Y. Laligant

Designing fast oxide-ion conductors based on La2Mo2O9

The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite, perovskites, intergrowth perovskite/Bi2O2 layers and pyrochlores. Here we report a family of solid oxides based on the parent compound La2Mo 2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors, this material undergoes a structural transition around 580 °C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 × 10 -2 S cm-1 at 800 °C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with β-SnWO 4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation that does not have a lone pair—and which has a higher oxidation state—could be used as an original way to design other oxide-ion conductors.

Reducibility of fast oxide-ion conductors La2−xRxMo2−yWyO9(R = Nd, Gd)

The substitutional range and cell parameter evolution of fast oxide-ion conductors La2−xRxMo2−yWyO9 (R = Nd, Gd) are investigated. In the whole series, the cubic β-La2Mo2O9 structural type is stabilized at room temperature. The effects on reducibility of both single and double substitutions are presented. Lanthanum substitution by rare earth appeared to be responsible for an increase in the reducibility and a strong but reversible amorphization under dilute hydrogen. On the contrary, the favourable role of tungsten on the compound stability under reducing conditions is evidenced: it depletes oxygen loss while making the La2Mo2O9 structural type less affected by it.

Crystal structure of Pd(NO3)2(H2O)2

Pd(NO3)2(H2O)2 is orthorhombic: SG: Pbca, Z = 4, a = 5.0036(3)A, b = 10.6073(7)A, c = 11.7223(8)A. The crystal structure was determined ab-initio from X-ray powder diffraction data and refined by the Rietveld method (37 parameters refined, 377 reflections, RB = 0.031, RWP = 0.079). Pd2+ adopts a square planar coordination, the nitrato-group is unidentate. The structure consists of isolated Pd(NO3)2(H2O)2 building units connected by hydrogen bonding in order to form two successive layers parallel to the ab plane. Layers are held together by van der Waals interactions.

Ordered Pd2+Cu2+ substitution in 1.2.3 superconductor: The oxide YBa2Cu(3−x)PdxOy (x ∼ 0.5) with Pd2+ in square planar coordination

Abstract The substitution of copper by palladium II in YBa 2 Cu 3 O 7 leads to a new compound Yba 2 Cu 2.5 Pd 0.5 O y with orthorhombic symmetry ( a = 3.841(1) A, b = 3.883(1) A, c = 11.671(3) A; space group Pmmm ) in which Pd exclusively occupies (1a) sites of the structure, corresponding to cations in square planar coordination. A resistive transition occurs at 49 K; magnetic measurements indicate less than 5% of the full Meissner effect.

A new refinement of the crystal structure of the inverse weberite Fe2F5(H2O)2

Abstract Fe2F5(H2O)2 is related to the weberite structure, whose space group is not clearly defined. A careful reexamination of the structure confirms and refines the previous results: Fe2F5(H2O)2 belongs to the space group Imma with cell parameters a = 7.477(1) A, b = 10.862(2) A, c = 6.652(1) A (Z = 4). The structure has been refined from 379 reflections to R = 0.029 (Rw = 0.034). Fe2F5(H2O)2 must be considered as an antiweberite structure since M2+ and M3+ positions are inverse of those of the weberite structure.

Ordered magnetic frustration: VII. Na2NiFeF7: Reexamination of its crystal structure in the true space group after corrections from renninger effect and refinement of its frustrated magnetic structure at 4.2 and 55 K

Abstract A new refinement of the crystal structure at 300 K and of the magnetic structure at 4.2 and 55 K of the ferrimagnetic weberite Na2NiFeF7 is undertaken in order to fully reveal both the true space group of this compound and its magnetically frustrated character. The reflections which previously obliged us to choose space groupImm2 are only due to Renninger effect. The true space group isImma (a = 7.2338(3)A,b = 10.3050(3)A,c = 7.4529(3)A,Z = 4) at 300 K. The structure was refined from 1148 independent reflections toR = 0.025 (Rw = 0.030). The ferrimagnetic behavior is confirmed (Tc = 88(2)K). Neutron powder diffraction shows that the nuclear and magnetic cells are identical and that there is an accident in the thermal evolution of the intensity of some magnetic peaks. Among the different modes given by the Bertauts macroscopic theory, the best fit is obtained for both temperatures with the modes−Fx andFx,Gz for Fe3+ and Ni2+ sublattices, respectively, instead of−Fx and+Fx in the solution previously proposed by Heger. The corresponding moments are 4.93(11) and 1.36(21) μB at 4.2 K (Rmag = 0.045) and 4.34(12) and 0.97(22) μB at 55 K (Rmag = 0.052). The slight anomaly in the thermal variation of the intensity of some magnetic reflections at 50 K is due to a significant change in the spin canting at this temperature, without any modification of the magnetic modes. A Mo¨ssbauer study confirms the anomaly from the thermal variation of the magnetic hyperfine field at the Fe nucleus.

Synthesis and single-crystal refinement of Ba2Y2CuPtO8

Ba2Y2CuPtO8 is orthorhombic (space group Pnma) with a = 13.207(2), b = 5.680(2) and c = 10.321(2) A. The structure was refined from 2654 x-ray independent reflections to R = 0.050 (Rw = 0.059). The lattice is built up from double chains of corner-sharing platinum octahedra and copper pyramids running along the b-axis. Yttrium and barium ions ensure the cohesion of the structure. The crystal chemistry of Ba2Y2CuPtO8 is discussed and compared to the structure of KPbCr2F9.

Li+ ionic conduction in the layered perovskite Li2La2/3Ta2O7

The Li+ ionic conduction properties of the Li2La2/3Ta2O7 layered perovskite compound have been investigated by complementary techniques: impedance spectroscopy, 7Li NMR and thermal neutron powder diffraction. Up to 770 K, the results are consistent with an electrical conductivity dominated by Li+ ions jumping between Li1 and Li2 sites, the adjacent centers of the two kinds of LiO4 tetrahedra constituting the interlayer region of the structure. The sudden event, observed near 770 K on the curve log(σT) = f(1000/T), is associated with significant changes to the 7Li NMR signal and to a structural modification followed by neutron powder diffraction up to 973 K. All these facts are consistent with the displacement at 770 K of the lithium ions residing in the Li1 sites. Neutron powder diffraction showed that, in the interlayer region, the location of the lithium ions in the Li2 sites remains unchanged. However, 17% of the Li1 population leaves its tetrahedral position to occupy a new Li3 site inside the perovskite cages, very close to their four O2− bottlenecks. The other Li+ ions (83%) remain in the interlayer but their coordination changes from tetrahedral to a five fold one.

The structure of Li3Cu2O4, a compound with formal mixed valence

The structure of Li3Cu2O4 was solved from X-ray powder diffraction data and refined from a multiphase specimen using the Rietveld method. The cuprate crystallizes in C2/m with Z = 2. The compound has a small homogeneity range. Typical parameters are a = 9.946(5) A, b = 2.778(2) A, c = 7.260(5) Aand β = 119.10(2)°. The structure may be described as an ordered intergrowth of slabs of Li2CuO2 and hypothetical LiCuO2 of NaCuO2 type. The formula suggests a copper(II, III) mixture, but there is only one crystallographic copper site which implies a more appropriate formulation as Li3+[Cu(II)O2]232 with parallel strands of edge-coupled CuO4 units running along the b axis.

Crystal structure of Li3ThF7 solved by X-ray and neutron diffraction

Abstract The crystal structure of Li 3 ThF 7 has been determined using both X-ray ( T = 300 K) and neutron ( T = 5 K) powder diffraction data, together with a X-ray single-crystal study ( R = 0.039, R w = 0.042). Li 3 ThF 7 is orthorhombic [space group Ccca (No. 68)], a = 8.7885(3), b = 8.7686(3), and c = 12.958(1) at T = 300 K (X-ray powder data). In this compound, Li + octahedra build up double layers with a defect NaCl arrangement. Two double layers are connected by vertices. Th 4+ are inserted between these double layers and are ninefold coordinated.