Vibronic splitting of Γ8 electronic states of Ir+4 in single crystals of Cs2ZrCl6 at 4.2 °K

Single cubic crystals of Cs₂ZrCl₆ containing Ir⁺⁴ exhibit two absorption bands with extensive vibronic structure at 4.2 °K. Two progressions in the symmetric stretch mode (ν₁) show a splitting of the absorption lines as a function of the number of quanta of the ν₁ mode. Analysis shows that the electronic transitions must be from the ground ${\Gamma }_7{(}^2{T}_{\text{2g}}{\mathrm{,\hspace{0.17em}t}}_{\text{2g}}^5)$ state to excited Γ₈ quartet states, which are coupled to a threefold degenerate vibration mode of symmetry Γ₄ or Γ₅ and n modes of the ν₁ mode. Zeeman measurements at 70 kG confirm the assignment for one progression. Arguments are presented to show that the threefold degenerate vibration mode corresponds to the transverse components of the ν₄(Γ₄u) mode of the face‐centered cubic lattice.