Magnetic circular dichroism and absorption spectra of Cs2ZrCl6:Mo4+

Absorption and magnetic circular dichroism (M.C.D.) spectra of Mo4+ in Cs2ZrCl6 have been recorded under high resolution at liquid helium temperature over the range 20 000–38 000 cm-1. All of the bands can be assigned to parity-allowed, ligand-to-metal charge-transfer transitions in both MoCl6 2-(4d2) and an impurity ion which has been identified as MoOCl5 2-(4d1). The d1 and d2 systems are easily distinguished by the temperature dependence of their M.C.D. spectra, and detailed assignments are proposed for the latter. The absorption spectrum in the region 23 000–26 000 cm-1 has recently been attributed to a d→d transition in the ion MoCl6 -(4d1) [1], but the M.C.D. spectrum proves this assignment to be incorrect, and we find no evidence for the presence of this singly-charged species. From the temperature dependence of the M.C.D. spectrum of MoCl6 2-, we estimate the second-order spin-orbit splitting of the E and T 2 components of the 3 T 1g ground state to be 27 ± 5 cm-1 with E lowest. The electrostatic splitting of the 3 T 2u and 5 T 2u charge-transfer states arising from the t 1u (π + σ) →t 2g excitation is estimated to be close to 515 cm-1.