2 NEW DIPHOSPHATES WITH SRV2(P2O7)(2) STRUCTURE - MERCURY AND LEAD PHASES

Two new diphosphates of the AV(2)((PO7)-O-2)(2) series involving a tra ns configuration of the bidentate (PO7)-O-2 groups have been synthesiz ed for A=Hg or Pb. Their structure, determined from a single-crystal X -ray diffraction study is similar to that of the Sr-phase. They crysta llize in the P1 space group, Z = 1, n = 4.848(1) Angstrom, b = 6.892(1 ) Angstrom, c = 8.077(2) Angstrom, alpha = 92.65(1)degrees, beta = 93. 26(1)Angstrom, gamma = 106.23(1)degrees, for the Hg phase and a = 4.80 4(1)Angstrom, b = 7.113(1) Angstrom, c = 7.898(2) Angstrom, alpha = 89 .78(1)degrees, beta = 92.62(1)degrees gamma = 106.10(2)degrees, for th e Pb phase. The crystal structures, solved by the heavy-atom method, w ere refined. The final agreement factors are R = 0.027 and 0.033 for t he Hg and Pb phases, respectively. A comparison with the other isotypi c members of this series A = Sr, Cd and (NaMoPO7)-Mo-x-O-2 (0.25 < x < 0.50) is presented. The great similarity between the lead and stronti um phases is shown, demonstrating the absence of a lone-pair effect of Pb-parallel to in this structure. The particular behaviour of the Hg phase is emphasized, due to the ability of this element to adopt the d umbbell (or 2 + 4) coordination; the strong covalent character of Hg-p arallel to induces a significant distortion of the PO, tetrahedra, rar ely observed to date in transition-metal phosphates.