✍ Scribed by Thomas J. Mormann; Wolfgang Jeitschko
No coin nor oath required. For personal study only.
A new (b-)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 P2/c, a = 600.9( ) pm, b = 388.7(1) pm, c = 1428.4(2) pm, b = 105.88(1)°, Z = 2, R = 0.052 for 797 structure factors and 52 variable parameters. In the superstructure of this high-temperature b-modification the a and the b axes of the subcell are doubled: C2/c, a = 1201.9(2) pm, b = 777.3(1) pm, c = 1428.4(2) pm, b = 105.88(1)°, Z = 8, R = 0.040 for 1490 F values and 110 variables. Like the previously reported low-temperature a-modification, the b-modification consists of two-dimensionally in-finite sheets of edge-and corner-sharing MoO 6 octahedra. These sheets are linked by Hg 2 pairs. Thus, the structures of the two modifications (a and b) 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 bmodification 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 b-Hg 2 Mo 2 O 7 is discussed.
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