Dimorphism in mercury(II) tellurite(IV) tellurate(VI): preparation and crystal structures of α- and β-Hg₂Te₂O₇

Abstract

The mixed-valent mercury(II) tellurite(IV) tellurate(VI), Hg₂Te₂O₇, is dimorphous and crystallizes in two modifications with slightly different density, named α-(high density) and β-(low density). Colourless single crystals of both modifications were prepared simultaneously by chemical transport reactions, starting from stoichiometric mixtures of HgO, TeO₂ and TeO₃ in sealed and evacuated silica glass ampoules. A temperature gradient of 600 → 550°C was applied and small amounts of HgCl₂ served as transport agent. The crystal structures (α-Hg₂Te₂O₇: C2/c, Z = 8, a = 12.910(4), b = 7.407(2), c = 13.256(4) Å, β = 112.044(5)°, 1711 structure factors, 73 parameters, R[F² > 2σ(F²)] = 0.0325, wR(F² all) = 0.0796; β-Hg₂Te₂O₇: Aea2, Z = 16, a = 7.4405(12), b = 23.713(4), c = 13.522(2) Å, 3528 structure factors, 132 parameters, R[F² > 2σ(F²)] = 0.0430, wR(F² all) = 0.0738) are not directly correlated by a supergroup-subgroup relation but show similar building units which are also found in the structure of the mixed-valent Te(IV,VI) compound BaCuTe₂O₇. Characteristic structural feature of both polymorphs is a slightly distorted hexagonal array made up of isolated ¹_∞[Te^VIO_4/1O_2/2] chains running parallel to [010] for the α- and to [100] for the β-modification. The chains are interconnected via [Te^IVO₄] polyhedra to form infinite Te₂O₇^4- layers with a different topology for both structures. In α-Hg₂Te₂O₇ the layers are linearly ordered and spread parallel to (101̅) whereas in β-Hg₂Te₂O₇ the zig-zag arranged layers extend parallel to (001). The lone-pairs of the Te^IV atoms are stereochemically active and protrude into the cavities of the structure. For both structure types the mercury atoms are situated in-between the anionic sheets and interconnect the layers into the three-dimensional network. The coordination numbers of the mercury atoms range from 6–8 in both modifications with the typical Hg–O distances between 2.03–3.0 Å.