PHASE EQUILIBRIA AND STRUCTURES OF SOME CRYSTALLINE PHASES IN Rb2O-B2O3 SYSTEM. S.K.Filatov, R.S.Bubnova*, M.G.Krzhizhanovskaya, I.G.Polyakova*. St.Petersburg University, Dept. of Crystallography, University Emb. 7/9, St.Petersburg, 199034, *Institute of Silicate Chemistry RAS, Ul. Odoevskogo, 24, korp.2, St.Petersburg, 199155, Russia
The paper presents new data on phase equilibria and crystal structures of rubidium borates and summarises some common features of borate crystal structures, which may throw some light on the marked tendency of the borates to supercool to vitreous phase. Alkali borate systems are rich in chemical compounds, especially in the boron anomaly interval (70-75 mol. % B2O3), where boron atoms are three- and fourfold coordinated. Tetrahedra and triangles condensed to rigid arrangements (diborate, triborate groups and others) form various (sometimes interlocking) frameworks. Up to now none of the rubidium borate structures has been determined.
Crystal data have been obtained by X-ray analysis and immersion and crystal optic method. Thermal expansion, phase transformations and melting processes of borates have been studied by powder high-temperature X-ray diffraction. Potentialities of this method and DTA for phase equilibria studying in glass forming systems are considered. Samples were prepared by solid state reaction as well as crystallisation of glasses.
New compounds Rb3BO3 and Rb3B7O12 and new modifications [[alpha]]-, [[beta]]-RbB3O5 have been discovered. The new compound Rb3B7O12 is formed in the course of solid-state reaction above 620oC and melts according to peritectic reaction. The new low-temperature modification [[alpha]]-RbB3O5 transforms to [[beta]]-RbB3O5 on heating (at about 675oC).
New crystal data on Rb2B4O7, [[alpha]]- and [[beta]]-RbB3O5 obtained by X-ray analysis are presented. Distribution of boron-oxygen bond lengths and angles within arrangements is of special interest. Bond lengths show systematic variations depending on the location of adjacent bonds. The review of these variations within rigid arrangements is given for various structures of borates. The changes in angles between arrangements are also considered.
Rubidium diborate Rb2B4O7 is triclinic, space group P`1. This compound is isostructural to K2B4O7. [[alpha]]-RbB3O5 crystallises in orthorhombic space group P 212121. It is isostructural to CsB3O5. [[beta]]-RbB3O5 belongs to orthorhombic space group P 212121.