PREPARATION AND CRYSTAL-STRUCTURE OF K6[AL2O6] AND RB6[AL2O6]

Colourless single crystals of K6[Al2O6] have been prepared from intimate mixtures of KAlO2 and K2O (550-degrees-C, 90 d). The structure determination from four-circle diffractometer data (MoKalphaBAR, 742 I(o)(hkl), R = 2.2%, R(w) = 2.1%) confirms the space group C2/m with Z = 2; a = 698.25 pm, b = 1 103.54 pm, c = 646.49 pm, beta = 102.49-degrees. Colourless single crystals of hitherto unknown Rb6[Al2O6] have been prepared from intimate mixtures of RbAlO2 and Rb2O (520-degrees-C, 120 d). The structure determination from four-circle diffractometer data (MoKalphaBAR, 1240 I(o)(hkl)) results in the residual values R = 7.2%, R(w) = 4.9%; space group C2/m; a = 725.92 pm, b = 1143.33 pm, c = 678.06 pm, beta = 104.05-degrees; Z = 2. K6[Al2O6] and Rb6[Al2O6] are isostructural with K6[Fe2O6]. A characteristic structure unit is the anion [Al2O6]6- consisting of two edge-sharing [AlO4] tetrahedra. Effective Coordination Numbers (ECoN), Mean Fictive Ionic Radii (MEFIR), the Madelung Part of Lattice Energy (MAPLE) and the Charge Distribution (CHARDI) are calculated and discussed.