Metastable polar order and phase competition in Carpy-Galy LaTaO4

Abstract

Carpy-Galy LaTaO4 undergoes a structural sequence linking the antipolar monoclinic P21/c ground state and the polar orthorhombic Cmc21 phase through an incommensurately modulated regime. First-principles calculations identify a metastable polar monoclinic P21 phase as a commensurate intermediate in the phase-transition landscape. As a common subgroup of P21/c and Cmc21, P21 provides a symmetry-connected route between the established polymorphs. Mode-resolved energy surfaces for these symmetry-breaking pathways show that coupling between the respective primary order parameters and an isosymmetric Gamma_1+ relaxation stabilizes the P21 minimum along both pathways. Consequently, the P21/c -> P21 transformation has a finite energy barrier, while a restricted soft-mode model retains competing P21 and Cmc21 basins over an illustrative range of harmonic stiffness. Lattice-metric and neutron-diffraction comparisons indicate a shared S_2+-dominated displacement character between the calculated commensurate state and the experimental IC-o modulation, supporting the use of P21 as a commensurate structural reference for IC-o. Finally, composition-dependent energetics suggest chemical routes for tuning polar-antipolar competition.