Crystal structure and thermal expansion of Rb5B19O31

The crystal structure of Rb5B19O3, monoclinic, sp. gr, C2/c, a = 18.211(3), b = 11.519(2), c = 13.310(2) Angstrom, beta = 95.63(1)degrees, V = 2778(1) Angstrom(3), Z = 4, d(calc) = 2.714 g/cm(3). It is determined by X-ray dif fraction analysis (P2(1) Syntex diffractometer, MoKalpha radiation, 2280 re flections, R = 0.050) starting from the atomic coordinates for the isotruct ural K5B19O31. The thermal vibrations of Rb and O atoms are considered in t he anisotropic approximation. The structure is a three-dimensional framewor k built of complicated anions of four types: triangles; tetrahedra; [(BB2O5 )-B-IV-O-III](-) rings consisting of two triangles and a tetrahedron; and d ouble [(BB4O8)-B-IV-O-III](-) rings consisting of two simple rings with sha ring a tetrahedron. Alternating ordinary and double rings form helical chai ns about the 2(1) screw axes. Contraction of these chains during heating is presumably the main reason for a strong anisotropy of thermal strain revea led in the crystals by high-temperature X-ray diffraction method.