Characterization of the charge transfer luminescence of the [WO6]6− octahedron in Ca3WO6 and the [WO5]4− square pyramid in Ca3WO5Cl2

Abstract

Charge transfer (CT) luminescence of different types of polyhedra, [WO₅]⁴⁻ in Ca₃WO₅Cl₂ and [WO₆]⁶⁻ in Ca₃WO₆, is characterized by spectroscopic experiments and ab initio calculations. According to the geometry optimization, W⁶⁺ ions form five-fold [WO₅]⁴⁻ square pyramids in Ca₃WO₅Cl₂ because of a large interatomic distance between W⁶⁺ and Cl⁻ of 3.266 Å. The analysis of the density of electronic states reveals the ionic character of Cl⁻ ions to the W⁶⁺ ions in the Ca₃WO₅Cl₂ lattice, resulting in the observed broad luminescence band peak at 488 nm of the single-crystal Ca₃WO₅Cl₂ sample being assigned to the CT transition in the [WO₅]⁴⁻ square pyramid. Compared with the [WO₆]⁶⁻ octahedron in Ca₃WO₆, the [WO₅]⁴⁻ square pyramid shows an inconsistent CT energy shift: higher CT absorption and lower luminescence energies. The larger bandgap brings about higher absorption energy due to the structural and compositional features of the orthorhombic Ca₃WO₅Cl₂. The redshifted CT luminescence band and small activation energy for the thermal quenching of the Ca₃WO₅Cl₂ sample are explained, assuming that the CT states of the anisotropic [WO₅]⁴⁻ square pyramid take a larger offset in the configurational coordinate diagram than the [WO₆]⁶⁻ octahedron.