Order-Disorder and Mobility of Li+ in the beta- and beta-LiZr2(PO4)3 Ionic Conductors: A Neutron Diffraction Study

Single-crystal X-ray crystallography studies have shown that diphosphates Na3.64Mg2.18(P2O7)2 and Na3.64Ni2.18(P2O7)2 crystallize with the same structural type and the same space group P . Their triclinic lattice parameters are equal to a=10.901 (2), b=9.765 (2), c=6.382 (1) ?, alpha=112.43 (1)°, beta=99.64 (1)°, gamma=107.53 (1)°, Z=2 and a=10.889 (5), b= 9.705 (4), c=6.358 (4) ?, alpha=112.46 (4)°, beta=99.92 (4)°, gamma=107.54 (4)°, Z=2, respectively. The structure could be regarded as a packing of diphosphate groups [P2O7]4- and [MO6] octahedra (M=Mg, Ni) delimiting cavities and tunnels which host sodium cations. The tunnels are running along [001]. The structure is characterized by mixed (Na, M) sites with an occupation factor ratio equal to ca. 0.82/0.18. Sodium cations are located in five different sites: two cavities (one penta- and the other octa-coordinated) totally occupied and three octahedral interstices, which are partially filled by Na(3), Na(4), and Na(5) according to the respective occupation factors of 0.15, 0.42, and 0.25 for Na3.64Mg2.18(P2O7)2 and 0.13, 0.40, and 0.29 in the case of Na3.64Ni2.18(P2O7)2.