STRUCTURE OF A MODULATED MONOCLINIC PHASE OF NA4TIP2O9

The crystal structure of tetrasodium titanium nona oxodiphosphate, Na4 TiP2O9, has been investigated a 293 K from X-ray diffraction data coll ected on CAD-4F diffractometer using Cu Kalpha radiation, lambda = 1.5 418 angstrom. The structure is commensurate modulated monoclinic, with the basic cell parameters: a = 8.631 (1), b = 7.541 (1), c = 7.049 (1 ) angstrom, beta = 116.10(2)-degrees, Z = 2. The superspace group is P 1 sBAR(P2/c.), the wavevector is q = 0.2a + 0.2c*. D(x) = 2.786 Mg m- 3; M(r) = 345.779; mu(Cu Kalpha) 15.202 mm-1; F(000) = 336. The final R factors are as follows: R = 0.080 for all 3056 reflections, R0 = 0.0 68 for 729 main Bragg reflections, R1 = 0.076 for 1329 first-order sat ellite reflections, R2 = 0.113 for 998 second-order satellite reflecti ons. The crystal structure of Na4TiP2O9 at 293 K consists of {Ti2[PO4] 4O2}infinity chains parallel to the [001] direction. The voids between the chains are occupied by Na atoms. The positions of these Na atoms, which in superionic high-temperature modifications of Na4TiP2O9 are i nvolved in ionic transport, have been studied in detail. Unlike the su perionic phases, where the sites of these atoms are occupied statistic ally, in the monoclinic phase these sites are either unoccupied or ful ly occupied. The latter leads to the filling of the main conductivity channels, which causes the loss of conductivity in the monoclinic phas e.