Characterization of the charge transfer luminescence of the [WO 6 ] 6- octahedron in Ca 3 WO 6 and the [WO 5 ] 4- square pyramid in Ca 3 WO 5 Cl 2 .

Charge transfer (CT) luminescence of different types of polyhedra, [WO 5 ] 4- in Ca 3 WO 5 Cl 2 and [WO 6 ] 6- in Ca 3 WO 6 , is characterized by spectroscopic experiments and ab initio calculations. According to the geometry optimization, W 6+ ions form five-fold [WO 5 ] 4- square pyramids in Ca 3 WO 5 Cl 2 because of a large interatomic distance between W 6+ and Cl - of 3.266 Å. The analysis of the density of electronic states reveals the ionic character of Cl - ions to the W 6+ ions in the Ca 3 WO 5 Cl 2 lattice, resulting in the observed broad luminescence band peak at 488 nm of the single-crystal Ca 3 WO 5 Cl 2 sample being assigned to the CT transition in the [WO 5 ] 4- square pyramid. Compared with the [WO 6 ] 6- octahedron in Ca 3 WO 6 , the [WO 5 ] 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 Ca 3 WO 5 Cl 2 . The redshifted CT luminescence band and small activation energy for the thermal quenching of the Ca 3 WO 5 Cl 2 sample are explained, assuming that the CT states of the anisotropic [WO 5 ] 4- square pyramid take a larger offset in the configurational coordinate diagram than the [WO 6 ] 6- octahedron.