A neutron diffraction study of mercury(II)chromate hemihydrate, HgCrO4 · 1/2 H2O

Abstract

Abstract The crystal structure of mercury(II)chromate hemihydrate, HgCrO4 · 1/2H2O, has been refined from neutron single-crystal data (870 independent reflections) to R = 0.030 and R w = 0.023. The crystals are monoclinic, space group C 2/c with a = 11.832(1), b = 5.2616(6), c = 14.637(2) Å, β = 121.01(1)° and Z = 8. The mercury atom is bonded only to two oxygen atoms, each belonging to a separate Chromate tetrahedron, at the distances 2.055(2) and 2.064(2) Å, the angle O–Hg–O being 179.95(5)°. As each Chromate group is bonded to two mercury atoms, fundamental building elements of the structure are endless chains of the composition (HgCrO4) n running approximately parallel to the diagonals of the ab plane. The chains are fused by hydrogen bonds to blocks of composition (HgCrO4 · 1/2 H2O) n with thickness 1/2 c. There are weak interactions between the blocks, five mercury to oxygen distances varying between 2.655–2.743 Å. In the water molecule the distances O(W)–H (2 × ) and H–H are 0.945(4) and 1.487(6) Å respectively. The angle H–O(W)–H is 103.7(4)°. In the hydrogen bonds the distance O(W)–H…O between water and Chromate oxygen atoms is 2.961(2) Å with a distance H…O of 2.133(4) Å and an angle O(W)–H…O of 145.6(3)°.