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Understanding and control of point defect populations in lithium tantalate

Ivy, Jacob M.
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Abstract
Lithium tantalate (LiTaO3, LT) is a ferroelectric material that is used in single crystal form for a variety of electro-optic, nonlinear optic, photo-refractive, and acoustic devices and applications. Commercial LiTaO3 single crystals are grown at a congruently melting composition which is lithium deficient, resulting in the formation of point defect complexes of lithium vacancies (VLiʹ) and tantalum antisite ions (TaLi••••). These defects play an important role in material properties such as Curie temperature (TC), ferroelectric coercive field, and thermal conductivity. Removing the intrinsic point defect clusters in LiTaO3 requires extensive thermal treatment in Li2O-rich atmospheres, the fundamentals of which are poorly understood. In this thesis, diffusion coefficients of lithium are measured via Raman spectroscopy to provide insight into the vapor transport equilibration (VTE) process that supplies lithium to lithium-deficient LiTaO3. This information is used to develop a sample set of single crystals possessing different concentrations of VLiʹ and TaLi•••• clusters to quantify the effects of such defect clusters on acoustic phonon velocity and thermal conductivity. Acoustic velocities measured by inelastic neutron diffraction and ultrasonic transducers indicate that increasing point defect concentrations increase acoustic velocity. Thermoreflectance measurements show that thermal conductivity decreases by a factor of 2 between stoichiometric and congruent LiTaO3 crystals as point defect populations increase. Polycrystalline LiTaO3 fabrication, something not yet commercially accessible and rarely demonstrated in the literature, is also undertaken. Successful sintering of LiTaO3 is hindered by an anisotropic coefficient of thermal expansion (CTE) and by ionic species with large differences in mobility and volatility. Informed by our work on Li2O diffusion in single crystals and guided by thermodynamic calculations from collaborators, we demonstrate the first pressureless sintering of single-phase LiTaO3 to up to 95% relative density. We also take initial steps towards using Templated Grain Growth to mitigate CTE-induced stresses in sintered polycrystalline LiTaO3.
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