Crystal Structures of Three MB5O8 (M=Cs, Rb) Borates (alpha-CsB5O8, gammaCsB5O8, and beta-RbB5O8)

Crystal structures of alpha-CsB5O8, gammaCsB5O8, and beta-RbB5O8 have been determined from single-crystal X-ray diffraction data. alpha-CsB5O8 crystallizes in the monoclinic space group P21/n with the following parameters: a=7.117(2) ?, b=9.634(3) ?, c=10.391(4) ?, beta=101.160(4)°; Z=4; the structure was refined from 1392 reflections until R1=0.0489. gamma-CsB5O8 and beta-RbB5O8 have been found to be isostructural with alpha-KB5O8 and beta-KB5O8 (and TlB5O8), respectively. They crystallize with eight formula units in the Pbca space group. Unit cell parameters are a=8.697(3) ?, b= 8.431(2) ?, c=21.410(6) ? for gamma-CsB5O8 and a=7.553(2) ?, b=11.857(3) ?, c=14.813(4) ? for beta-RbB5O8; their structures were refined from 1085 reflections until R1=0.0712, and from 1271 reflections until R1=0.0290, respectively. The structures of these compounds can be described on the basis of a new two- (alpha-CsB5O8) and three-dimensional (gamma-CsB5O8, beta-RbB5O8) network of pentaborate units [B5O8]-; these units are formed by two B3O3 cycles linked by a boron atom, which contain two groups of two BO3 triangles (delta) on each side of a BO4 tetrahedron (T); their shorthand notations are 5:infinity2 (4delta+T) for alpha-CsB5O8 and 5:infinity3 (4delta+T) for gamma-CsB5O8 and betaRbB5O8. Distortions of the [B5O8]- units found in these compounds are presented.