Abstract

The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO4), anglesite (PbSO4), and barite (BaSO4) were refined by Rietveld methods using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Their structural model was refined in space group Pbnm. The unit-cell parameters are a = 6.87032(3), b = 8.36030(5), c = 5.34732(1) Å, and V = 307.139(3) Å3 for SrSO4; a = 6.95802(1), b = 8.48024(3), c = 5.39754(1) Å, and V = 318.486(1) Å3 for PbSO4; and a = 7.15505(1), b = 8.88101(3), c = 5.45447(1) Å, and V = 346.599(1) Å3 for BaSO4. The average <M-O> [12] distances are 2.827(1), 2.865(1), and 2.953(1) Å for SrSO4, PbSO4, and BaSO4, respectively, and their corresponding average <S-O> [4] distances are 1.480(1), 1.477(3), and 1.471(1) Å. The geometrical features of the SO4 and MO12 polyhedra become more symmetrical from SrSO4 to BaSO4. Across the series, the a, b, and c parameters vary non-linearly with increasing V. The radii of the M2+ cations, rM, <M-O> [12], and <S-O> [4] distances vary linearly with V. These structural trends arise from the effective size of the M2+ cation (rM: Sr < Pb < Ba) that is coordinated to 12 O atoms.

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