ON THE BONDING BEHAVIOR OF TRANSITION-METAL IONS IN INORGANIC SOLIDS - OPTICAL AND EPR SPECTROSCOPIC STUDIES ON ANHYDROUS PHOSPHATES AND PHOSPHATE-SILICATES OF TI3+

TiPO4 (green), Ti4P6Si2O25 (purple-red) and TiP3O9 (blue) show a remar kable variety of colours. We compare the results of optical and e.p.r. spectroscopic measurements on these compounds with d-electron energy levels for Ti3+ obtained by using the angular-overlap model. Thus, a u nified set of angular-overlap model bonding parameters for the descrip tion of the spectroscopic behaviour of Ti3+ in anhydrous phosphates ca n be derived. The sigma-interaction was assumed to depend on the nth p ower of the Ti-O bond distance, and it was found that optimum agreemen t with experiment occurred with n = -4.5. For the correct reproduction of the experimentally observed ground-state splittings it is necessar y to allow for anisotropic pi-bonding interactions between titanium an d the oxygen ligands linking Ti-O coordination polyhedra.