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

Author:

Weil Matthias

Abstract

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[F 2 > 2σ(F 2)] = 0.0325, wR(F 2 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[F 2 > 2σ(F 2)] = 0.0430, wR(F 2 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 [TeVIO4/1O2/2] chains running parallel to [010] for the α- and to [100] for the β-modification. The chains are interconnected via [TeIVO4] 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 TeIV 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.

Publisher

Walter de Gruyter GmbH

Subject

Inorganic Chemistry,Condensed Matter Physics,General Materials Science

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