A reinvestigation of the crystal structures of HgSO4 and CdSO4

Abstract

Abstract The crystal structures of the isomorphous compounds CdSO4 and HgSO4 have been refined from diffractometer data to R = 0.013 (791 reflections) and R = 0.033 (1092 reflections). The crystals are orthorhombic, space group Pn21 m (No 31) with Z = 2 and a = 4.6982(2), b = 4.7191(2), c = 6.5578(4) Å for CdSO4 and a = 4.7779(2), b = 4.8119(2), c = 6.5720(3) Å for HgSO4. The metal atoms are coordinated to four oxygen atoms of different sulphate groups forming very distorted MeO4 tetrahedra. The angles O – Me – O vary for Cd from 88.1 to 142.7° and for Hg from 84.4 to 143.4°. The bond distances Me – O are for Cd 2.23 (3 ×) and 2.29 Å and for Hg 2.22 (2 ×), 2.29 and 2.42 Å. For Hg there is a tendeny to the usual 2 or possibly a (3 + 1) coordination. Next nearest neighbours are four O atoms; for Cd at 2.58 – 2.92 Å and for Hg at 2.70 – 2.89 Å. The sulphate tetrahedron is very regular for CdSO4 and somewhat distorted for HgSO4. The structures are built up of three-dimensional networks of MeO4 and SO4 tetrahedra sharing corners. The absolute atomic configuration was determined for both crystals used by refinements including anomalous dispersion corrections for the metal and sulphur atoms.