The non-centrosymmetric borate chlorides Ba2TB4O9Cl (T = Al, Ga)

The structures of the novel borate chlorides Ba2TB4O9Cl (T = Al, Ga) have been determined by single crystal X-ray diffraction using crystals grown from a BaCl2 flux. The compounds are isomorphous representatives of a new non-centrosymmetric tetragonal (P42nm) borate structure type related to triclinic hilgardite, Ca2B5O9Cl·H2O, and its orthorhombic anhydrous derivatives, such as Ba2B5O9Cl. The Ba2TB4O9Cl structure consists of a TB4O9 framework with Ba2+ and Cl− ions accommodated in tunnels parallel to the c axis. The framework is based on a novel TB4O12 fundamental building block built of a tetraborate cluster of two BO3 triangles and two BO4 tetrahedra sharing a corner with a TO4 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 (deff) for a powder sample of Ba2GaB4O9Cl gave a value of 0.95 relative to a KH2PO4 standard.

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