Tl1-xNaxNb2PO8 and Related Compounds, Isostructural with Ca0.5+xCs2Nb6P3O24

Compounds A2/3Aʹ1/3M2XO8 (A=Tl, Rb, Cs; Aʹ=Na, Ag; M=Nb, Ta; X=P, As) have been synthesized using the ceramic method. The sodium and potassium compounds (A= Na and K) have been prepared by an ion exchange reaction starting from their thallium analogues. These materials are isotypic with Tl1-xNaxNb2PO8 (x=0.21) the structure of which has been determined by using X-ray single-crystal data. The space group is R32, the cell constants are aH=13.369(2), cH=10.324(3) ? and z=9. This compound is isostructural with Ca0.5+xCs2 Nb6P3O24. Its three-dimensional framework [Nb2PO8]n, built up from NbO6 octahedra and corner-sharing PO4 tetrahedra, delimits tunnels running along cH and cavities accommodating Tl+ and Na+ cations, respectively. The K2/3Na1/3Nb2PO8 structure, refined using X-ray powder data, showed that K+ cations are spread like the Tl+ ones over many sites, but more excentred from the tunnel axis. The isotypy of these compounds is also revealed by the similarity of the infrared and Raman spectra. The nonlinear optical study showed a behavior similar to that of the KDP for all the compounds. The ionic conductivity measurements gave high activation energies and low conductivity values for these materials.