Structural phase transitions in the geometric ferroelectric LaTaO4

The recent report of an intermediate incommensurately modulated orthorhombic phase in $\mathrm{LaTa}{\mathrm{O}}₄$ has prompted a re-examination of the phase transition sequence in $\mathrm{LaTa}{\mathrm{O}}₄$ as a function of temperature. With falling temperature, the sequence of phases examined is (orthorhombic) $Cmc{2}₁(\mathrm{C})\ensuremath{\leftrightarrow}Cmc{2}₁(\mathrm{IC})\ensuremath{\leftrightarrow}(\mathrm{monoclinic})P{2}₁/c$, with C and IC denoting commensurate and incommensurate phases, respectively. The orthorhombic to monoclinic transition, ${T}_{\mathrm{m}\ensuremath{-}\mathrm{o}}$, is a first order reconstructive transition occurring at 440 K and ${T}_{\mathrm{IC}\ensuremath{-}\mathrm{C}}$ is a first-order displacive transition occurring at 500--530 K. Strain and elasticity data confirm a first-order transition between the basic and modulated $Cmc{2}₁$ phases, with similarities to the isostructural fluoride $\mathrm{BaMn}{\mathrm{F}}₄$. A Raman spectroscopic study of the $\mathrm{LaTa}{\mathrm{O}}₄$ phase transition indicates that the IC-C phase transition is driven by a soft zone-boundary phonon (unstable) of the commensurate orthorhombic ($Cmc{2}₁$) phase. The soft phonon is found to appear (underdamped) above 443 K and vanishes (overdamped) around 528 K. A large supercell of the monoclinic phase below ${T}_{\mathrm{m}\ensuremath{-}\mathrm{o}}$ is proposed based on the Raman spectroscopic results.