Magnetic circular dichroism and absorption spectra of Cs2ZrCl6:Mo4+

Abstract

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.