CRYSTAL-STRUCTURE AND SPECTROSCOPIC CHARACTERIZATION OF MAS(2)O(6) (M=PB, CA) - 2 SIMPLE SALTS WITH ASO6 GROUPS

The crystal structures of MAs(2)O(6) (M = Pb, Ca) have been redetermin ed using X-ray powder diffraction data by the Rietveld method. These c ompounds are trigonal (space group P $($) over bar$$ 31m) with a = 4.8 75(1) Angstrom and c = 5.504(1) Angstrom for M = Pb, and a = 4.826(1) Angstrom and c = 5.082(1) Angstrom for M = Ca. The agreement factors w ere R(WP) = 11.8% and R(F) = 3.5% for M = Pb and R(WP) = 10.3% and R(F ) = 1.4% for M = Ca. The diffraction peaks show a very strong broadeni ng anisotropic peak shape that has been fitted and results in a remark able improvement of the description of the structures: This series of compounds has arsenic atoms in a pseudooctahedral environment of oxyge ns. AsO6 groups are very poorly spectroscopically characterized; hence , we also report the IR and Raman data. The results of the vibrational study are in agreement with the crystal structure, and an initial ass ignment of the spectra is proposed. We observe a displacement of the i ntense bands for the AsO4 groups at approximate to 880 cm(-1) (IR acti ve) and approximate to 840 cm(-1) (Raman active) to approximate to 740 and approximate to 750 cm(-1), respectively, for the AsO6 groups.