Pressure induced solid-solid reconstructive phase transition in LiGaO2 dominated by elastic strain

Abstract

Pressure induced solid-solid reconstructive phase transitions for graphite-diamond, and wurtzite-rocksalt in GaN and AlN occur at significantly higher pressure than expected from equilibrium coexistence and their transition paths are always inconsistent with each other. These indicate that the underlying nucleation and growth mechanism in the solid-solid reconstructive phase transitions are poorly understood. Here, we propose an elastic-strain dominated mechanism in a reconstructive phase transition, $\ensuremath{\beta}\ensuremath{-}\mathrm{LiGa}{\mathrm{O}}_{2}$ to $\ensuremath{\gamma}\ensuremath{-}\mathrm{LiGa}{\mathrm{O}}_{2}$, based on in situ high-pressure angle dispersive x-ray diffraction and single-crystal Raman scattering. This mechanism suggests that the pressure induced solid-solid reconstructive phase transition is neither purely diffusionless nor purely diffusive, as conventionally assumed, but a combination. The large elastic strains are accumulated, with the coherent nucleation, in the early stage of the transition. The elastic strains along the $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}001\ensuremath{\rangle}$ directions are too large to be relaxed by the shear stress, so an intermediate structure emerges reducing the elastic strains and making the transition energetically favorable. At higher pressures, when the elastic strains become small enough to be relaxed, the phase transition to $\ensuremath{\gamma}\ensuremath{-}\mathrm{LiGa}{\mathrm{O}}_{2}$ begins and the coherent nucleation is substituted with a semicoherent one with Li and Ga atoms disordered.