Approaches to understanding structure—property relationships in nonlinear optical materials

Abstract

Ab initio calculations of the electronic structure of TiO6 octahedra are used to provide some insight into the nonlinear optical properties of KTP (potassium titanyl phosphate) crystals. Such octahedra tield doubly charged anions which, when abstracted from the unit cell, are stable relative to the isolated atoms once hydrogen are attached to the four oxygens normally coordinated to phosphorus. Initial results suggest that at least two configurations may contribute to the ground-state electronic structure. In the configuration with lowest energy, the two highest occupied molecular orbitals establish weak dπ-pπ bonds between titanium and an oxygen located unusually close to the metal. The next orbital shows a a bond between the titanium and the nearby oxygen, while the distant (trans) oxygen remains nonbonded. In the second configuration, the π bond is absent but there is a significant transfer of charge between the two oxygens trans to Ti(2) in the -O-Ti-O-Ti- chains, with the shift effected through a d-p σ bond. Optical properties of KTP and related systems also are investigated experimentally by photoelectron spectroscopy and SHG (second harmonic generation) measurements on powders.