ABSTRACT Ceramics of Li0.98?xTa1.004?x/5O3 solid solutions with 0 ≤ x ≤ 0.20 are studied by a new theoretical approach. From the experience, we have proposed the new vacancy models which are able to describe substitutional mechanism in Ni-doped lithium tantalate. Calculations of the Curie temperature in Ni-doped non stoichiometric lithium tantalate reveal good correspondence with experimental results. The substitution mechanism of the doped compositions Ni in LiTaO3 crystal is discussed. So, the mechanism of phase transition due to thermal expansion of crystal is described.
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