CRYSTAL STRUCTURE AND THERMAL EXPANSION OF Rb₅B₁₉O₃₁

Maria Krzhizhanovskaya¹, Irina Bannova¹, Stanislav Filatov¹, Rimma Bubnova²

¹Dept. of Crystallography, St.Petersburg University, University Emb. 7/9, 199034, Russia,
E-mail: flt@cryst.geol.pu.ru
²Institute of Silicate Chemistry of the Russian Academy of Sciences, Ul. Odoevskogo, 24/2, St.Petersburg, 199155, Russia

Keywords: alkali borates, X-ray structural analysis, powder high-temperature X-ray diffraction

The chemical formula has been revised and the crystal structure has been determined (R = 0.050) for Rb₅B₁₉O₃₁, previously known as Rb₂B₈O₁₃. The compound is isostructural to K₅B₁₉O₃₁ (J.Krogh-Moe, 1974). It is monoclinic, space groop C2/c, with cell parameters: a = 18.211(3), b = 11.519(2), c = 13.310(2) A, b = 95.63(2)^o, V = 2778(1) A³, Z = 4, r_calc. = 2.71 g/cm³.

The crystal structure like recently determined ones of Rb₂B₄O₇ [1] and b-RbB₃O₅ [2] contains a single three-dimensional borate framework, composed of four different rigid arrangements: single tetrahedra, single triangles, single rings, [B^III₂B^IVO₅]^-, and double rings, [B^III₄B^IVO₈]^-. Double and single rings, by sharing corners, form infinite spiral chains around the twofold screw axes along b axis. The framework contains large cavities in which the Rb atoms are located.

The thermal expansion of the structure was investigated by powder high-temperature X-ray diffraction. Under heating the compound is stable up to 730^oC and then melts.The expansion of the compound is a sharply anisotropic. The principal coefficients of thermal expansion for Rb₅B₁₉O₃₁ are a₁₁ = 27, a₂₂ = 3 and a₃₃ = 11.10^{-6 o}C^-1. Anisotropy of the thermal expansion of Rb₅B₁₉O₃₁ is correlated to the thermal reconstruction of the structure and may be caused by anisotropic thermal vibrations of such heavy atoms as rubidium. As a result the spiral boron-oxigen chains are shortened along b-axis and extended in ac plane. The average coefficient of thermal expansion a = 14.10^{-6 o}C^-1 is less than one of the other rubidium borates (a ~ 30.10^{-6 o}C^-1).

1. M.G.Krzhizhanovskaya, R.S.Bubnova, I.I.Bannova, S.K.Filatov, Crystallogr. Repor. 42 (1997) 1 2 226-231.
2. M.G.Krzhizhanovskaya, R.S.Bubnova, V.S.Fundamenskii, I.I.Bannova, I.G.Polyakova, S.K.Filatov, Crystallogr. Repor. 43 (1998) 1 1 26-31.