Structure-property relationships in the nonlinear optical crystal KTiOPO4 investigated using NMR and a initio DFT calculations

O-17, K-39, and Ti-47,Ti-49 solid-state NMR studies of the nonlinear optical crystal potassium titanyl phosphate, KTiOPO4 (KTP), are reported. The results are compared with ab initio calculations using the WIEN2k program and discussed in specific relation to the bond-polarizability approach for the calculation of second-order nonlinear optical properties. It is found that the oxygen atoms in the anomalously short bonds to the titanium atoms (denoted by OT) have a considerably more ionic environment than the oxygen atoms (denoted by OP) that are involved in bridging TiO6 octahedra and PO4 tetrahedra. This result is consistent with the fractional covalencies computed using bond-polarizability principles for the Ti-OT and Ti-OP bonds, respectively. The NMR experiments are shown to validate independently the bond-polarizabitity approach to the calculation of nonlinear activity in KTP and demonstrate conclusively that the original notion that strong covalency in the short Ti-OT bonds is the principal source of nonlinear activity is incorrect. The field gradients at the Ti sites are estimated from NMR experiments to be similar to40 MHz, in agreement with WIEN2k calculations. These values are much larger than any found in previous studies of titanates and cannot be explained by models based on the distortion of the local octahedral oxygen coordination environment. It is suggested that both the large field gradient at the Ti sites and the variation of characteristics displayed by the different oxygen atoms in the structure may arise from other significant interactions, such as delocalization of charge or charge transfer, in significant structural chains of polyhedral units.