On the Bonding Behaviour of Transition Metal Ions in Inorganic Solids—Optical and Epr Spectroscopic Studies on Anhydrous Phosphates and Phosphate-Silicates of Ti³⁺

Abstract

TiPO₄ (green), Ti₄P₆Si₂O₂₅ (purple-red) and TiP₃O₉ (blue) show a remarkable 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 unified set of angular-overlap model bonding parameters for the description of the spectroscopic behaviour of Ti³⁺ in anhydrous phosphates can be derived. The σ-interaction was assumed to depend on the nth power of the Ti-O bond distance, and it was found that optimum agreement with experiment occurred with n = -4.5. For the correct reproduction of the experimentally observed ground-state splittings it is necessary to allow for anisotropic π-bonding interactions between titanium and the oxygen ligands linking Ti-O coordination polyhedra.