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

Phase-pure ScF[SeO3] and YF[SeO3] were synthesized via solid-state reactions of the corresponding rare-earth metal sesquioxides and trifluorides with selenium dioxide in excess of fluxing cesium bromide at 700 degrees C for 7 d. To avoid the formation of oxosilicates, these reactions took place in corundum crucibles embedded inside of evacuated fused silica ampoules. Upon cooling down below 13 degrees C, a pseudo-second-order phase transition (P2(1)/m -> P (1) over bar or P1) for ScF[SeO3] occurs, which is similar to the high-temperature phase transition of LuF[SeO3]. By heating YF[SeO3] above 319 degrees C, it undergoes a first-order phase transition (P2(1)/c -> P2(1)/m) without passing the intermediate P (1) over bar modification in contrast to LuF[SeO3]. Both the high-temperature phase transition of YF[SeO3] and the low-temperature phase transition of ScF[SeO3] were characterized by in-situ X-ray powder diffraction, X-ray single-crystal diffraction, thermal analysis DSC, and distortion-mode analysis. For both cases the driving forces for the symmetry-breaking phase transitions from the high- to the low-temperature phases are discussed.