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, rcalc. = 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, [BIII2BIVO5]-, and double rings, [BIII4BIVO8]-. 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 730oC 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
oC-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 oC-1 is less
than one of the other rubidium borates (a ~ 30.10-6 oC-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.