New polymorphs of A2VP2O8 (A = Na, Rb):: Structure determination and ionic conductivity

New polimorphs of the compounds Na2VP2O8 and Rb2VP2O8 have been prepared by a direct fusion of the reactants. Na2VP2O8 (NVP) crystallises in the tetragonal P4mb space group with a = 8.108(4) Angstrom and c = 4.943(6) Angstrom. Rb2VP2O8 (RVP) is orthorhombic, space group Pnma, a = 12.790(3) Angstrom, b = 7.09(3)(8) Angstrom and c = 9.1786 Angstrom. Both compounds are formed by the association of P2O7 groups and VO5 square pyramids. NVP is isotypical with K2VP2O8 and consists of [VP2O8](infinity) layers parallel to the ab plane intercalated by Na atoms that are located into pentagonal tunnels running along the c-axis. RVP is isotypical with Cs2VP2O8 and formed by the assemblage of [VP2O10] units leading to the two-dimensional [VP2O7](infinity) layers laying in the bc plane; the Rb+ are located between the layers in the interstitial spaces defined by the seven-sided windows which do not give rise to tunnels. The second part of this paper is devoted to the ionic conductivity properties of these materials using the complex impedance method, the measurements were made in the frequency range 10 MHz-1 Hz. The activation energies calculated were E-a(NVP) = 0.49 eV and E-a(RVP) = 0.85 eV.