Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

A wide range of technological applications of lithium tantalate LiTaO3 (LT) is closely related to the defect chemistry. Several cation substitution defect models have been considered in literature since decades. Here we report a combinational approach based on DFT calculations of electric field gradients (EFG) and solid-state NMR in order to reveal the defect structure in congruent lithium tantalate. Using this approach, we were able to identify the cation substitution structure attributed to the Empty site defect model in one of two congruent LT crystals studied in our work. This observation is supported by the calculation of the energy for defect formation as well as by evaluation of the defect models based on the structural refinements and chemical reasonability reported in literature. After thermal treatment, hydrogen out diffusion and homogenation of other defects in lithium tantalate have been observed by electron spin resonance (ESR), NMR and FTIR spectroscopies. Identification of the defect structure in the second LT sample seems to be more challenging, as the extrinsic defects for balancing other impurities and/or an inhomogeneous defect distribution have to be taken into account. We have found that, although grown by the same method; the two congruent LT samples provided by two manufacturers show rather different defect structures, which is manifested not only in the distribution of EFGs at 7Li, but also in the FTIR and ESR spectra and in the 7Li spin-lattice relaxation behaviour. This observation has to be taken into account when attempting to favour one specific defect model within the scientific community and when studying the physical – particularly, magneto-optical – phenomena in the systems where lithium tantalate is used as a substrate.