FT-Raman and powder XRD analysis of the Ba(SO(4))(x)(CrO(4))(1-x) solid solution

The Ba(SO(4))(x)(CrO(4))(1-x) solid solution has been described in nature, forming the mineral Hashemite. From the geochemical point of view, however, anionic solid solutions have much interest because they are suitable systems to probe order-disorder phenomena. The solid solution analysed in the present study has, moreover, a special incentive in its possible use for the extraction from water, and immobilisation, of the pollutant Cr(VI) ion. The orthorhombic (space group Pnma) unit cell parameters of the solid solution change linearly with the mole fraction of both anions, decreasing with increase in the sulfate anion concentration. The vibrational spectroscopic study is centred on the behaviour of the anionic symmetric stretching band (nu(1),A(1)), whose characteristics are examined in detail. While the chromate anion band retains its wavenumber along the full compositional range, the sulfate anion band is shifted toward lower wavenumbers with decrease in the corresponding mole fraction. The positional disorder induced by the random anionic substitution results in strong increase of the halfwidth in both bands, which becomes greatest in the central member of the series.