Characterization of the charge transfer luminescence of the [WO] octahedron in CaWO and the [WO] square pyramid in CaWOCl.

Charge transfer (CT) luminescence of different types of polyhedra, [WO] in CaWOCl and [WO] in CaWO, is characterized by spectroscopic experiments and calculations. According to the geometry optimization, W ions form five-fold [WO] square pyramids in CaWOCl because of a large interatomic distance between W and Cl of 3.266 Å. The analysis of the density of electronic states reveals the ionic character of Cl ions to the W ions in the CaWOCl lattice, resulting in the observed broad luminescence band peak at 488 nm of the single-crystal CaWOCl sample being assigned to the CT transition in the [WO] square pyramid. Compared with the [WO] octahedron in CaWO, the [WO] 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 CaWOCl. The redshifted CT luminescence band and small activation energy for the thermal quenching of the CaWOCl sample are explained, assuming that the CT states of the anisotropic [WO] square pyramid take a larger offset in the configurational coordinate diagram than the [WO] octahedron.