P. Hagenmuller

Electrochemical intercalation and deintercalation of NaxMnO2 bronzes

Abstract Various NaxMnO2 bronzes have been electrochemically deintercalated. Na0.40MnO2 has a channel structure which is maintained for a large intercalation range (0.30 ≤ × ≤ 0.58). In order to explain the upper intercalation limit, an ordered sodium distribution between two types of Na+ sites is proposed. Na0.70MnO2 and α-NaMnO2 have lamellar structures of P2 and 0′3 types. During intercalation the original P2 type is maintained for 0.45 ≤ × ≤ 0.85 while two reversible structural transitions are observed from α-NaMnO2. A similar behavior occurs during the deintercalation of the high-temperature β-NaMnO2 variety. In each case of the structural transition the double octahedral layers remain unchanged. Electronic localization (increased by Mn3+ Jahn—Teller effect) tends to trap the Na+ ions and therefore increases the relaxation time of the investigated materials.

Structure and thermal expansion of LiGe2(PO4)3

Abstract LiGe2(PO4)3 belongs to the Nasicon-type family. Room-temperature structure has been determined on a single crystal from 3D X-ray data. Thermal evolution of this structure has been established from neutron powder diffraction data between 300 and 1000 K using the Rietveld method. The thermal expansion is positive along the c-axis, whereas shrinking is observed along the a-axis below 900 K, followed by dilatation. The atomic displacements noted with increasing temperature are consistent with a model proposed for NaZr2(PO4)3.

On the preparation and characterization of a new polymorph of V2O5

Abstract A new metastable γ′ variety of V 2 O 5 has been prepared by chemical or electrochemical deintercalation of lithium from a γ-LiV 2 O 5 bronze. The structure of the new phase has been determined with a Rietveld method and compared to those of the normal V 2 O 5 oxide and of γ-LiV 2 O 5 . γ′-V 2 O 5 transforms into the normal form of V 2 O 5 at 340°C.

Anisotropie des proprietes electriques de l'oxyde de fer Fe2O3α

Abstract The electrical conductivity and the thermoelectric power of trivalent iron oxide α-Fe 2 O 3 , studied in the crystallographic plane (001) and along the [001] axis, have shown typical anisotropic behaviour. In the same material, the existence of two electronic transitions at 2.14 and 2.50 eV respectively has been found after optical absorption measurements. At high temperature (T > 1020 K) the electrical conductivity has an essentially intrinsic origin, and is practically independent of the oxygen pressure, while the extrinsic contribution predominates below 720 K. According to the model developped for the interpretation of the above experimental results, the peculiar behaviour of the electrical conductivity results from variations in the mobility of the carriers, depending on the electronic levels they are likely to occupy. These levels were identified as the results of a split in the t 2g orbitals of iron under the influence of the rhombohedral. distortion of the crystal field. Furthermore, the anisotropy of mobility is strongly dependent on the magnetic order below Neel temperature.

Proprietes physiques et structurales de la phase CrxV1−xO2

Abstract The CrxV1−xO2 solid solution has been studied by D.T.A., X-ray diffraction, magnetic susceptibility and electrical conductivity measurements. Three phases with related structures are made in evidence at any composition; they are respectively monoclinic, orthorhombic and tetragonal with the rutile structure. The extension of the homogeneity ranges with composition and temperature and the evolution of the conduction mechanisms are explained.

A structural interpretation of the Pb2+, Eu3+ and Nd3+ optical spectra in doped sodium borate glasses

Abstract The study of sodium borate glasses over the compositional range 10–35 mol.% Na2O by using Pb2+, Eu3+ and Nd3+ as local sounds shows the existence of two possible situations depending on whether the Na2O content is lower or higher than about 25 mol.%. In the first composition range the simultaneous increase of optical basicity and of modifier ion site asymmetry implies rather a 2D-character of the network former. The properties of the glasses with high Na2O content involve more 3D-character for the former sublattice.

Stabilization of unusual electronic configurations of transition elements in elongated six-coordinated oxygen sites of a K2NiF4 structure

Abstract The A ′ x A 2− x Li 0.5 M 0.5 O 4 matrix deriving from the K 2 NiF 4 structure has an elongated M O 6 octahedron; it has been used to stabilize anisotropic electronic configurations such as high-spin Fe 4+ , low-spin Co 4+ , medium-spin Co 3+ , low-spin Ni 3+ , or low-spin Cu 3+ . The choice of ions to be stabilized was based on a simple model for predicting the electronic configuration of a d n ion in a tetragonally distorted octahedral environment.

Elaboration and characterization of lithium conducting thin film glasses

Abstract Thin films obtained in the B 2 O 3 -Li 2 O, B 2 O 3 -SiO 2 -Li 2 O or B 2 O 3 -Li 2 O-Li n X (n = 1, X = N; n = 2, X = SO 4 ) systems, using vacuum evaporation and r.f. sputtering, have been investigated. They have a small electrical resistance and their use as electrolytes in microbatteries present a great deal of interest. The electrochemical cell Li/thin film glass/TiS 2 has been investigated, the open circuit voltage and current density are respectively 2.45 V and 10 μA/cm 2 .

Growth and electrical properties of pure and Ni-doped Co3O4 single crystals

Abstract Single crystals of pure and Ni-doped Co 3 O 4 spinel-type phases have been grown by C.V.T. Electrical conductivity and thermopower measurements have been performed between 25 and 950 K. Ni-doping gives rise to a significant increase of electrical conductivity at room temperature by three orders of magnitude. The obtained results suggest that the Ni atoms are located in the octahedral sites of the spinel lattice with both oxidation states 2+ and 3+, as illustrated by the formula : Co 2+ 1−y Co 3+ y [Co 3+ 2−x Ni 2+ y Ni 3+ x−y ]O 4 .

On the electrical properties of polycrystalline delafossite-type AgNiO2

Abstract AgNiO 2 is prepared by reaction of molten silver nitrate with NaNiO 2 . Powder X-ray diffraction shows that the compound has the delafossite structure of space group R3m . Magnetic data give evidence for the localization of the 3 d -electrons of most Ni atoms. Electrical conductivity and thermoelectric measurements performed between 4.2 and 300 K on polycrystalline samples show that AgNiO 2 behaves like a semimetal. N -type and p -type charge carriers are created by a slight overlapping of the d band of silver atoms with the σ* antibonding band resulting from the overlapping of the Ni 3 d - and O 2 p -orbitals.