Structural trends for celestite (SrSO4), anglesite (PbSO4), and barite (BaSO4): Confirmation of expected variations within the SO4 groups

The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO 4 ), anglesite (PbSO 4 ), and barite (BaSO 4 ) 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) Å 3 for SrSO 4 ; a = 6.95802(1), b = 8.48024(3), c = 5.39754(1) Å, and V = 318.486(1) Å 3 for PbSO 4 ; and a = 7.15505(1), b = 8.88101(3), c = 5.45447(1) Å, and V = 346.599(1) Å 3 for BaSO 4 . The average <M-O> [12] distances are 2.827(1), 2.865(1), and 2.953(1) Å for SrSO 4 , PbSO 4 , and BaSO 4 , 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 SO 4 and MO 12 polyhedra become more symmetrical from SrSO 4 to BaSO 4 . Across the series, the a, b, and c parameters vary non-linearly with increasing V. The radii of the M 2+ cations, r M , <M-O> [12], and <S-O> [4] distances vary linearly with V. These structural trends arise from the effective size of the M 2+ cation (r M : Sr < Pb < Ba) that is coordinated to 12 O atoms.