Crystal structure, thermal and compositional deformations of β-CsB5O8

The crystal structure of beta-CsB5O8 has been determined from X-ray powder diffraction data using synchrotron radiation: Pbca, a=7.8131(3) angstrom, b=12.0652(4) angstrom, c=14.9582(4) angstrom, Z=8, rho(calc) = 2.967 g/cm(3), R-p=0.076, R-wp=0.094. beta-CsB5O8 was found to be isostructural with beta-KB5O8 and beta-RbB5O8. The crystal structure consists of a double interlocking framework built up from B-O pentaborate groups. The crystal structure exhibits a highly anisotropic thermal expansion: alpha(a)=53, alpha(b)=16, alpha(c)=14 center dot 10(-6)/K; the anisotropy may be caused by partial straightening of the screw chains of the pentaborate groups. The similarity of the thermal and compositional (Cs-Rb-K substitution) deformations of CsB5O8 is revealed: increasing the radius of the metal by 0.01 angstrom leads to the same deformations of the crystal structure as increasing the temperature by 35 degrees C. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.