Crystal chemical study of the high-temperature phase R(O,F) 1.93± δ with R=Tm, Yb and Lu

Abstract

The crystal chemistry of the non-stoichiometric compounds R(O,F) x with R=Tm, Yb, and Lu with x≈1.93±0.02, which are formed by peritectoid reaction of R 2O 3 with RF 3 at ca. 900 K, ca. 950 K and 1076 K, respectively, has been studied by X-ray powder diffraction (Guinier method). Samples of the common composition R(O,F) 1.93, which were obtained by quenching from 1375±15 K to room temperature, displayed diffraction patterns of a rather complex superstructure based on the fluorite-related parent structure. The corresponding body-centered basis structure with Z=2 was found to be orthorhombic with the following lattice parameters: R a (Å) b (Å) c (Å) Tm 3.7508(2) 5.3772(3) 3.8210(2) Yb 3.7341(2) 5.3609(4) 3.8129(3) Lu 3.7008(2) 5.3371(4) 3.8170(2) This type of compound could not be found in the system Er–O–F. It is therefore concluded that its existence is restricted to the systems with R=Tm, Yb, Lu and Sc, which are characterised by the occurrence of the monoclinic, baddeleyite-type modification of stoichiometric ROF. It could be shown that for the Tm–O–F system the parent structure formula volumes of the orthorhombic vernier phase Tm(O,F) 2.11, cubic α-TmOF and orthorhombic Tm(O,F) 1.93 depend approximately linearly on composition. The question, whether monoclinic baddeleyite-related TmOF, which has been described in the literature, exists as a thermodynamically stable compound, is tentatively discussed.