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%, R_w = 1.8%). RbNa₂ [BO₃]: a = 613.3(2), b = 568.0(2), c = 554.9(2) pm (R = 6.0%, R_w = 4.0%). A characteristic feature of the structure is face-sharing of trigonal planar [BO₃] groups with distorted anticuboctahedra of [MO_I2]. 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].