Structural trends for celestite (SrSO (sub 4) ), anglesite (PbSO (sub 4) ), and barite (BaSO (sub 4) ); confirmation of expected variations within the SO (sub 4) groups

The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO (sub 4) ), anglesite (PbSO (sub 4) ), and barite (BaSO (sub 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) Aa, and V=307.139(3) Aa (super 3) for SrSO (sub 4) ; a=6.95802(1), b=8.48024(3), c=5.39754(1) Aa, and V=318.486(1) Aa (super 3) for PbSO (sub 4) ; and a=7.15505(1), b=8.88101(3), c=5.45447(1) Aa, and V=346.599(1) Aa (super 3) for BaSO (sub 4) . The average <M-O> [12] distances are 2.827(1), 2.865(1), and 2.953(1) Aa for SrSO (sub 4) , PbSO (sub 4) , and BaSO (sub 4) , respectively, and their corresponding average <S-O> [4] distances are 1.480(1), 1.477(3), and 1.471(1) Aa. The geometrical features of the SO (sub 4) and MO (sub 12) polyhedra become more symmetrical from SrSO (sub 4) to BaSO (sub 4) . Across the series, the a, b, and c parameters vary non-linearly with increasing V. The radii of the M (super 2+) cations, r (sub M) , <M-O> [12], and <S-O> [4] distances vary linearly with V. These structural trends arise from the effective size of the M (super 2+) cation (r (sub M) : Sr<Pb<Ba) that is coordinated to 12 O atoms.