The non-centrosymmetric borate chlorides Ba_2TB_4O_9Cl (T =Al, Ga)

摘要

The structures of the novel borate chlorides Ba_2TB_4O_9Cl (T = Al, Ga) have been determined by single crystal X-ray diffraction using crystals grown from a BaCl_2 flux. The compounds are isomorphous representatives of a new non-centrosymmetric tetragonal (P4_2nm) borate structure type related to triclinic hilgardite, Ca_2B_5O_9Cl centre dot H_2O, and its orthorhombic anhydrous derivatives, such as Ba_2B_5O_9Cl. The Ba_2TB_4O_9Cl structure consists of a TB_4O_9 framework with Ba~(2+) and Cl~- ions accommodated in tunnels parallel to the c axis. The framework is based on a novel TB_4O_(12) fundamental building block built of a tetraborate cluster of two BO_3 triangles and two BO_4 tetrahedra sharing a comer with a TO_4 tetrahedron. The large cross-sections of the tunnels within the framework are associated with Ba and Cl positional disorder. The measurement of the second harmonic generation efficiency (d_(eff)) for a powder sample of Ba_2GaB_4O_9Cl gave a value of 0.95 relative to a KH_2PO_4 standard.