La10W2O21: an anion-deficient fluorite-related superstructure with oxide ion conduction.

The crystal structure of La10W2O21, which has to be reformulated (La5.667W0.333)LaWO14□2, is best described, on average, by a 2 × 2 × 2 anion-deficient fluorite-related superstructure cubic cell, with space group F4 3m, Z = 4, and a = 11.17932(6) Å, similar to Y7ReO14--δ. The 32 cations are distributed with lanthanum on the 4a-site, tungsten on the 4b-site, and a partial occupancy of the 24g-site by La (94%) and W. The 56 oxygen atoms occupy four 16e-sites, three of them fully and with an occupancy of 1/2 for the fourth one. Others M10W2O21 (M = Er, Y) adopt a 3 × 2 × 2 fluorite superstructure with W in octahedral sites, whereas W is mainly in tetrahedral sites in La10W2O21. Several powerful techniques such as crystal image furnace synthesis, (139)La nuclear magnetic resonance (NMR) and convergent beam electron diffraction (CBED) were used to achieve our results. Transmission electron microscopy (microdiffraction, CBED, and Tanaka patterns) brought us the real symmetry, showing that indeed classical cubic twinning along the 3-fold axis does take place. The surprising La/W mixed site is nicely confirmed by (139)La NMR. This compound exhibits interesting fast oxide ion conducting properties, comparable with LAMOX (Lacorre et al. Nature 2000, 404, 856-858) at low temperature. As opposed to many ionic conductors, no temperature structural transition is observed. Its conductivity is about 6.4 × 10(-4) S·cm(-1) at 700 °C.

Structure determination of La18W10O57.

We report the synthesis, structure determination, and structure analysis of La(18)W(10)O(57), which presents 52.65 WO(3) mol % in the binary system La(2)O(3)-WO(3) in place of the previous reported compound: La(14)W(8)O(45), 53.3 WO(3) mol %. The structure has been determined from synchrotron and laboratory X-ray, neutron, and electron diffraction data. This tungstate crystallizes in the non-centrosymmetric hexagonal space group (no. 190) P62c, with Z = 2, a = 9.0448(1) A, c = 32.6846(3) A, and a measured density of 7.28(3) g x cm(-3). The structure consists of [WO(6)] octahedra, isolated or sometimes sharing a face, and unusual tungsten [WO(6)] trigonal prisms. This compound presents a subcell c' = c/6 = 5.45 A. Polytypism has been observed concerning this compound. Transmission electronic microscopy, ionic conductivity, and thermal expansion have been explored.

Structure determination of beta-Pb2ZnF6 by coupling multinuclear solid state NMR, powder XRD and ab initio calculations.

The results from one-dimensional multinuclear (19F, 207Pb and 67Zn) magic-angle spinning nuclear magnetic resonance experiments combined with the use of the ISODISPLACE program allow for the space group determination of beta-Pb2ZnF6 (no. 138 P4(2)/ncm). The structure was refined from X-ray powder diffraction data (a = 5.633 (1) A and c = 16.247 (1) A, Z = 4). beta-Pb2ZnF6 has one six-fold coordinated Zn, one eleven-fold coordinated Pb and five F non-equivalent crystallographic sites and is built from alternated layers parallel to the (a, b) plane; tilted ZnF4(2-) layers of corner sharing ZnF6(4-) octahedra and FPb+ layers of edge sharing FPb4(7+) tetrahedra. The structure of beta-Pb2ZnF6 was then optimized using the ab initio code WIEN2k and the calculated 67Zn EFG is in agreement with the NMR results. 19F-19F proximities and 19F-207Pb connectivities were evidenced using through-space and through-bond NMR correlation experiments, respectively, and support the proposed structure. 19F-207Pb J-coupling was also used to select fluorine resonances depending on the number of neighbouring lead ions, leading to an unambiguous assignment of the different 19F resonances.

Neutron powder diffraction, multinuclear, and multidimensional NMR structural investigation of Pb5Ga3F19.

The room temperature structure of Pb5Ga3F19 is investigated by combining neutron diffraction and multinuclear 19F, 71Ga, and 207Pb one-dimensional and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments. Two models built in space group I4cm are reported for the description of the crystalline structure of Pb5Ga3F19. The structure is built from a network of both opposite corner-sharing Ga2F6(3-) octahedra forming infinite chains along the c-axis and isolated Ga1F6(3-) octahedra. The two models present two slightly different views of the strong static disorder of the fluorine ions belonging to the Ga2F6(3-) octahedra. 71Ga NMR results show that the local environment of all Ga2 ions is identical, which indicates a tilt of the Ga2F6(3-) octahedra within the chains. 207Pb NMR experiments confirm that the environment of only one of the two lead sites, Pb1, is strongly affected by the disorder, which gives rise to three broad distinct 207Pb NMR lines for this site. All 19F NMR lines are assigned using the 19F DQ-SQ MAS experiment. 19F-207Pb through-bond and through-space heteronuclear correlation experiments are carried out for the first time, supporting assignment of both the 19F and 207Pb NMR spectra. These correlation experiments also show that both models correctly describe the medium-range order of the structure of Pb5Ga3F19.