Crystal Structure and Thermal Behavior of Rb2[B4O5(OH)4]·3.6H2O

The crystal structure of synthetic Rb2[B4O5(OH)4]·3.6H2O 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) ?, b=13.097(14) ?, c=16.751 (5) ?; V=2474(3) ?3; Z=8. The structure was determined from 1201 independent reflections and refined until R=0.049 and Rw=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 Cs2[B4O5(OH)4]·3H2O: a=8.424 ?, b=11.378 ?, c=13.160 ?, BETA=92.06°; V=1260.5 ?3; Z=4; space group P21/c; in fact, the unit cell volume of rubidium borate is doubled but the symmetries of these two compounds are quite different. However, they contain identical [B4O5(OH)4]2polyanions. The dehydration of Rb2[B4O5(OH)4]·3.6H2O consists of a continuous loss of water until 300°C, forming anhydrous Rb2B4O7, which is amorphous to X-ray diffraction. Rb2B4O7 crystallizes at about 475°C and melts at 717°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.