Two new diphosphates with SrV2(P2O7)2 structure: mercury and lead phases
Two new diphosphates of the AV2(P2O7)2 series involving a trans configuration of the bidentate P2O7 groups have been synthesized 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 crystallize in the P space group, Z= 1, a= 4.848(1)Å, b= 6.892(1)Å, c= 8.077(2)Å, α= 92.65(1)°, β= 93.26(1)°, γ= 106.23(1)°, for the Hg phase and a= 4.804(1)Å, b= 7.113(1)Å, c= 7.898(2)Å, α= 89.78(1)°, β= 92.62(1)°, γ= 106.10(2)°, for the Pb phase. The crystal structures, solved by the heavy-atom method, were refined. The final agreement factors are R= 0.027 and 0.033 for the Hg and Pb phases, respectively.
A comparison with the other isotypic members of this series A = Sr, Cd and NaxMoP2O7(0.25 < x < 0.50) is presented. The great similarity between the lead and strontium phases is shown, demonstrating the absence of a lone-pair effect of PbII in this structure. The particular behaviour of the Hg phase is emphasized, due to the ability of this element to adopt the dumbbell (or 2 + 4) coordination; the strong covalent character of HgII induces a significant distortion of the PO4 tetrahedra, rarely observed to date in transition-metal phosphates.