Theoretical and Experimental Analysis of Structural Phase Transitions for ScF[SeO3] and YF[SeO3]

摘要: Phase-pure ScF[SeO3] and YF[SeO3] were synthesized via solid-state reactions of the corresponding rare-earth metal sesquioxides trifluorides with selenium dioxide in excess fluxing cesium bromide at 700 °C for 7 d. To avoid formation oxosilicates, these took place corundum crucibles embedded inside evacuated fused silica ampoules. Upon cooling down below 13 °C, a pseudo-second-order phase transition (P21/m P or P1) occurs, which is similar to high-temperature LuF[SeO3]. By heating above 319 it undergoes first-order (P21/c P21/m) without passing intermediate modification contrast Both low-temperature characterized by in-situ X-ray powder diffraction, single-crystal thermal analysis DSC, distortion-mode analysis. For both cases driving forces symmetry-breaking transitions from high- phases are discussed.