Preparation and Crystal Structure of CsNa2 [BO3] and RbNa2 [BO3]

Abstract

Colourless single crystals of hitherto unknown CsNa2 [BO3] and RbNa2 [BO3] have been prepared by exchange reactions from intimate mixtures of Li3 [BO3], Na2O, CszO and Rb2O respectively. The new compounds are isostructural with KNa2 [BO3]. The structure determination from four-circle diffractometer data confirms the orthorhombic space group Pmmn with Z = 2. CsNa2 [BO3]: a = 626.33(4), b = 579.67(4), c = 560.09(5) pm (R = 2.0%, Rw = 1.8%). RbNa2 [BO3]: a = 613.3(2), b = 568.0(2), c = 554.9(2) pm (R = 6.0%, Rw = 4.0%). A characteristic feature of the structure is face-sharing of trigonal planar [BO3] groups with distorted anticuboctahedra of [MOI2]. The Madelung Part of the Lattice Energy (MAPLE), Effective Coordination Numbers ( ECoN ), Mean Fictive Ionic Radii (MEFIR) and Charge Distribution (CHARDI) are calculated and compared with results for KNa2 [BO3].