Complex distribution of diphosphate groups in the structure of V(IV) phosphates A(VO)3(P2O7)2 (A = Sr, Pb, and Ba)

New V(IV) diphosphates A(VO)(3)(P2O7)(2) have been synthesized for A = Ba, Sr, and Pb. Their X-ray diffraction and electron microscopy studies show their complex crystal chemistry. The polycrystalline samples of the Ba, Sr, and Pb phases and the single crystals of Sr and Pb phases exhibit a P2(1)2(1)2(1)-type structure (called alpha form) similar to that observed for K(VO)(3)(P2O5)(2). For a single crystal of Ba(VO)(3)(P2O7)(2), a new structural form, called beta, has been stabilized. It crystallizes in the P11b space group, with cell parameters similar to those of the alpha form. This structure, closely related to that of the alpha form, differs from the latter by the distribution of the barium cations and the surrounding P2O7 groups. The HREM study of polycrystalline beta-Ba(VO)(3)(P2O7)(2) shows the great flexibility of this structure. It demonstrates the coexistence of the two structures, P2(1)2(1)2(1) and P11b, in many crystallites, in the form of P11b intergrowth defects in the P2(1)2(1)2(1) matrix.