Philippe Lacorre

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 κ-alumina: an X-ray powder diffraction,TEM and NMR study

The crystal structure of κ-alumina (κ-Al 2 O 3 ) has been determined ab initio from an X-ray powder diffraction pattern (reliability factors: R Bragg =0.046, R p =0.090, R wp =0.115, χ 2 =11.7). The acentric structure (orthorhombic system, space group Pna2 1 , a=4.8437(2) A, b=8.3300(3) A, c=8.9547(4) A, Z=8) is built up from a pseudo-close-packed stacking ABAC of oxygen atoms, with aluminium in octahedral and tetrahedral environments in a 3:1 ratio, which form zigzag ribbons of edge-sharing octahedra and corner-sharing tetrahedra. κ-Alumina was characterized using magic angle spinning (MAS) 27 Al NMR at two fields and multiple quantum magic angle spinning (3Q MQ MAS) 27 Al NMR. A high-resolution electron microscopy study confirmed the structure and showed the presence of two types of defects: antiphase boundaries and 120 ° disorientations. A model is proposed for these two types of defects, which leaves unchanged the pseudo-close-packed arrangement of the oxygen atoms and assumes a shift or 120 ° twinning of aluminium ions.

The LPS concept, a new way to look at anionic conductors

Abstract A new approach to anionic conduction is presented, and illustrated using already well-known anion conductors, as well as the recently discovered fast oxide-ion conductor La 2 Mo 2 O 9 . The so-called LPS (lone-pair substitution) concept could be used to seek for novel families of anion conductors, issued from already known compounds with lone-pair elements. It is based on the similarity in volume between an electronic lone pair and oxide or fluoride anions, and on the vacancy created by the replacement of a lone-pair by a non-lone-pair cation. A tentative list of the most appropriate substituting elements for given lone-pair cations is presented.

Cooling by adiabatic pressure application in Pr1−xLaxNiO3

A novel principle for cooling by adiabatic pressure application in the mixed crystalline compound Pr1−xLaxNiO3 is described and experimentally verified. Cooling occurs in the vicinity of the structural phase transition where the electronic ground state of the Pr3+ ions changes from a singlet to a doublet state. By properly choosing the La concentration x, the cooling effect can be achieved down to some 100 mK. Furthermore, Pr1−xLaxNiO3 can be used for second and third stage cooling down to the μK region by classical paramagnetic and nuclear demagnetization techniques, respectively.

Phase transition process in oxide-ion conductor β-La2Mo2−xWxO9 assessed by internal friction method

The oxygen ion diffusion and phase transition in La2Mo2−xWxO9 (x=0, 0.25, 0.75, 1.0, and 1.4) have been investigated by the internal friction method. In addition to the low-temperature relaxation peak associated with oxygen ion diffusion, an internal friction peak of phase transition type is observed around 350°C in all tungsten substituted La2Mo2O9 compounds. Based on the behavior of this peak and the ionic conduction properties, the mechanism of this peak is suggested to be associated with a transition from static disordered state to dynamic disordered state of oxygen ion distribution in anion sublattice that most probably results in a transition of the ionic conduction from the Arrhenius type to the Vogel-Tamman-Fulcher type.

Crystal and magnetic structures of LiCoF4: The first compound with a dirutile structure

Abstract The nuclear and magnetic structures of the antiferromagnet LiCoF 4 ( T N = 150(2) K) were solved by neutron powder diffraction at 170 and 2 K, respectively. The nuclear structure ( R I = 0.047) provides the first explicit example of a dirutile structure with a Li + Co 3+ cationic ordering within the chains. In order to explain the magnetic properties, it can also be described from (CoF 4 ) − pervoskite-like layers between which Li + ions are inserted. The relationship between dirutile and A FeF 4 structures is discussed. The magnetic cell is 2 a , b , c . After the refinement of the data at 2 K ( R nuc = 0.041; R mag = 0.065), the moments of Co 3+ (μ = 3.62(8) μ B ) are found in the (010) plane ( G x , A y , G z mode) of the monoclinic cell. In terms of perovskite layers, the moments, perpendicular to the planes, indicate a negative value of the anisotropy term of the spin Hamiltonian.

A new technique to assess electrical behaviour by microwave measurements. Application to perovskites RNiO3 (R = Nd, Sm)

Abstract An original technique to measure conductivity and Hall effect on powdered samples at microwave frequency is presented. A microwave electric field E and a perpendicular magnetic field B induce an effect on the motion of electric charges and a component of the movement appears along the direction E Λ B . This motion generates, in a cavity, a new mode of oscillation. The analysis of the signal allows the study of the charge carriers, especially the sign and the parameters τ m and n. This new method has been used to measure the conductivity behaviour of perovskites Sm1 − xNdxNiO3. It proved possible to detect the metal-insulator transition at room temperature, as well as the nature of charge carriers.

Ordered magnetic frustration: X. Magnetic structure of α-KCrF4 at 1.5 K

Abstract The magnetic structure of orthorhombic α-KCrF4 at 1.5 K (S.G. Pnma, a=15.76 A, b=7.43 A, c=18.38 A, Z=24) was solved from powder neutron diffraction data. Magnetic constraints, due to antiferromagnetic coupling inside the triangular cycles of Cr3+F6 corner sharing octahedra, lead approximately to a star configuration of moments in the (010) plane (μCr1=2.11(11)μB, μCr2=1.91(12)μB, μCr3=2.19(6)μB). The magnetic coupling between Cr3+ along [010], which corresponds to the direction of infinite columns, is strictly antiferromagnetic.

A novel principle for cooling by adiabatic pressure application in rare-earth compounds

Abstract A novel principle for cooling by adiabatic pressure application in rare-earth compounds is introduced. The principle is based on the occurrence of a pressure-induced structural and/or magnetic phase transition where the point symmetry at the rare-earth site is changed involving a change of the degeneracy of the crystal-field ground state and/or a complete lifting of the degeneracy of the crystal-field states by the Zeeman effect. Cooling can then be achieved under adiabatic application (or removal) of pressure as will be demonstrated for the rare-earth compounds Pr 1− x La x NiO 3 and CeSb.