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Subcell and Superstructure of a High‐Temperature (β‐) Modification of the Mercury(I) Dimolybdate(VI) Hg 2 Mo 2 O 7
Author(s) -
Mormann Thomas J.,
Jeitschko Wolfgang
Publication year - 2001
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/1521-3749(200102)627:2<224::aid-zaac224>3.0.co;2-o
Subject(s) - superstructure , octahedron , crystallography , crystal structure , molybdate , materials science , single crystal , chemistry , metallurgy , thermodynamics , physics
A new (β‐)modification of the mercury molybdate Hg 2 Mo 2 O 7 , thermodynamically stable at temperatures above 390 ± 10 °C, was prepared by solid state reaction of HgO with MoO 2 in sealed silica tubes. Its crystal structure, determined from single‐crystal X‐ray data, has a very pronounced subcell: space group P 2/ c , a = 600.9(1) pm, b = 388.7(1) pm, c = 1428.4(2) pm, β = 105.88(1)°, Z = 2, R = 0.052 for 797 structure factors and 52 variable parameters. In the superstructure of this high‐temperature β‐modification the a and the b axes of the subcell are doubled: C 2/ c , a = 1201.9(2) pm, b = 777.3(1) pm, c = 1428.4(2) pm, β = 105.88(1)°, Z = 8, R = 0.040 for 1490 F values and 110 variables. Like the previously reported low‐temperature α‐modification, the β‐modification consists of two‐dimensionally infinite sheets of edge‐ and corner‐sharing MoO 6 octahedra. These sheets are linked by Hg 2 pairs. Thus, the structures of the two modifications (α and β) differ essentially only in the orientation of the Hg 2 pairs, which are located between the sheets of the MoO 6 octahedra. The superstructure of the β‐modification differs from the subcell‐structure by the puckering of the sheets of MoO 6 octahedra. A hypothetical displacive phase transition between the subcell‐structure (corresponding to the potential high‐temperature structure) and the superstructure of β‐Hg 2 Mo 2 O 7 is discussed.