Structural phase transitions in the geometric ferroelectric LaTaO4

Funding: School of Chemistry, University of St Andrews for funding of a studentship to GWH through the EPSRC doctoral training grant (Grant No. EP/N509759/1). This work was also facilitated by funding provided by the EPSRC (Grant No. EP/P024637/1). K.K.M. and R.S.K. acknowledge financial support from the Department of Defense, USA (DoD Grant No. FA9550-20-1-0064). The RUS component of this work was funded by EPSRC Grant No. EP/P024904/1. The recent report of an intermediate incommensurately modulated orthorhombic phase in LaTaO4 has prompted a re-examination of the phase transition sequence in LaTaO4 as a function of temperature. With falling temperature, the sequence of phases examined is (orthorhombic) Cmc21(C)↔Cmc21(IC)↔(monoclinic)P21/c, with C and IC denoting commensurate and incommensurate phases, respectively. The orthorhombic to monoclinic transition, Tm-o, is a first order reconstructive transition occurring at 440 K and TIC-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 Cmc21 phases, with similarities to the isostructural fluoride BaMnF4. A Raman spectroscopic study of the LaTaO4 phase transition indicates that the IC-C phase transition is driven by a soft zone-boundary phonon (unstable) of the commensurate orthorhombic (Cmc21) 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 Tm-o is proposed based on the Raman spectroscopic results. Publisher PDF