Preparation and defect structure analysis of near-stoichiometric lithium tantalate wafers

Abstract

A vapour transfer equilibrium (VTE) method has been used to prepare near-stoichiometric lithium tantalate (NSLT) crystals with different Li contents. The NSLT crystals were tested and analyzed by differential thermal analysis (DTA) and X-ray photoelectron spectroscopy (XPS) to investigate the effect of Li content on the Curie temperature and internal defects of NSLT crystals. This study found that when the Li content increased in the NSLT wafer, the binding energy corresponding to the peak of the Ta_4f electron layer in the XPS spectrum first decreased and then increased, indicating that the proportion of Ta valence states was different in wafers with different Li contents. From XPS energy spectrum analysis, it can be seen that the lithium tantalate crystal contains Ta⁵⁺, Ta⁴⁺, Ta³⁺ and lower-valence Ta. As the Li content increases in the NSLT wafer, Ta⁴⁺ disappears and the proportion of Ta⁵⁺ decreases initially, follows by a later increase and then subsequent further decrease. However, the change in proportion of Ta³⁺ and lower-price Ta is completely opposite to that of Ta⁵⁺, showing a trend of first rising, then falling and then finally rising again. Moreover, when the Li content is 49.751% in the NSLT wafer, the proportion of Ta⁵⁺ reaches a maximum, showing that at this Li concentration the NSLT crystal has a more perfect lattice structure. In this study, we propose a mixed defect model in which polarons coexist with Li vacancies and Ta inversion, explaining the change in Ta valence state in lithium tantalate crystals. This model is more in line with the observed results in this work. The new hybrid defect model and the variation law of Ta valence state with Li concentration proposed in this paper provide a new direction and experimental proof for the defect study of NSLT crystals, and also provide a theoretical basis to explore the Li content at the best physical properties of NSLT crystals.