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.     

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.     

A series of rare earth silicates RE3+2M3+[SiO4]2 (OH)

A series of isotypic silicates of composition RE₂M[SiO₄]₂ (OH) with RE = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, and M = Al³⁺, Fe³⁺ has been synthesized under hydrothermal conditions. Lattice constants of two members as determined from single crystal X-ray diffraction data are: La₂Al[SiO₄]₂ (OH) (La₂Fe[SiO₄]₂ (OH)) $\text{a}{}_{\mathrm{o}}\text{= 7.401 (7.346)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 5.702 (5.862)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 17.072 (97.196)}\text{A}\text{?}$, gb = 112.4 (112.5°), $\text{P}\text{2}{}_1\text{c}$, Z=4.     

Influence of local-site symmetry on fluorescence lifetime in high-Nd-concentration laser materials

We have correlated the radiative fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ level of Nd³⁺ in laser crystals, including the recently developed high-Nd-concentration materials such as NdP₅O₁₄, with the local Nd³⁺ site symmetry. At Nd³⁺ concentrations low enough for non-radiative interactions to be negligible, the fluorescence decay rate is determined by the deviation from local inversion symmetry, which admixes even- and odd-parity electronic wavefunctions, allowing ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ radiative decay by electric-dipole transitions.     

Electronic and spin configurations of Co3+ and Ni3+ ions in oxides of K2NiF4 structure: A magnetic susceptibility study

In La₄LiCoO₈, Li⁺ and Co³⁺ ions are ordered in two dimensions and Co³⁺ ions undergo transitions from the low-spin to the intermediate as well as the high-spin states. Both Sr₄TaCoO₈ and Sr₄NbCoO₈ exhibit low to intermediate-spin state transitions of Co³⁺ ions. In the system LaSr_1?xBa_xNiO₄, the e_g electrons are essentially in extended states forming a band. With increase in x, the band width decreases accompanying an increase in unit cell volume; high-spin Ni³⁺ ions are formed to a small extent with increasing x, but there is no spin-state transition. In LaSrAl_1?xNi_xO₄, at small x, there is a small proportion of high-spin Ni³⁺; when x ≈ 0.6, there is an abrupt decrease in the $\text{c}$/$\text{a}$ ratio, signalling the formation of the band. In LnSrNiO₄, the $\text{c}$/$\text{a}$ ratio decreases sharply between Ln = La and Nd; this is likely to be accompanied by a broadening of the band.     

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).     

Synthesis and electrical properties of Ln2(V43W23)O7 pyrochlores

Mixed oxides of the type $\text{Ln}{}_2\text{(V}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{W}{}^{\mathrm{6+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{)O}{}_7$, Ln=GdLu, Y have been synthesized in vacuum via solid state reactions of the constituent oxides at 1150–1300°C. The compounds, which crystallize in cubic pyrochlore structure ($\text{a ～ 10.2}\text{A}\text{?}$), have been characterized by X-ray, density and wet chemical analysis. Electrical resistivity and Seebeck coefficient measurements show the phases to be p-type extrinsic semiconductors with low energies of activation possibly exhibiting hopping-type behavior.     

Crystal structure and fluorescence lifetime of a laser material NdNa5(WO4)4

The structure of NdNa₅(WO₄)₄, determined by single-crystal x-ray analysis, is tetragonal with space group I 4₁/a and cell parameters $\text{a}\text{= 11.559(2)}\text{A}\text{?}$, $\text{b}\text{= 11.453(2)}\text{A}\text{?}$, z = 4. A full-matrix, least-squares refinement gives R = 0.077. The W atoms and the Nd atoms occupy isolated tetrahedra and dodecahedra, respectively. These polyhedra are connected by the Na atoms, which are located at two different sites, tetrahedral and octahedral. The fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ transition of Nd⁺³ is reported for the system La_1?xNd_xNa₅(WO₄)₄. The lifetime of NdNa₅(WO₄)₄ is 85 ± 5 μsec, which is comparable to NdP₅O₁₄. Concentration quenching is considerably reduced in both host structures, which we conclude is due to the isolation of the rare-earth polyhedra.     

Crystal structure and fluorescence lifetime of NdAl3(BO3)4, a promising laser material

The structure of NdAl₃(BO₃)₄, determined by single-crystal x-ray analysis, is rhombohedral with space group R32 and cell parameters $\text{a}\text{= 9.3416 (6)}\text{A}\text{?}$, $\text{c}\text{= 7.3066 (8)}\text{A}\text{?}$, Z = 3. A full-matrix, least-squares refinement gives R = 0.033 and R_w = 0.037. The Nd atoms, Al atoms and B atoms occupy trigonal prisms, octahedra, and triangles of oxygen, respectively. Edge-shared Al octahedra form helices along the c-axis. These helices are connected by isolated B triangles and isolated Nd trigonal prisms. The fluorescence lifetime of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ transition of Nd⁺³ is reported for the system Nd_xGd_1?xAl₃(BO₃)₄. The lifetime of NdAl₃(BO₃)₄ is 19us, and concentration quenching is reduced in the series, as happens in NdNa₅(WO₄)₄ (85μs) and NdP₅O₁₄ (115μs). The shorter lifetime is attributed to noncentrosymmetry; the reduced concentration quenching to isolation of Nd polyhedra.     

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.     

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.     

Concentration quenching of Tb3+ luminescence in LaOBr and Gd2O2S phosphors

The concentration quenching of trivalent terbium ${}^5\text{D}{}_{\mathrm{3,4}}\text{→}{}^7\text{F}{}_{\mathrm{J}}$ emissions from UV-excited (La, Tb) OBr and (Gd, Tb)₂O₂S phosphors was studied. The activation concentration x was varied from 5·10^?5 to 0.2 for (La_1?xTb_x) OBr and from 10^?3 to 0.1 for (Gd_1?xTb_x)₂O₂S. ${}^5\text{D}{}_3\text{→}{}^7\text{F}{}_{\mathrm{J}}$ emissions (blue) were observed to quench first and the Tb³⁺ concentration giving rise to maximum intensity was 0.003 in (La, Tb) OBr and between 0.005 and 0.01 in (Gd, Tb)₂O₂S. The optimum concentration for ${}^5\text{D}{}_4\text{→}{}^7\text{F}{}_{\mathrm{J}}$ (green) emissions was 0.05 in (La, Tb) OBr and 0.03 in (Gd, Tb)₂O₂S. Dipole-dipole and dipole-quadrupole interactions are possible mechanisms for the quenching of emissions from the ⁵D₃ and ⁵D₄ levels.A method for determining the Tb³⁺ concentration in these phosphors, based on the intensity ratios of the ${}^5\text{D}{}_3\text{→}{}^7\text{F}{}_{\mathrm{J}}$ and ${}^5\text{D}{}_4\text{→}{}^7\text{F}{}_{\mathrm{J}}$ transitions, is also presented.     

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.     

Distribution coefficients in (YSmLuCa)3(FeGe)5O12 film growth

The composition of bubble garnet films has been analyzed by Inductively Coupled Plasma Emission Spectroscopy (ICP) to determine the distribution coefficients for different growth conditions. Under typical growth conditions, the distribution coefficients, k, of each element are as follows: k^Y = 2.15, k^Sm = 1.56, k^Lu = 1.32, k^Ca = 0.45, k^Fe = 0.98, k^Ge = 1.10. As the supercooling temperature (growth rate) increases, k^Ca, which is the smallest and deviates most from 1.0, changes in the direction approaching 1.0. For charge compensation, k^Ge also increases, consequently, k^Fe must decrease. Also k^Y, which is the largest, $\text{k}{}^{\mathrm{Sm}}\text{k}{}^{\mathrm{Y}}$ and $\text{k}{}^{\mathrm{Lu}}\text{k}{}^{\mathrm{Y}}$ change in the direction approaching 1.0. On the other hand, as the melt parameter R₁ (≡ $\text{Fe}{}_2\text{O}{}_3\text{Σ}\text{Ln}{}_2\text{O}{}_3$) increases, k^Fe decreases, and k^Y, k^Sm and k^Lu increase, whereas $\text{k}{}^{\mathrm{Sm}}\text{k}{}^{\mathrm{Y}}$ and $\text{k}{}^{\mathrm{Lu}}\text{k}{}^{\mathrm{Y}}$ remain constant at 0.73 and 0.61, respectively.     

Conductivite electrique et spectres de diffusion raman des verres mixtes AgPO3 − MI2 AVEC M = Cd,Hg, Pb. Correlation entre conductivite et structure

The glass-forming regions in the AgPO₃ ? MI₂ systems with M = Cd,Pb,Hg were determined. Electrical conductivity measurements and Raman spectra were carried out. A maximum conductivity value of $\text{10}{}^{\mathrm{?2}}\text{(Ω}\text{cm}\text{)}{}^{\mathrm{?1}}$ at 25°C is obtained for a mole fraction of 0,19 in PbI₂ or in CdI₂, whereas a value of $\text{3×10}{}^{\mathrm{?5}}\text{(Ω}\text{cm}\text{)}{}^{\mathrm{?1}}$ at 25°C is found for a mole fraction of 0,5 in HgI₂. The conductivity results and Raman spectra are examined and compared with those of AgPO₃ ? AgI. An exchange between Ag⁺ and M²⁺ ions is proposed leading to AgI species in AgPO₃ ? CdI₂ and AgPO₃ ? PbI₂ glasses. It could explain the high conductivity values obtained and the similarities observed in Raman spectra.     

L'auto-extinction de Nd3+ : Son mecanisme fondamental et un critere predictif simple pour les materiaux minilaser

Crystal field is demonstrated to play an important role in the self quenching process of neodymium ⁴F_3/2 level. It arises essentially from the compensation by the crystal field of the difference in energy gravity center differences for states F_3/2, ⁴I_15/2, ⁴I_9/2 of the Nd³⁺ free ion. Using a new parameter Nv derived from the usual B^k_q parameters as an ordering parameter, a limit to the easily available ground state splitting is given : $\text{Δ}\text{E}\text{(}{}^4\text{I}{}_{\mathrm{9/2}}\text{) < 470}\text{cm}{}^{\mathrm{?1}}$. This is equivalent to Nv < 1800 cm^?1. In this instance melting or decomposition temperature is also considered : t_m,d < 1200°C.     

Contribution a l'etude du polymorphisme de Ga2S3: Structure cristalline de Mn0,23Ga1,85S3

The Mn_0.23Ga_1.85S₃ phase belongs to the solid solution $\text{ф}{}_0$, stable at low temperature in the Ga₂S₃MnS system. It is hexagonal superstructure of the wurtzite, with the Ga₂S₃α′ type ($\text{a}\text{= 6.397}\text{A}\text{?}$; $\text{c}\text{= 18.027}\text{A}\text{?}$Z = 6; space groupe P6₁ or P6₅). Its crystal structure has been refined by the least squares method to a final R = 0.06 with 323 independant reflections. This structure is closely related to Ga₂S₃ α described by Hahn and Frank, and differs only by the partial occupation of the vacant metal site of Ga₂S₃ by Mn atoms in statistical disorder.     

Vaporization reactions of manganese gallium sulfide

The vaporization of manganese gallium sulfide was investigated in the temperature range 500–975°. Manganese gallium sulfide was found to vaporize incongruently by the reaction:

$\text{3}\text{MnGa}{}^2\text{S}{}^4\text{(s) =}\text{Mn}{}^3\text{Ga}{}^2\text{S}{}^6\text{(s) + 2}\text{Ga}{}^2\text{S}\text{(g) + 2}\text{S}{}^2\text{(g)}$

and

$\text{Mn}{}^3\text{Ga}{}^2\text{S}{}^6\text{(s) = 3}\text{MnS(s)}\text{+}\text{Ga}{}^2\text{S}\text{(g) +}\text{S}{}^2\text{(g)}$

Approximate vapor pressures were measured by the Knudsen effusion technique. Lattice parameters, density, and chemical analysis of the new ternary compound Mn₃Ga₂S₆ are reported.     

The synthesis of the first stage graphite salt C8+AsF6− and its relationship to the first stage graphite/AsF5 intercalate

Oxidation of graphite with excess O₂⁺AsF₆^?, in suspension in SO₂C1F, produces the blue first-stage graphite salt of composition C₈AsF₆, which X-ray single crystal photographs show is hexagonal with a = 4.92(2), c = 8.10(2), $\text{V}\text{= 170}\text{A}\text{?}{}^3$. The blue first-stage material of approximate composition C₁₀AsF₅ obtained from graphite and AsF₅ has a related pseudo cell. Arsenic X-ray absorption-edge spectra show that C₈AsF₆ contains AsF₆^? alone, and that the graphite/AsF₅ intercalte contains AsF₆^? and AsF₃ in accord with the AsF₅ reduction:

$\text{3 AsF}{}_5\text{+ 2 e}{}^{\mathrm{?}}\text{→ 2 AsF}{}_6{}^{\mathrm{?}}\text{+ AsF}{}_3$

Treatment of the graphite/AsF₅ compound with F₂ gas results in conversion of all of the intercalated arsenic to AsF₆^?.