Kinetics and mechanism of the cation-ordering transformation in Li2TiO3–Li3NbO4 solid solutions

Abstract

Solid solutions Li_{2 +x}Ti_{1 – 4x}Nb_3xO₃ have the cation-disordered, rock-salt structure at high temperature and the cation-ordered, low Li₂TiO₃ structure at lower temperatures. The ordering transition was studied by X-ray powder diffraction on samples heated isothermally as a function of time, temperature and composition and subsequently quenched to room temperature. A nucleation and growth mechanism was observed. In the early stages, ordered domains were surrounded by cation-disordered material; in the later stages, all of the material was ordered but was divided into domains that were in antiphase relationship to each other. Reduction in total area of antiphase boundaries provided the driving force for continued domain growth in the later stages. The kinetic data fitted a cubic growth law, similar to that used to fit grain growth during sintering of ceramics. The activation energies were large, 1.8–2.3 eV and indicated that diffusion of the multivalent cations, Ti⁴⁺ and/or Nb⁵⁺, provided the rate-limiting step.