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Complex Distribution of Diphosphate Groups in the Structure of V(IV) Phosphates A(VO)3(P2O7)2 (A = Sr, Pb, and Ba)

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journal contribution
posted on 1999-11-05, 00:00 authored by M. M. Borel, M. Hervieu, A. Leclaire, C. Michel, J. Chardon, J. Provost, B. Raveau
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 P212121-type structure (called α form) similar to that observed for K(VO)3(P2O7)2. For a single crystal of Ba(VO)3(P2O7)2, a new structural form, called β, has been stabilized. It crystallizes in the P11b space group, with cell parameters similar to those of the α form. This structure, closely related to that of the α form, differs from the latter by the distribution of the barium cations and the surrounding P2O7 groups. The HREM study of polycrystalline β-Ba(VO)3(P2O7)2 shows the great flexibility of this structure. It demonstrates the coexistence of the two structures, P212121 and P11b, in many crystallites, in the form of P11b intergrowth defects in the P212121 matrix.