F. d'Yvoire

Phase transitions and ionic conduction in 3D skeleton phosphates A3M2(PO4)3 : A = Li, Na, Ag, K ; M = Cr, Fe

Abstract Phosphates A 3 M 2 (PO 4 ) 3 (AMP) with A = Li, Na, Ag, K and M = Cr, Fe have been prepared by direct synthesis, flux methods and cation exchange in molten salts. The Li, Na and Ag-compounds are solid electrolytes structurally related to Nasicon (NaMP, AgMP and I-LiMP) or to monoclinic Fe 2 (SO 4 ) 3 (II-LiMP). Their conductivities are about 10 −2 Ω −1 cm −1 at 300°C. Below 350°C, NaMP and II-LiMP undergo several reversible phase transitions accompanied by slight changes of the crystal lattice and probably associated with a disordering of the A + ions. These transitions range from first order ones to transitions spread over a wide range of temperature. Their effect upon conductivity is presented. KFeP has a particular structure with a 3D network of linked PO 4 , FeO 6 and FeO 5 polyhedra. The K + ions occupy an interstitial space which presents too narrow bottlenecks to allow a fast ion transport.

Crystal structure and cation transport properties of the ortho - diphosphates Na7(MP2O7)4PO4 (M = Al, Cr, Fe)

Abstract The isotypic ortho-diphosphates Na7(MP2O7)4PO4 (M = Al, Cr, Fe) have been synthesized. They are tetragonal, space group P421c. The structure of the iron member has been solved and refined to a final R = 0.037 for 577 independent reflections. It consists of (FeP2O7)4PO4 units interconnected so as to form a 3D framework into which the sodium ions are inserted. The units are made of a PO4 tetrahedron linked to four FeO6 octahedra; each octahedron is connected to a P2O7 group by two oxygens. Ion exchange experiments and conductivity measurements give evidence of some mobility of the sodium ions at high temperature. However, the σ-values ( ∼ 10 −5 Ω −1 cm −1 at 300°C) are relatively low. The narrowness of the windows between the sodium sites as well as the presence of an oxygen atom linked to only one polyhedron of the framework are unfavourable features to fast cation transport.

A new family of sodium ion conductors: the diphosphates and diarsenates Na7M3 (X2O7)4; (M=Al, Ga, Cr, Fe; X=P, As)

Abstract Seven monoclinic isotypic compounds with the general formula Na7M3 (X2O7)4 have been synthesized by crystallization in a flux of sodium phosphates or arsenates. They exhibit high ionic conductivity (σ≈10−3-10−2ω−1cm−1 at 300°C) and good cation exchange properties particularly with Ag+ ions. At temperatures varying from −20°C to 240°C, depending on the particular compound, they undergo a reversible phase transition, α⇌β, which is associated with a loss of long range ordering of the sodium ions and results in an increase of the conductivity. The crystal structures of β-Na7Fe3 (As2O7)4 and α-Na7Fe3 (P2O7)4 have recently been determined and a model of sodium diffusion paths has been proposed. This model indicates a two-dimensional conduction within planes parallel to (001). Conductivity measurements performed on single crystals confirm this two-dimensional character with a σ|/σ⊥ ratio of about 2×103.

NASICON type materials - Na3M2(PO4)3 (M=Sc, Cr, Fe): Na+-Na+ correlations and phase transitions

Abstract The thermal behaviour of Nasicon type materials Na 3 M 2 (PO 4 ) 3 (M=Sc, Cr, Fe) is examined. Three types of phases are found. a) A high temperature superionic rhombohedral γ phase, with only local order, which is the same for all three M elements. b) An intermediate phase in which the average lattice is either rhombohedral with a superstructure 2a,2a,c for M=Sc and Fe (β phase) or monoclinic for M=Cr (αphase). This intermediate phase corresponds to highly correlated Na + 2 type ions while Na 1 + ions are strongly disordered. c) A low temperature α phase which is different for M=Cr or Fe and corresponds to an ordering while single crystals tend to Cr α type one because of strong strains. The transitions are sharp for M=Fe (and Sc powders), much smoother for M=Cr diffuse for M=Sc crystals. They are discussed in terms of ion transfer.

Preparation of textured alumina films by the sol-gel route

Alumina films were fabricated by the sol-gel route under various preparation conditions. The peptization rate and the structure of colloidal particles depend on these conditions. In particular, the peptization of a dried boehmite precipitate prepared from aluminum alkoxide was achieved within a few seconds without heating. The resulting sol is made of highly dispersed crystallites; this allows the formation of well-textured xerogel films as evidenced by the Weissenberg x-ray method.

Crystal structure, non-stoichiometry and conductivity of II-Na3M2(AsO4)3 (M=Al, Ga, Cr, Fe)

Abstract The arsenates Na 3 M 2 (AsO 4 ) 3 (M=Al, Ga, Cr, Fe) in their II-modification exhibit fast sodium ion transport properties. Their structure consists of a 3D framework [M 4 (AsO 4 ) 6 ] ∞ with cavities in which the sodium ions are located according to the formula Na(1) Na(2) 6 □ M(1) M(2) 3 (AsO 4 ) 6 . The framework contains M 4 O 18 clusters made up of three M(2)O 6 octahed a sharing an edge with a central M(1)O 6 octahedron. Two models of conduction paths are proposed: one involves both the Na(1) and Na(2) sites, the other only the Na(2) site. Each of them forms a 3D network of intersecting channels. A non-stoichiometry phenomenon can occur which, for M=Fe, results from partial substitution of Na + and Fe 2+ for Fe 3+ in the Fe(1) site compensated by a higher occupancy of the Na(2) site according to the formula Na(1) Na(2) 5+2 v + v □ v −2 v − v [Fe 1− v − v III Fe v II Na v ](1) Fe III (2) 3 (AsO 4 ).

Structure and ion transport properties of Na2OGa2O3P2O5 glasses

Abstract The structure of glasses prepared in the Na 2 Oue5f8Ga 2 O 3 ue5f8P 2 O 5 system was investigated by vibrational (IR, Raman) and X-ray absorption spectroscopies. For the orthophosphate compositions, gallium is found in four-fold coordination, whereas it is found in six-fold coordination in a metaphosphate glass. For pyro-, tri and tetraphosphate glasses, gallium atoms are distributed on tetrahedral and octahedral sites. Gallium atoms found on octahedral sites are likely chelated by pyrophosphate groups. The ionic conductivity of the glasses depends mainly on their sodium content.

Xanes study of five-fold coordinated iron (III) in Fe3PO7 and FeAsO4-I

Abstract Fe 3 PO 7 and FeAsO 4 -I are appropriated model compounds for an X-ray absorption spectroscopy study of the 5-fold coordination of iron (III) with oxygen. Their XANES spectra were compared with those of FePO 4 (4-fold coordination), ZnFe 2 O 4 (6-fold coordination) and NiFe 2 O 4 (4 and 6-fold coordination). The main features exhibited by these spectra depend on the symmetry of the iron site. They are associated with electronic transitions from the core level (1s) to 3d, 4s and 4p-like molecular levels and with photoelectron scattering resonances. The energy gap between two given transitions is different from one compound to another: it mainly depends on the mean Feue5f8O distance in the FeOn polyhedra. Furthermore, our data show that XANES makes it possible to clearly distinguish 5-fold from 6-fold coordination of Fe III but not from 4-fold coordination.

Electrochemical oxidation of CoO-ZrO2(CaO) lamellar eutectic structures to form aligned Co3O4-ZrO2(CaO) material: Mechanism and crystallography of the reaction

Abstract Oriented lamellar eutectic composites of CoO and ZrO 2 (CaO) platelets were oxidized in oxygen at 880°C to form Co 3 O 4 between the zirconia plates. The relatively rapid longitudinal oxidation of CoO can be described by an electrochemical local cell model whereby oxygen ions are conducted from the external surface to the internal oxidation front through the zirconia electrolyte. The crystallographic relationship between the Co 3 O 4 and ZrO 2 (CaO) resulting phases is the same as that found for CoO and ZrO 2 (CaO) in the starting material.

XAFS study of sixfold coordinated arsenic(V) in LiAsO3 and CaAs2O6

Abstract Conversely to previously reported data, EXAFS indicate that the Asue5f8O distances are similar in LiAsO3 and CaAs2O6. The main features of the XANES spectrum of LiAsO3, except the intense white line, come from single and multiple scatterings on atoms found in a small cluster surrounding the absorbing As atom ( R A ).