HYDROTHERMAL SYNTHESIS AND STRUCTURAL STUDY OF BARIUM ZINC ARSENATES. Sue-Lein Wang* and Fen-Ling Liao, Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043

In comparison with the rich structural chemistry of transition metal phosphates, reports on the arsenates are rare. Recently we have investigated the structural chemistry of transition metal arsenates and have synthesized a number of new compounds in the A-M-As^V-O system (A = alkali or alkaline-earth metals and M = V, Fe, Co, Ni, Cu, and Mo) by using hydrothermal methods. Most of these arsenates adopt new structural types and have stoichiometries not found in the phosphates. To explore novel frameworks built up from various polyhedra, we have extended our work to zinc-based arsenate systems. With respect to vari-ous coordination geometries adopted by ZnO_x (x = 4, 5, 6), the struc-tural chemistry of zinc-based oxy compounds is considered unique. This has been shown by a great number of ternary and quaternary zinc phosphates. This presentation reports the hydrothermal synthesis and characterization of two barium zinc arsenates, BaZnAs₂O₇ and BaZn₂(HAs₂O₇) AsO₄. They are the first examples in the Ba-Zn-As^V-O system. BaZnAs₂O₇ is isostructural with the BaZnP₂O₇ analogs and reveals new features concerning the layered framework. BaZn₂(HAs₂O₇) AsO₄ contains the unusual Zn₄O₁₆ cluster units in which four ZnO₅ square pyramids linked together via common vertices and edges. It is the first experimental evidence that the hydrogen-diarsenate anion, HAs₂O₇^2- exists in metal arsenates.