HYDROTHERMAL SYNTHESIS AND CRYSTAL-STRUCTURES OF NACO3(PO4)(HPO4)2 AND NACO3(ASO4)(HASO4)2 - SYNTHETIC MODIFICATIONS OF THE MINERAL ALLUAUDITE

Two cobalt-containing analogues of the mineral alluaudite, NaCo3(XO4)( HXO4)2(X = P,As), have been synthesized by hydrothermal methods and ch aracterized by single-crystal X-ray diffraction. Crystal data: NaCo3(P O4)(HPO4)2, monoclinic, C2/c, a = 11.865(2) angstrom, b = 12.137(2) an gstrom, c = 6.512(1) angstrom, beta = 114.12(1)degrees, V = 855.9(2) a ngstrom3, Z = 4, R = 0.0373; NaCo3(AsO4)(HAsO4)2, as above except a = 12.054(1) angstrom, b = 12.378(1) angstrom, c = 6.780(2) angstrom, bet a = 113.03(1)degrees, V = 931.0(3) angstrom3, R =0.0177. The two compo unds are isostructural. The framework consists of paralle infinite cha ins of CoO6 octahedra sharing their skew edges, which are linked by XO 4 and HXO4 groups to form a three-dimensional architecture with two ty pes of channels running along the c-axis. Sodium ions are located in s ites within one channel with the other channel being occupied by OH gr oups. The structure is compared with that of a natural alluaudite. Mag netic susceptibility measurements on a powder sample of NaCo3(PO4)(HPO 4)2 were performed.