Nonstoichiometry in Inorganic Fluorides. IV: The Initial Stage of Anionic Nonstoichiometry in RF3 (R = Y, La, Ln)

• Published in: Crystallography reports 2021-05, Vol.66 (3), p.349-360
• Main Author: Sobolev, B. P.
• Format: Article
• Language: English
• Subjects: Crystallography and Scattering Methods; Erbium; Fluorides; Fluorine; Gadolinium; Heat exchange; High temperature; Lanthanum fluorides; Melts; Physics; Physics and Astronomy; Pyrohydrolysis; Rare earth elements; Reviews; Solid phases; Substitutes
• ISSN: 1063-7745; 1562-689X
• DOI: 10.1134/S1063774521030238

The anionic nonstoichiometry in inorganic fluorides is at result of substitution of F 1– for O 2– in the anionic sublattice. All families of fluorides exhibit the initial stage of anionic nonstoichiometry (ISAN), which was previously studied for trifluorides of rare-earth elements (REEs), R F 3 . Partial substitution of F 1– for O 2– in R F 3 occurs in reactions with H 2 O vapor upon heating (pyrohydrolysis), exchange reactions of R F 3 and R 2 O 3 in melts, hydrothermal solutions, solid phase, and during mechanochemical synthesis. The ISAN is based on the formation of R F 3 – 2 x O x oxofluorides, the type and structure of which depend on the crystalline R F 3 forms. Congruently melting tys - R F 3 – 2 x O x compounds are formed based on the tysonite forms tys - R F 3 ( R = La–Gd, the LaF 3 type). Berthollide phases ~ tys - R F 3 – 2 x O x , which are isostructural to the above-mentioned phases and melts incongruently above the corresponding R F 3 compounds, are formed with R = Tb–Ho. The effect of stabilization of tys - R F 3 – 2 x O x when moving up the temperature scale (+Δ T fus ) changes nonmonotonically along the REE series, exhibiting a maximum of ~100°C in the range of Gd–Tb. There are no F 1– → O 2– substitutions in the β- R F 3 forms ( R = Er–Lu, Y) of the β-YF 3 type. The α- R F 3 – 2 x O x phases of the α-YF 3 (α-UO 3 ) type melt incongruently and decompose at high temperatures. The ISAN products in R F 3 may serve as sources of new congruently melting fluorine–oxygen materials.