Crystal chemistry of new descloizite-type compounds containing mercury

Up to date the only mercury-containing phase with a descloizite-type of structure was synthetic HgZn(OH)[AsO₄]. During phase formation studies under hydrothermal conditions, six new synthetic phases with composition HgM^II(OH)[AsO₄] (M^II = Mg, Co, Ni, Cu) or HgM^III(O)[AsO₄] (M^III = Mn, Fe) could be obtained. All phases adopt the descloizite-type of structure, as revealed by crystal-structure analysis using single-crystal X-ray diffraction data (space group Pnma, Z = 4, a ≈ 7.8, b ≈ 6.2, c ≈ 8.6 Å). Whereas the crystal structures of the Mg-, Co- and Ni-members show a high degree of similarity with the known Zn-member, the crystal structures of the Cu-, Mn- and Fe-members show much higher displacements of atoms relative to the Zn-member. In case of the Cu-member and to a minor extent in the Mn-member, this is ascribed to Jahn-Teller distortions of the [MO₆] octahedra, and, in case of the Mn- and Fe-members, to missing hydrogen bonding interactions. The replacement of OH⁻ by an O²⁻ anion in the two HgM^III(O)[AsO₄] (M^III = Mn, Fe) structures is unprecedented for descloizite-type phases and represent rare examples where an OH⁻ → O²⁻ substitution does not change the adopted structure family and/or the space-group type.