Order–Disorder Structural Transformations in Highly Imperfect Fluorite-Derived R2TiO5-Based (R = Tm, Er) Solid Solutions

Abstract

Order–disorder structural transformations in xR2O3 ∙ (1 – х)TiO2 (R = Tm, Er; х = 0.5−0.6) solid solutions with a highly imperfect fluorite-derived structure at 1600°C have been studied using monochromatic synchrotron X-ray diffraction and Raman spectroscopy. The results demonstrate that the synthesis process leads to the formation of two cubic phases identical in composition: a disordered fluorite-like (F) phase (Fm3m) and an ordered pyrochlore-like (P) phase (Fd3m), which is coherent with the disordered phase and consists of nanoscale (<100 A) and nanocrystalline domains. The lattice parameters of these phases have been determined. In the stability range of the solid solutions ((0.5 ≤ x ≤ 0.6)), the lattice parameter of the fluorite-like phases follows Vegard’s law. The Raman spectra of the R2TiO5-based (R = Tm, Er) solid solutions contain broad bands at low and high frequencies, with peaks at 100, 171, 291, 355, 385, and 723 cm–1 for R = Tm and at 100, 169, 292, 355, 390, and 720 cm–1 for R = Er, which correspond to the P- and F-phases, respectively. The formation of pyrochlore-like phases with different degrees of order in a fluorite matrix is due to the internal stress induced by the high density of structural defects in their unit cells. The materials obtained in this study have a large specific surface area and can be used as catalysts and catalyst supports.