Crystal chemistry of mercury oxo- and chalcohalides

The features of mercury oxo- and chalcohalide crystal structure, including Hg₃ Y ₂ X ₂ (Y?=?O, S, Se, Te; X?=?Cl, Br, I), chromates, phosphates and related compounds, are analyzed in terms of building blocks, their symmetry and stability. Building blocks are found, which are rigid atomic groups, namely, oxo-centered [Hg₄O] tetrahedra, [Hg₆O₂] r-octahedra, [Hg₂]²⁺, [Hg₃]⁴⁺, [Hg₃O]²⁺, [Ag₃Hg]⁴⁺ cluster cations, etc. Bonded by the strongest chemical bonds, these groups keep their geometry unchanged in crystal structures of different composition. Thus cluster cations can be considered as single large cations, while their environment may be described by pseudo-coordination polyhedra, constructed around the centroid of each cation. This tendency was found for the example of atoms joined to pairs of [CrO₄] tetrahedra, according to which the geometry of mutual arrangement of rigid atomic groups tends not to change. It is shown that the symmetry of the rigid atomic groups is a subgroup of the space group symmetry, and partly predetermines it. In crystal structures of some Hg₃ Y ₂ X ₂ chalcohalides, the structure-forming role of packing of halogen atoms is revealed.