FRAMEWORK ALKALI-METAL ZINC ORTHOBORATES - AZN4(BO3)3 (A = K, RB, CS)

The new family of alkali metal zinc orthoborates AZn4(BO3)3 (A = K, Rb, Cs) have been synthesized, and the structures of the K and Rb analogs have been established by single-crystal X-ray methods. Each compound crystallizes in a monoclinic cell, space group P2/c, with Z = 2. Cell parameters for the K analog: a = 6.876 (2) angstrom, b = 4.990 (1) angstrom, c = 12.574 (1) angstrom, beta = 92.92 (1)-degrees, V = 430.9 (2) angstrom3. Cell parameters for the Rb analog: a = 6.849 (2) angstrom, b = 5.000 (2) angstrom, c = 12.718 (2) angstrom, beta = 92.53 (2)-degrees, V = 435.1 (4) angstrom3. For the Cs analog, a cell of dimensions a = 6.871 (2) angstrom, b = 5.019 (2) angstrom, c = 12.961 (3) angstrom, beta = 92.50 (3)-degrees, and V = 446.5 (4) angstrom3 has been obtained from refinement of powder X-ray data. The structure is a new type composed of tetrahedral ZnO4 and triangular BO3 groups that are arranged in a three-dimensional framework by sharing O vertices. The framework affords channels having rectangular cross sections of dimensions 3.5 x 5.6 angstrom that extend in the [010] direction; the alkali metal ions occupy sites in these channels. The ions in these channels are not exchanged with other cations.