Crystallization behavior of Li1–5x Ta1+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 Li1–5xTa1+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.