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 Γ7(2T2g, t2g5) 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.