CRYSTAL-CHEMICAL STUDY OF THE HIGH-TEMPERATURE PHASE R(O,F)(1.93+ -DELTA) WITH R=TM, YB AND LU - PROFESSOR DR. BRAUER,GEORG ON THE OCCASIONOF HIS 90(TH) BIRTHDAY/

The crystal chemistry of the non-stoichiometric compounds R(O,F)(x) wi th R=Tm, Yb, and Lu with x approximate to 1.93 +/- 0.02, which are for med by peritectoid reaction of R2O3 with RF, at ca. 900 K, ca. 950 K a nd 1076 K, respectively, has been studied by X-ray powder diffraction (Guinier method). Samples of the common composition R(O,F)(1.93), whic h were obtained by quenching from 1375 +/- 15 K to room temperature, d isplayed diffraction patterns of a rather complex superstructure based on the fluorite-related parent structure. The corresponding body-cent ered basis structure with Z=2 was found to be orthorhombic with the fo llowing lattice parameters: [GRAPHICS] This type of compound could not be found in the system Er-O-F. It is therefore concluded that its exi stence is restricted to the systems with R=Tm, Yb, Lu and Sc, which ar e 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 ve rnier phase Tm(O,F)(2.11), cubic alpha-TmOF and orthorhombic Tm(O,F)(1 .93) depend approximately linearly on composition. The question, wheth er monoclinic baddeleyite-related TmOF, which has been described in th e literature, exists as a thermodynamically stable compound, is tentat ively discussed. (C) 1998 Elsevier Science S.A. All rights reserved.