Crystallization behavior of Li_1–5x Ta_1+xO₃ glasses synthesized from liquid precursors

The crystallization of amorphous oxides synthesized by pyrolytic decomposition of mixed Li and Ta 2-ethylhexanoates and alkoxides has been investigated. The study was motivated by thermodynamic considerations that, in the light of experience in other systems, suggest the potential for metastable extension of the LiTaO₃ homogeneity range. Materials investigated are described by the general stoichiometry Li_1–5xTa_1+xO₃ and include Li₂O contents from 0 to 70 mol% (20.18 ≤ x ≤ 0.2). The samples were prepyrolyzed at 400 °C and subsequently crystallized by heat treatments in air at 550–700 °C for 0.1–100 h. The first product of crystallization for compositions from 30 to 65% Li₂O is always the LiTaO₃ phase. Extensive characterization by x-ray and neutron diffraction, as well as ⁷Li-NMR spectroscopy, revealed that this phase evolves with a structure and stoichiometry close to equilibrium. For Li⁺-deficient compositions, excess Ta⁵⁺ is rejected to the amorphous constituent during crystallization and eventually gives rise to the evolution of Ta₂O₅, with the LiTa₃O₈ phase suppressed in all cases. Similar observations were made for the Li⁺-rich compositions, but the evolving second phase is the equilibrium Li₃TaO₄. The absence of solubility extension in LiTaO₃, which appears feasible from a thermodynamic viewpoint, is ascribed to the large differences in mobilities of the Li and Ta species in the parent amorphous oxide resulting from pyrolysis.