\mathrm{\to \; E}}_g^{\prime }{(}^2{T}_{\text{1g}})$ whose lower spin—orbit component, $E_g^{\mathrm{\prime \prime }}{\mathrm{\to \; U}}_g^{\prime }{(}^2{T}_{\text{1g}})$ , corresponds to a pair of weak absorptions $\text{∼ 12\hspace{0.17em}000\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . The present results are consistent with our previous assignment of the intense band system $\text{∼ 13\hspace{0.17em}000\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ to the ligand‐to‐metal charge‐transfer transition $E_g^{\mathrm{\prime \prime }}{\mathrm{\to \; U}}_u^{\prime }{(}^2{T}_{\text{1u}})$ split by Jahn—Teller interaction. The other component of this transition, $E_g^{\mathrm{\prime \prime }}{\mathrm{\to \; E}}_u^{\prime }{(}^2{T}_{\text{1u}})$ , is orbitally forbidden but is nominally assigned to an intense band overlapping the high energy side of the $E_g^{\mathrm{\prime \prime }}{\mathrm{\to \; E}}_u^{\mathrm{\prime \prime }}{(}^2{T}_{\text{2u}})$ transition with which it is strongly mixed. Aside from some very weak absorption in the 15 000 cm⁻¹ region, there seems to be no basis for assigning any of the spectral features observed to ligand field transitions.