Crystals of different members of the (Ba,Sr)HAsO4·H2O solid solution have been synthesized, and the first structural studies indicate that they crystallize in the same space group, Pbca, with Z = 8. The unit-cell parameters are a = 7.436(2), b = 8.481(1), c = 14.348(6) Å, and a = 7.752(1), b = 8.759(1), c = 14.668(3) Å for the strontium and barium end-members, respectively. Both end-members have a layered structure with slices parallel to (001) linked by hydrogen bonds from the water molecules. These features are consistent with both the perfect cleavage on {001} and the morphological importance of this form in the crystals obtained. However, the two end-members are not isostructural and show differences in both the anionic hydrogen positions and number of hydrogen bonds. Complementary powder-diffraction measurements indicate that the cell parameters increase in a non-linear way with the barium content indicating that the solid solution is complete but could be non-ideal. Preliminary data suggest that barium partitions preferentially into the solid phase when crystallizing this solid solution from aqueous solutions.

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