Nature de la fluorescence de l'europium divalent dans les fluorures

The levels of the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ (4f⁷) and 4f⁶5d¹ states of europium (+II) have been determined from fluorescence spectra in a series of fluorides M_xB_yF_z (M = alkaline-earth element, B = Li, Be, Mg, Y, Si). As in BaY₂F₈, SrSiF₆ and BaSiF₆, the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ level lies even at room temperature far below the 4f⁶5d¹ band, only the f → f emission is observed. The influence of the choice of the alkaline-earth and the B cations and of the coordination of europium on the relative positions of the energy levels is discussed.     

Influence de la substitution oxygene-fluor dans les materiaux ferroelectriques de structure derivee de la chiolite

All oxide fluorides with ferroelectric properties so far investigated have been obtained by slight substitution of oxygen by fluorine in ferroelectric oxides (less than 10 %) : the Curie temperature decreases very rapidly as the substitution rate rises. A solid solution with formula Na₅(W_3?xNb_x)O_9?xF_5+x (0 ? x ? 2) and a chiolite-related structure has been identified in this work. Three phase transitions have been observed. The corresponding temperatures decrease slowly with increasing x, but the high Curie temperature ($\text{T}{}_{\mathrm{<mtext>C</mtext>}}\text{?800}\text{K}$) observed for a relatively large substitution rate (for $\text{x}\text{= 0 :}\text{F}\text{O+F}\text{= 0.36}$) shows that the material belongs to a family of true ferroelectric oxide fluorides. This conclusion is supported by the fact that neither the pure oxide nor the fluoride chiolites are ferroelectric.     

Determination de la structure cristalline de la variete a des hydroxycarbonates de terres rares LnOHCO3 (Ln = Na)

A probable model for the structure of the orthorhombic A - type neodymium hydroxycarbonate is presented. It relies on an optical determination of the site symmetry of OH^?, CO^2?₃, and Nd³⁺ atoms from infrared and visible absorption spectra, together with a computation of 50 X-ray diffraction lines intensities of a high resolution Guinier orthorhombic powder pattern ($\text{a}\text{= 4,953}\text{A}\text{?}$, $\text{b}\text{= 8,477}\text{A}\text{?}$, $\text{c}\text{= 7,210}\text{A}\text{?}$; space group Pmcn (n^o62)). The CO^2?₃ groups lies flat between mettalic planes as in the aragonite type of structure and are linked to OH^? by hydrogen bond ($\text{d(CO}{}^{\mathrm{2?}}{}_3\text{?}\text{OH}{}^{\mathrm{?}}\text{) = 2,52}\text{A}\text{?}$). The rare earth coordination is nine fold: two OH^? groups being closer (2,58 Å) than the seven oxygen from the CO^2?₃ groups (2,58 to 2,70 Å).     

Etude par spectrometrie d'absorption infra-rouge des facteurs influencant la transformation ordre-desordre de ferrites γ lacunaires derivant de l'oxydation de spinelles ferreux

The order-disorder phenomenon in defect γ-spinels of type $\text{(Fe}{}_{\mathrm{<mtext>(8?8y)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{<mtext>8y</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{□}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ and $\text{Fe}{}_{\mathrm{<mtext>(8?2x)</mtext><mtext>3</mtext>}}{}^{\mathrm{3+}}\text{M}{}_{\mathrm{x}}{}^{\mathrm{2+}}\text{□}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ with M³⁺ = A1³⁺, Cr³⁺ and M²⁺ = Zn²⁺, Co²⁺, Mn²⁺ obtained from ferrous spinels was investigated using IR spectrometry. These compounds possess a vacancy ordering on octahedral sites for substitution extents of less than 0.30 and no ordering has been observed for substitution extent > 0.40. The ordering process is also influenced by the nature and position of the trivalent or divalent cation, particle size, heating temperature and oxidation time.     

Spectres vibrationnels,transitions electroniques et structures a courtes distances dans les verres: Sulfures de terres rares - sulfure de gallium

Absorption spectra of chalcogenide glasses of general formula Ln₂S₃-1,8 Ga₂S₃ (Ln = La⁺³, Ce⁺³, Pr⁺³, Nd⁺³, Gd⁺³ were investigated continously from far infrared to ultra-violet region. Raman spectra were recorded in the 5cm^?1–600 cm^?1 range. Vibrational spectra of glasses have been compared to those of polycrystalline compounds $\text{Ln}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{Ga}{}_6\text{S}{}_{14}$ (Ln = La⁺³, Ce⁺³) and $\text{Ln}{}_6\text{Ga}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{S}{}_{14}\text{(}\text{Ln = La}{}^{\mathrm{+3}}\text{to Gd}{}^{\mathrm{+3}}\text{)}$.From the observed spectra it could be concluded that the short distance order around gallium atom is strongly modified in going from crystalline to glassy state.Electronic transitions of Ce⁺³, Pr⁺³ and Nd⁺³ ions have been also identified in the visible region and in the mid infrared absorption spectra.     

Système ternaire La2Se3-Ga2Se3-GeSe2 diagramme de phase — Etude des verres

Existence of a solid solution, hexagonal $\text{Ce}{}_6\text{Al}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{S}{}_{14}$-type, between $\text{La}{}_6\text{Ga}{}_{\mathrm{<mtext>10</mtext><mtext>3</mtext>}}\text{Se}{}_{14}$ and La₆Ge_2.5Se₁₄ compounds. Location of the eutectic valleys and ternary eutectic (650°C, for La₂Se₃, 3 Ga₂Se₃, 14 GeSe₂). Level curves of the liquidus.Obtention of glasses in a large domain of composition, along the eutectic valley. Measurements of Tg and crystallisation temperature.     

Etude par conductivite electrique et par spectrometrie d'absorption infra-rouge de la precipitation d'hematite a partir de ferrites lacunaires

The hematite precipitation from zinc or cobalt-substituted defective spinels of type $\text{(M}{}_{\mathrm{x}}{}^{\mathrm{2+}}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>(8?2x)</mtext><mtext>3</mtext>}}\text{■}{}_{\mathrm{<mtext>(1?x)</mtext><mtext>3</mtext>}}\text{)O}{}_4{}^{\mathrm{2?}}$ has been investigated using electrical conductivity and infrared spectrometry. The experimental data show that the temperature and kinetic of hematite precipitation depend of substitution extent x and might be closely associated with the cation vacancy in the spinel lattice. Stabilization of spinel solid solution by the cation is more pronounced for zinc than for cobalt and the effect of the cation is interpreted in terms of cation distribution in the spinel lattice.     

Structure cristalline et proprietes physiques de Cu0.75 VS2

A new compound with composition Cu_0.75 VS₂ has been prepared. Its preparation, X-Ray structure, electrical and magnetic properties are reported. The structure is related to the CdI₂-type, as in the case of the previously described Cu_xTiS₂ (1); in Cu_0.75 VS₂, Cu atoms are ordered in tetrahedral sites between the CdI₂-type subunits, whereas in Cu_xTiS₂ Cu atoms are disordered in two independent sites. The vanadium atoms are shifted with respect to the titanium sites which leads a monoclinic distortion of the hexagonal cell. The relation between the CdI₂ unit cell and the true monoclinic cell of Cu_0.75 VS₂ is:

$\text{a}{}_{\mathrm{<mtext>mono</mtext>}}\text{= 2}\text{a}{}_{\mathrm{<mtext>hex</mtext>}}\text{3}\text{;}\text{b}{}_{\mathrm{<mtext>mono</mtext>}}\text{=}\text{4}\text{3}\text{a}{}^2{}_{\mathrm{<mtext>hex</mtext>}}\text{+}\text{c}{}^2{}_{\mathrm{<mtext>hex</mtext>}}\text{9}\text{;}\text{c}{}_{\mathrm{<mtext>mono</mtext>}}\text{= 2}\text{a}{}_{\mathrm{<mtext>hex</mtext>}}$

In Cu_0.75 VS₂, vanadium atoms occupy two independent sites, three vanadium atoms forming a triangular cluster (V₂—V₃ distances are 2.91 Å and V₃—V₃ are 2.92 Å) while one vanadium atom is isolated ($\text{V}{}_1\text{?}\text{V}{}_2\text{= 3.36}\text{A}\text{?}$ and $\text{V}{}_1\text{?}\text{V}{}_3\text{= 3.37}\text{A}\text{?}$. The physical properties exhibit a transition at 50°K approximately, the magnetic susceptibility being temperature-independent above and temperature dependent below the transition (Curie-Weiss behavior). Resistivity and Hall measurements confirm the metallic nature of the compound and show the existence of the low temperature transition. The observed properties could be interpreted as a result of the low temperature localisation of the 3d electron of V₁.     

Verres fluores au tetrafluorure de zirconium proprietes optiques d'un verre dope au Nd3+

Glass formation has been observed in the ZrF₄BaF₂NaF system around the composition 50 % ZrF₄, 25 % BaF₂, 25 % NaF. A Nd³⁺ doped glass obtained by dissolving NdF₃ in the previous material has been studied. Absorption spectra in the region ${}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, I.R. fluorescence spectra and lifetime measurement of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level show a good protection of Nd³⁺ against the fluorescence quenching.     

Synthese et caracterisation du pentafluorotellurate (IV) de sodium NaTeF5

Sodium pentafluorotellurate (IV) NaTeF₅ has been prepared by slow evaporation of a $\text{1}\text{1}$ mixture of sodium fluoride and tellurium dioxyde in 50 % aqueous hydrofluoric acid.NaTeF₅ has an orthorhombic structure, parameters of which are $\text{a}\text{= 9.10}{}_5\text{A}\text{?}$$\text{b}\text{= 11.30}{}_6\text{A}\text{?}$$\text{c}\text{= 10.12}\text{A}\text{?}$     

Macle et structure cristalline de Mn0,75Ga2,17S4

Twins of $\text{Mn}{}_{\mathrm{<mtext>1?</mtext><mtext>x</mtext>}}\text{Ga}{}_{\mathrm{<mtext>2+</mtext><mtext>2</mtext><mtext>3</mtext><mtext>x</mtext>}}\text{S}{}_4$ were used for crystal structure determination. Twinning is explained by a reticular pseudomerihedry. Axis, plane and obliquity of the twin have been determined. Cell dimensions are: $\text{a = b}\text{= 5.456(2)}\text{A}\text{?}$; $\text{c}\text{= 10.220(4)}\text{A}\text{?}$; α = β = γ = 90°; space group 14; Z = 2. The final R value is 0.059. The material is isostructural with CdGa₂S₄.     

Stability of the perovskite phase LaBO3 (B = V,Cr, Mn,Fe, Co,Ni) in reducing atmosphere I. Experimental results

The chemical stability of perovskites LaBO₃ where B = V, Cr, Mn, Fe, Co, Ni was studied by thermogravimetry at 1000°C in gas mixtures of $\text{CO}{}_2\text{H}{}_2$, $\text{O}{}_2\text{CO}{}_2$ and $\text{O}{}_2\text{Ar}$ at 1 bar.The stability limits of the perovskite phases expressed in terms of -log Po₂★ (Po₂★ = critical oxygen partial pressure in bar) were for LaCrO₃ and LaVO₃ (greater than 21.1), LaFeO₃ (16.95), LaMnO₃ (15.05), LaCoO₃ (7.0) and for LaNiO₃ (～0.6). The changes in standard enthalpy ΔH° and entropy ΔS° of the following reactions were obtained.$\text{LaVO}{}_4\text{=}\text{LaVO}{}_3\text{+}\text{1}\text{20}{}_2\text{δ=328}\text{kJ}\text{mol}\text{°=135}\text{J}\text{mol·deg.}$,$\text{LaMnO}{}_3\text{=}\text{1}\text{2}\text{La}{}_2\text{O}{}_3\text{+}\text{MnO}$     

Proprietes optiques et distribution cationique de la phase CaLa1-xGdxGao4α(0 ⩽ x ⩽ 1)

The luminescence of Gd³⁺ in CaLa_1?xGd_xGaO₄α, which is olivine type structure, is reported. Additional information on the crystal structure can be deduced from the emission and excitation spectra : the Ca²⁺ and Gd³⁺ ions are randomly distributed in the (4a) and (4c) sites. Furthermore the thermal variation of spectral energy distribution of the emission may be explained by a Gd³⁺ → Gd³⁺ energy transfer from (4c) to (4a) sites. The $\text{6}\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ and $\text{6}\text{P}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ energy levels are discussed on the base of crystal chemical considerations.     

Third-order bounds on the effective bulk and shear modulus of a dispersion of fully penetrable spheres

We evaluate the third-order Beran-Molyneux bounds on the effective bulk modulus K_e and the third-order McCoy bounds on the effective shear modulus $\text{μ}{}_{\mathrm{e}}$ of a model of a two-phase composite in which one of the phases consists of spherical inclusions (or voids), with bulk and shear moduli, K₂ and $\text{μ}{}_2$, respectively, and volume fraction $\text{φ}{}_2$, dispersed randomly throughout a matrix phase, with bulk and shear moduli, K₁ and $\text{μ}{}_1$, respectively, and volume fraction $\text{φ}{}_1$. We tabulate the two fundamental microstructural parameters I₁ and L₁ required to evaluate the bounds, which depend upon the three-point matrix probability function of the model, for the aforementioned fully-penetrablesphere model. We compare the third-order bounds on K_e and $\text{μ}{}_{\mathrm{e}}$ to the second-order bounds due to Hashin and Shtrikman and to Walpole. We find that the third-order bounds for our model are always more restrictive than the corresponding second-order bounds. When the moduli of the phases differ by an order of magnitude, the third-order bounds are sharp enough to provide quantitatively useful estimates of K_e and $\text{μ}{}_{\mathrm{e}}$ for all $\text{φ}{}_2$. The third-order bounds are very restrictive at low $\text{φ}{}_2$ values (e.g., $\text{φ}{}_2\text{= 0.1}$) where they remain useful for cases in which the moduli of the phases differ by two orders of magnitude. Experimental values of $\text{μ}{}_{\mathrm{e}}$ measured by Corson for a tungstenlead composite are found to lie within the McCoy bounds for our model, with the lower bound giving a good estimate of the data.     

Lithium organic electrolyte cells using the copper chevrel phase as cathode

The copper Chevrel phase, Cu_xMo₆S_8?z (x=0–4, z=0-0.2), was evaluated as the cathodes in lithium organic electrolyte cells. The cells $\text{Li}\text{Cu}{}_{\mathrm{x}}\text{Mo}{}_6\text{S}{}_{\mathrm{<mtext>8?</mtext><mtext>z</mtext>}}$ showed a good discharge performance for the charge transferred range of 0.5 to 3.5 $\text{electron(e)}\text{Cu}{}_{\mathrm{x}}\text{Mo}{}_6\text{S}{}_{\mathrm{<mtext>8?</mtext><mtext>z</mtext>}}$ under a high current density such as 3mA/cm². The charge and discharge cycle tests of the $\text{Li}\text{Cu}{}_2\text{Mo}{}_6\text{S}{}_{\mathrm{7·8}}$ cell indicated a good rechargeability for the range of 0.5 to $\text{2.5}\text{e}\text{Cu}{}_{\mathrm{x}}\text{Mo}{}_6\text{S}{}_{\mathrm{<mtext>8?</mtext><mtext>z</mtext>}}$. The electrode reaction mechanism was discussed with the help of x-ray diffraction analysis of the cathode reaction products.     

Growth of Ca3Fe2Ge3O12 and Ca3Fe(Al,Cr) Ge3O12 garnets in molten bismuth germanate glasses

Additions of Fe₂O₃ to CaO·Bi₂O₃·2 GeO₂ cause Ca₃Fe₂Ge₃O₁₂ garnets to precipitate from the resultant melt at 1250°C. Garnets with the composition Ca₃Fe(Al, Cr) Ge₃O₁₂ are also precipitated by adding either $\text{Al}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ or $\text{Cr}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ mixtures. The well-formed crystals range from several to 100 μm in size and are obtained in 50 to 70% yields at $\text{Fe}\text{Bi}\text{= 0.4}$. Additions of Fe₂O₃ (up to $\text{Fe}\text{Bi}\text{= 1.0}$) to compositions containing ZnO, CdO, SrO, and BaO yield only dark glasses. The physical properties of these glasses suggest that Fe(III), in contrast to AL(III) & Ga(III), prefers octahedral coordination.     

Sur un nouveau materiau pour l'enregistrement magnetique derive du dioxyde de chrome

A new ferromagnetic ternary oxide of formula Cr_1?xIr_xO₂ (0 < x < 0.3) has been prepared and ajusted. Thanks to iron doping a high coercive field ($\text{H}{}_{\mathrm{<mtext>c</mtext>}}\text{? 600}\text{Oe}$) can be obtained in appropriate conditions for an irridium content as low as $\text{x}\text{? 5.10}{}^{\mathrm{?4}}$. Such a material is promising for magnetic recording.     

Models for the effect of chemisorbed gases on the electrical resistance of metal films

Three hypotheses are considered for the proportionate change of resistance $\text{ΔR}\text{R}{}_1$ when a continuous metal film of resistance R₁ adsorbs gas: (a) a loss of metallic properties at the surface; (b) a change in the electron scattering parameter at the surface; and (c) a change in the electron concentration throughout the film. Two models of the film structure are used to compare the predictions of these hypothesis with our experimental data for the systems $\text{O}{}_2\text{Er}$, $\text{CO}\text{Er}$, $\text{CO}\text{Ti}$ and $\text{O}{}_2\text{Ti}$. The flat plate model accounts for neither R₁ nor $\text{ΔR}\text{R}{}_1$. A simple version of the columnar model accounts for $\text{ΔR}\text{R}{}_1$ by either (a) or (b) for the first three systems and by (c) for the fourth, but it does not explain all the properties of the clean films. The linear approximation given by Mola and Heras for the columnar model of Mayadas and Shatzkes accounts satisfactorily for R₁ and $\text{ΔR}\text{R}{}_1$ for $\text{O}{}_2\text{Er}$, $\text{CO}\text{Er}$ and $\text{CO}\text{Ti}$ by increases in the electron scattering coefficient at the grain boundaries. It does not account for $\text{ΔR}\text{R}{}_1$ with $\text{O}{}_2\text{Ti}$ because that system incorporates gas into the metal lattice.