NONSTOICHIOMETRIC TUBULAR NICKEL(II) HYDROXYARSENATES OF THE DUMORTIERITE FAMILY - CRYSTAL-STRUCTURE AND TOPOCHEMICAL THERMAL REDUCTION OF NI12-X(ASO4)(8)(OH)(6)(X=1.16 AND 1.33)(XH6)

Arsenate derivatives of formula Ni12+xH6-2x(AsO4)(8)(OH)(6) (1.16 less than or equal to x less than or equal to 1.33) have been synthesized. The structures of the two extreme compositions (x=1.16 and x=1.33) ha ve been refined from X-ray powder diffraction data. Like the other mem bers of this family, these hydroxyarsenates crystallize in the hexagon al P6(3)mc space group (Z=1) with a= 12.7018(2) Angstrom and c=5.03223 (9) Angstrom [V=703.11(3) Angstrom(3)] and reliability factors R(F)=4. 5%, R(P)=8.6%, R(WP)=11.1% for x=1.16, and a=12.6953(2) Angstrom and c =5.0311(1) Angstrom [V=702.23(3) Angstrom(3)] and R(F)=3.7%, R(P)=6.4% , R(WP)=8.9% for x=1.33. Thermal studies have shown that these tubular dumortierite-like solids remain unaltered, or only slightly modified, up to 700 degrees C.