M. Danot

Structural aspects and magnetic properties of the lamellar compound Cu0.50Cr0.50PS3

Abstract Cu 0.50 Cr 0.50 PS 3 is a new lamellar compound obtained from the elements at 700°C in evacuated silica tubes. The unit cell is monoclinic with a = 5.916 (1) A; b = 10.246 (2) A; c = 13.415 (5) A; β = 107.09 (3)°. The structure is built up with S|Cu 0.33 Cr 0.33 (P 2 ) 0.33 |S slabs in which copper, chromium, and (P 2 ) pairs share the octahedral voids between two sulfur layers. Copper is not located at the center of its octahedral sites but is distributed among a continuous series of positions within these sites. This complex distribution has been simulated, attributing to copper two crystallographic eightfold positions with important thermal factors, especially perpendicular to the a - b plane. EPR studies and optical and magnetic measurements show that chromium is present as Cr 3+ ions. The magnetic study suggests that, below T N ∼ 30 K, this compound is a weakly anisotropic antiferromagnet consisting possibly of ferromagnetic layers which are antiferromagnetically coupled to adjacent layers. A good fit with the experimental results is obtained by means of calculations performed on the basis of a two-dimensional Heisenberg model.

Etude structurale des composés MxTiSe2 (M = Fe, Co, Ni)

Abstract New compounds M x TiSe 2 have been prepared with M = Fe ( x ⩽ 0.66), M = Co or Ni ( x ⩽ 0.50). The metal M is located in vacant octahedral sites of the TiSe 2 host lattice (hexagonal unit cell a ′, c ′). An ordering of vacancies occurs if x ⩾ 0.20. With M = Co or Ni ( x = 0.50) and with M = Fe (0.25 ⩽ x ⩽ 0.66) isotypic compounds of Ti 3 Se 4 can be obtained ( M 3 □ X 4 type; monoclinic unit cell a ≈ a ′ √3, b ≈ a ′, c ≈ 2 c ′). The compounds Fe 0.38 TiSe 2 and Co 0.38 TiSe 2 (hexagonal unit cell a ≈ a ′ √3, c ≈ 2 c ′) are of the M 2 □ X 3 type, variety 2 c ′. The Fe 0.25 TiSe 2 and Co 0.25 TiSe 2 monoclinic unit cells ( a ≈ 2 a ′ √3, b ≈ 2 a ′, c ≈ 2 c ′) allow us to assume, for these two compounds, a structure of the M 5 □ 3 X 8 type, variety 2 c ′, identical to the Ti 5 Se 8 one. The compound Ni 0.25 TiSe 2 has an hexagonal unit cell ( a ≈ 2 a ′, c ≈ 3 c ′); it belongs to a so-called 3 c ′ variety of the M 5 □ 3 X 8 type.

EXAFS identification of the active species in supported niobium sulfide hydrotreatment catalysts

Abstract We present a Nb K-edge EXAFS study of carbon- and alumina-supported niobium sulfide hydrotreatment catalysts. NbS 2 and NbS 3 entities have been evidenced on the carbon support, and NbS 2 and Nb 1− y S ones on the alumina support. EXAFS spectra of carbon-supported species, which are highly disordered, were analyzed through an asymmetric distance distribution model. Considering their poor dispersion, Nb 1− y S species can be thought to be intrinsically highly active. Their activity, and that of NbS 3 entities as well, can possibly be related to cation clustering. Niobium sulfides appear as very sensitive to support effects.

Thermal transformations of nanocomposite material consisting of molybdenum disulfide and nickel hydroxide layers

Abstract The structure of Ni(OH)2/MoS2 nanocomposite material obtained via MoS2 single-layer dispersion and the products of its annealing in the 100–400°C temperature range were studied using X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The transformation of the initial MoS2 superlattice structure with Mo–Mo bonding to the usual structure with unique Mo–Mo distance was found to occur upon annealing. The decomposition of two-dimensional Ni(OH)2-like species in the interlayer space of MoS2 at 400°C leads to complete substitution of O by S in the Ni local environment.

Ionic conductors NaxInxSn1−xS2: NMR study of 23Na and complex impedance measurements

The NaxInxSn1−xS2 intercalation — substitution compounds have been studied by NMR of 23Na and by complex impedance measurements. The obtained results — quadrupole splitting, T1 relaxation times, hopping frequencies, diffusion coefficients, activation energies — are discussed and compared to those relative to homologous series. The role of some factors — such as the sodium environment geometry, the host structure covalency, and the amount of vacancies — is discussed.

Crystal structure of a metal excess spinel : Cu1.10Cr1.30Sn0.70S3.90

The compound Cu1.10Cr1.30Sn0.70S3.90 has been studied by neutron diffraction. It belongs to the spinel structural type, but the formula indicates a copper excess with respect to the ideal spinel composition. The structural refinements have been performed according to the Rietveld method. Chromium and tin are located on the 16d octahedral sites (B-sublattice of the spinel structure). 0.95 copper occupy the 8a tetrahedral sites (A-sublattice) whereas 0.15 copper is found on the 16c octahedral sites which would be unoccupied in an ideal stoichiometric spinel. The R value is 3.6 %. The excess spinel is found to slowly release the copper excess leading to Cu2S formation. The question whether short metal-metal distances or epitaxy phenomenon are involved is discussed.

Investigation of europium-ammonia intercalation compounds of titanium disulfide

Abstract Euue5f8NH 3 intercalation compounds of titanium disulfide have been synthesized from Euue5f8NH 3 solutions at −40 °C, characterized and studied using Mossbauer spectroscopy. These materials are at least initially mixedvalence compounds, with Eu 2+ and Eu 3+ probably occupying distinct sites. Europium occurs predominantly in the trivalent oxidation state. However, the nature of the intercalation products depends stongly upon the method of preparation as well as annealing time and temperature.

Double coordinence du fer dans la phase FexZrSe2 (0 < x < 0,25): propriétés magnétiques et caractéristiques Mössbauer

The FexZrSe2 compounds have been studied from the structural, magnetic, and Mossbauer points of view. The structure of a Fe0.18ZrSe2 single crystal has been refined to a final 0.067 R factor value. Iron disorderly occupies both octahedral and tetrahedral voids of the Van der Waals gap of ZrSe2. The Curie constants are consistent with the Fe(II) state. Negative θp show that interactions are predominantly A.F. For x < 0.18, χ−1 tends toward zero with temperature. For x ≥ 0.18, a susceptibility maximum occurs for T ⋍ 7 K. Mossbauer spectra have been fitted considering two components: a singlet (tetrahedral FeII) and a doublet octahedral FeII) broadened by the □-Fe disorder. The Mossbauer parameters and their thermal evolution are discussed.