CRYSTAL STRUCTURE AND IR-SPECTROSCOPY OF THE MICRO-POROUS SELENITE Ni11(OH)6(SeO3)8ˇx(H2O,...) (x ~ 1.5)
G. Giester, M. Wildner
Institut für Mineralogie und Kristallographie,
Universität Wien - Geozentrum, Althanstr. 14, A-1090 Wien,
Austria
Single crystals of Ni11(OH)6(SeO3)8ˇx(H2O) (x ~1.5) were synthesized at low-hydrothermal conditions. The crystal structure was determined by direct methods from single-crystal X-ray diffraction data in space group P63mc: Z=1, a=12.686(2), c=4.914(1) A, V=684.9(2)A3 and refined to R1 = 0.030, wR2 = 0.046. The structure consists of infinite chains formed by face- and edgesharing NiO6 octahedra. These chains are linked by common corners as well as by two different selenite groups to a framework structure with hexagonal channels parallel [001]; the lone-pair electrons of one type of the Se(IV) atoms are directed into these channels. The structure refinement and microprobe analyses indicate that the Ni atom site is only partially occupied (11/12) such that the framework is neutral. The channels are most probably occupied by disordered H2O molecules and - depending on the method of synthesis - minor amounts of CO2 molecules.
The compound belongs to a family of hexagonal framework structures with infinite channels parallel [001]: the hydroxyphosphites M11(OH)6(HPO3)8 (M = Mn, Fe, Co, Ni, Zn) [1-3] have a neutral framework due to 1/12 octahedral M-vacancies; in the compounds M12+xH62x(XO4)8(OH)6 (M = Co, Ni; X = P, V, As) [4-5 and references therein] x additional M-cations, located within the channels, are necessary for the balance of charges; the tellurites M3(TeO3)2(OH)2 [= M12(OH)2(TeO3)8(OH)6] (M = Co, Ni) [6] also have been described with a fully occupied octahedral framework but with additional hydroxyl groups within the channels. Recently, Rietveld structure refinements of the hydroxo- and fluoro-selenites M12(X)2(SeO3)8(OH)6 (M = Co, Ni; X = OH, F) were published [7], describing a nickel hydroxo-selenite very similar (or perhaps even identical) to our title compound. Therein the authors [7] propose a model for balance of charges by additional OH- (resp. F-), analogous to the tellurites. This type of compensation would imply a rather unusual arrangement of hydroxyl groups and of the resulting hydrogen bonding system within the channels. In our single crystal X-ray study, we therefore focus on this problem of charge compensation and atomic arrangement within the channels, thus including polarized single crystal and temperature dependent powder IR-spectroscopy.