Michel Tournoux

TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17

TiO2(B) a new form of titanium dioxide, the octatitanate K2Ti8O17 and intergrowths between K2 Ti4 O9 and K2 Ti8 O17 have been prepared by hydrolysis of K2 Ti4 O9 followed by heating at 500°C. TiO2 (B) and K2Ti8 O17 have the host covalent framework of the bronzes Nax TiO2 and K3Ti8O17 respectively.

Positive electrode materials with high operating voltage for lithium batteries: LiCryMn2 − yO4 (0 ≤ y ≤ 1)

Reversible lithium deintercalation of chromium-substituted spinel manganese oxides LiCryMn2 − yO4 (0 ≤ y ≤ 1) in the voltage range 3.4–5.4 V versus Li, occurs in two main steps for 0 < y < 1: one at about 4.9 V and the other at about 4 V. The 4.9 V process capacity increases with the chromium content while the 4 V process capacity decreases at the same time. Excellent cyclability was observed for y ≤ 0.5 while materials with y ≥ 0.75 were loosing capacity rapidly upon cycling. Changing the chromium composition of these materials enables the control of the average intercalation voltage in the range 4.05–4.5 V versus Li, a voltage range where no material was known before. A low manganese to chromium substitution rate in LiMn2O4 was found to be beneficial to the specific capacity and energy and to the cyclability of the spinel materials. Due to the selected electrolyte composition with high stability against oxidation, extra capacity due to electrolyte oxidation at each cycle remained very low even though the charge voltage was highly oxidative.

Phases LixMnO2λ rattachees au type spinelle

Abstract Spinel related compounds Li x MnO 2 λ, 0.02 ⩽ x ⩽ 0.50, are obtained from the spinel type LiMn 2 O 4 by solid-liquid reaction with various oxidizing agents. A new tetragonal phase, LiMnO 2 λ, having the rocksalt structure has been prepared at ambiant temperature by lithium intercalation with n-butyl lithium into Li 0.02 MnO 2 λ or LiMn 2 O 4 . X-ray and electron diffraction studies and crystal structure refinements of both Li 0.02 MnO 2 λ and LiMnO 2 λ show that the manganese ions occupy the same octahedral sites in all compounds as in LiMn 2 O 4 .

La transformation TiO2(B) → anatase

Abstract The transformation from BTiO 2 to anatase either above 550°C under normal pressure or at room temperature under 60 kb is explained by a process of crystallographic shear with elimination of an equal number of anionic and cationic vacancies.

The amorphous oxides MnV2O6+δ (0 < δ < 1) as high capacity negative electrode materials for lithium batteries

MnV 2 O 6+δ .nH 2 O (0 < δ < 1) amorphous oxides were prepared by a two-step process including the precipitation of a crystallized precursor MnV 2 O 6 .4H 2 O and its ozonation at tempcratures below 100 °C. Their characterization by spectroscopic techniques ( XAS and EELS ) shows that V 5+ retains a VO 5 environment and that Mn, which remains in octahedral coordination, is oxidized to Mn 4+ mainly. MnV 2 O 6+δ compounds insert reversibly large amounts of Li per formula unit (e.g. Li 12 MnV 2 O 6.96 ) at low voltage. After the first insertion/de-insertion cycle, subsequent cycles correspond to fully reversible lithium insertion between the two compositions: Li α MnV 2 O 6+α (α≥2) and Li 12 MnV 2 O 6+δ (from 600 to 900 mAh/g). Investigations on the Li derivatives by XAS and EELS show that during the insertion/de-insertion cycles the V oxidation state varies reversibly between + 5 and + 3. Mn 4+ cations are reduced to Mn 2+ upon the first discharge. During the subsequent cycles the average Mn oxidation state varies reversibly between + 2 and about + 2.6. It clearly appears that the electron transfer from Li to the host matrix decreases with increasing x in Li x MnV 2 O 6+δ .

Structure et anomalie dielectrique de Tl2Ta2O6

Abstract Single crystals of the pyrochlore Tl2Ta2O6 have been grown in a flux of thallous carbonate under pressure of carbon dioxide at 900°C. The crystal structure has been refined in the cubic space group Fd3m from 93 independent reflections. The problem of thallium localization along [111] axis is discussed. The dielectric properties show that the real space group cannot be Fd3m at temperatures less than 560°K.

Crystal structure of KSb2PO8

Abstract KSb 2 PO 8 crystallizes in the monoclinic system, space group Co, with a = 12.306(4) A , b = 7.086(2) A , c = 15.037(5) A , β = 95.82(3)°, Z = 8. The structure was determined from 2149 reflexions collected on a NONIUS CAD4 automatic diffractometer with MoK \ ga radiation. The final R index and weighted R w index are 0.019 and 0.025 respectively. The structure is built up from SbO 6 octahedra sharing corners or edges and PO 4 tetrahedra sharing corners with octahedra to form a three-dimensional network. The potassium atoms are situated in the interconnected channellike cavities.

Les tantalates M5−xTa11 + x5O30 et la phase de type bronze hexagonal TlTa3,4O9

Abstract The ternary oxides M 5− x Ta 11 + x 5 O 30 (M = Tl, Rb, H 3 O, Na) have been synthetised by reaction of Ta 2 O 5 with Tl 2 CO 3 at 950°C and from the thallium phase by ionic exchange. The basic framework Ta 11 O 30 is built up from pyrochlore blocks and from octahedra sheets reminiscent of hexagonal tungsten bronze. Non-stoichiometry has been proved by density measurements, ionic conductivity study and T 1 (longitudinal relaxation time) determination for the thallium phase, for several x values, between 20 and 150°C. Partial substitution of Ta by W allows to obtain a bidimensional non-stoichiometric range in the Tl 2 O1bTa 2 O 5 1bWO 3 system. A phase with the hexagonal tungsten bronze structure is obtained in the tantalum rich part of the Tl 2 O1bTa 2 O 5 system. This phase TlTa 3,4 O 9 is isotyp with KTa 3,4 O 9 .

Structure de TlFeBr3: Distorsion du type perovskite hexagonale 2L

TlFeBr3 crystallizes in the hexagonal system with a = 12.444(7) A, c = 6.23(2) A, Z = 6. The crystal structure has been determined from 229 independent reflexions in the space group P63cm. The final R index and the weighted Rw are respectively 0.050 and 0.063. This structure is tightly related to the 2L (CsNiCl3) hexagonal perovskite type. Its a axis coincides the [110] direction of the 2L phase and its length is accordingly multiplied by √3. Furthermore the face-sharing chains of octahedra are shifted along the a axis.

Lamellar perovskites Bi2O2(An-1BnO3n+1) catalysts for oxidative coupling of methane

The oxidative coupling of methane has been studied in the presence of lamellar perovskites Bi2O2(An-1BnO3n+1) belonging to Aurivillius phases. These materials are selective and stable in the OCM reaction. Moreover, their substitution in perovskite (A) or octahedral (B) sites leads to significant changes of the C2+ selectivity or yield which are attributed to modifications of the basicity of A-O or B-O bonds.