Synthesis, crystal structure and thermal behavior of Rb3B7O12, a new compound

A new compound, Rb3B7O12, has been synthesized using solid state reaction and glass crystallization methods. It is triclinic, space group P-1, a = 6,603(1), b = 6.632(1), c = 30.085(6) Angstrom, alpha = 91.183(4), beta = 91.781(4), gamma = 119.293(3)degrees, V = 1147.5(5) Angstrom(3), Z = 4. The crystal structure of the compound has been solved by direct methods and refined to R-1 = 0.068 (wR = 0.142) on the basis of 1840 unique observed reflections (\F-o\ > 4sigma\F-o\). The structure is based upon a novel [B5O8.5](2-) layered anion. The layer is parallel to (001) and is composed of four types of rigid boron-oxygen groups: two [B5O8.5](2-) pentaborate groups, [B3O5](-) triborate groups and BO4 single tetrahedra. There are six symmetrically independent Rb atoms in the structure, of which four are located in voids within the borate layer and two are between the layers. High-temperature X-ray powder diffraction studies showed that the new compound forms above 620degreesC as a result of the solid state reaction Rb2B4O7 + alpha-RbB3O5 <----> Rb3B7O12. The compound melts at similar to 700degreesC according to the peritectic reaction Rb3B7O12 <----> beta-RbB3O5 + liquid. Thermal expansion of the structure of Rb3B7O12 is highly anisotropic (alpha(11) = 34, alpha(22) = 9, alpha(33) = 2 x 10(-6) degreesC(-1)). The thermal expansion is maximal in the direction perpendicular to the plane of the layer (alpha(11) = 34 x 10(-6) degreesC(-1)). (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.