Crystal structure and thermal behavior of Rb2[B4O5(OH)4]•3.6H2O

The crystal structure of synthetic Rb-2[B4O5(OH)(4)]. 3.6H(2)O has been established from single-crystal X-ray diffraction data. It crystallizes in the orthorhombic space group Pbcn with a unit cell of dimensions a = 11.276(5) Angstrom, b = 13.097(14) Angstrom, c = 16.751(5) Angstrom; V = 2474(3) Angstrom(3); Z = 8, The structure was determined from 1201 independent reflections and refined until R = 0.049 and R-w = 0.036; it is composed of isolated [B4O5(OH)(4)](2-) polyanions separated by free water molecules and Rb+ ions, Some of the positions of rubidium and oxygen atoms-the latter are attributed to water molecules-were localized with occupancy factors of 0.5 and 0.3. A similarity appears with the unit cell parameters of Cs-2[B4O5(OH)(4)]. 3H(2)O: a = 8.424 Angstrom, b = 11.378 Angstrom, c = 13.160 Angstrom, beta = 92.06 degrees; V = 1260.5 Angstrom(3); Z = 4; space group P2(1)/c; in fact, the unit cell volume of rubidium berate is doubled but the symmetries of these two compounds are quite different. However, they contain identical [B4O5(OH)(4)](2-) polyanions, The dehydration of Rb-2[B4O5(OH)(4)]. 3.6H(2)O consists of a continuous loss of water until 300 degrees C, forming anhydrous Rb2B4O7, which is amorphous to X-ray diffraction, Rb2B4O7 crystallizes at about 475 degrees C and melts at 717 degrees C. Two forms of Rb2B4O7 appear on the temperature resolved X-ray powder diffraction pattern. One corresponds to the triclinic form of the structure which has been solved recently; to this day, the other one has not been isolated. (C) 2000 Academic Press.