Investigation of the structure and optical properties of LiNb1-xTaxO3

The structure and and optical properties of lithium niobate-tantalate have been studied with a focus on the Ta-rich end of the solid solution series. Structural studies using a range of X-ray and neutron powder diffraction measurements between -230 and 820 C have been performed on the full compositional range. Rietveld refinements of the collected diffraction data have allowed the structural changes to be investigated as a function of temperature and Ta content. To complement the structural investigations, the effect of the changes in the Li content in lithium tantalate has also been investigated. As the Li content is decreased or the temperature is increased, the Ta displacement and the octahedral tilt decrease which coincides with an increase in the birefringence. Crystals of lithium niobate-tantalate, across the full compositional range have been made using a lithium vanadate flux growth technique. Single-crystal diffraction measurements confirmed the results of the powder diffraction measurements; the Nb/Ta displacement and octahedral tilt both decrease as the Ta content is increased. This also results in a decrease in the lattice parameters from lithium niobate to lithium tantalate. Birefringence measurements on the crystals as a function of temperature have been used to determine the point that the crystals become zero-birefringent and by comparison with the structural studies, have shown that the structures remain polar through the zero-birefringence points. Finally, a series of DFT calculations on the structures determined using Rietveld refinements of the neutron powder diffraction measurements on LT have been used to investigate the effect of each atom on the optical properties. The optical properties are insensitive to the position of the Li and depend on the O and Ta positions. The position of the Ta atom has the largest effect on the optical properties and the calculated birefringence increases with increasing temperature as expected from experimental measurements.