A reinvestigation of the crystal structures of HgSO₄ and CdSO₄

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 Pn2₁m (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.