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.     

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.     

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.     

Properties of bone at low temperatures and its potential as cryostat support material

The ultimate compressive strength, $\text{σ}{}_{\mathrm{<mtext>f</mtext>}}$, Young's modulus, E, and the integrated thermal conductivity k_T1–T2, of bone have been measured between 20 and 80 K. The two figures of merit indicating the quality for transmission of forces to low temperature apparatus are determined as $\text{σ}{}_{\mathrm{<mtext>f</mtext>}}\text{/}\text{k}{}_{\mathrm{4–77}}\text{= 42.5 ± 4 Ms m}{}^{\mathrm{?2}}$, and E/k_4–77 = 3.5 ± 0.2 Gs m^?2. According to these figures bone is comparable or superior to the best glass-fibre composites. Some observations on creep strength are added.     

Concentration quenching of Nd3+ fluorescence in ordered perovskite oxides and fluorides

New ordered perovskite-type fluorides, Rb₂KNdF₆ and Cs₂KNdF₆, have been synthesized and their luminescence properties investigated. Their neodymium emission shows weak self-quenching. This results both from the isolation of the neodymium in discrete NdF₆ octahedra and from the weak absorption associated with the ${}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}$ transitions in the region of the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}$ emission, which restricts cross-relaxation via the intermediate ${}^4\text{I}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}$ levels. In contrast the emission of neodymium, substituted for lanthanum in the homologous Ba₂LaNbO₆ oxide, shows a strong concentration self-quenching.     

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.     

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.     

Magnetic resonance of intercalation compounds of graphite: Questions of ionicity and mobility of inserted species

Graphite will react at room temperature with Lewis acids as PF₅ and BF₃ in the presence of the oxidant C1F to form intercalation compounds containing closed-shell anions. In the case of “C₁₆BF₄”, the chemical shifts of both ¹¹B and ¹⁹F nuclear magnetic resonances point to the existence of BF^?₄, rather than the initial BF₃, within the graphite planes. The existence of second order quadrupolar coupling of the ¹¹B resonance suggests, however, possible hybrid $\text{BF}{}_3\text{BF}{}^{\mathrm{?}}{}_4$ character, as in B₂F^?₇, a known dimeric anion of BF₄ and BF₃. NMR results on ¹⁹F and ³¹P in the two compounds “C₁₄PF₆” and “C₂₈PF₆” support this hypothesis, as “C₂₈PF₆” shows only the presence of PF^?₆, but the more concentrated “C₁₄PF₆” shows composite $\text{PF}{}_5\text{PF}{}^{\mathrm{?}}{}_6$ character. Our claim for intercalated anions in these systems is reinforced both by radical cation-type signals in the ESR and by deshielding effects in the ¹³C NMR. The narrow linewidths of the nuclear magnetic resonance absorptions of the intercalated species are suggestive of “liquid-like” behavior.     

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^?.     

Structure determination of (3D) P2NbS8

Tridimensional P₂NbS₈ crystallizes in P4?n2 tetragonal space group, with $\text{a}\text{= 12.0483(4)}\text{A}\text{?}$, $\text{c}\text{= 7.2070(5)}\text{A}\text{?}$, $\text{V}\text{= 1046.2(1)}\text{A}\text{?}{}^3$ and Z = 4. The structure was anisotropically refined down to R = 2.3% from 468 reflexions and 53 variables. It is built from [Nb₂S₁₂] biprismatic bicapped units (average $\text{d}{}_{\mathrm{Nb?S}}\text{= 2.571}\text{A}\text{?}$) made of S^?II and S^?II₂ anions (dianionic distance of 2.014(3) Å). The niobium atoms are found as isolated Nb^IV ? Nb^IV pairs ($\text{d}{}_{\mathrm{Nb?Nb}}\text{= 2.859(1)}\text{A}\text{?}$) in these niobium group otherwise linked to each other through (PS₄) tetrahedral units (average $\text{d}{}_{\mathrm{P?S}}\text{= 2.051}\text{A}\text{?}$) themselves constituting interbonded [P₄S₁₂] rings. The P₂NbS₈ three-dimensional network thus obtained is compared to the (2D) P₂NbS₈, layered phase already described.     

Copper doped zinc oxide Y2BaZnO5: ERS and optical investigation

In copper doped Y₂BaZnO₅ oxides, copper exhibits a distorted square pyramidal coordination which is consistant with the values of g and A tensors obtained from O band ERS spectrum for a sample containing about 1 % Cu. Three values for g and A are observed, g₁ = 2.049₅, g₂ = 2.051₅, g₃ = 2.275, $\text{¦}\text{A}{}_1\text{¦ = 13 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$, $\text{¦}\text{A}{}_2\text{¦ = 10 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$ and $\text{¦}\text{A}{}_3\text{¦ = 147.5 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$. Since $\text{g}{}_1\text{?}\text{g}{}_2$ an approximate C_4v point symmetry can be assumed for copper. The electronic spectrum shows three bands at 11700, 14500 and 20500 cm^?1 which can be assigned to the transitions A₁ → B₁, B₂ → B₁ and E → B₁ respectively. The orbital reduction parameters are calculated and the bonding covalency is discussed.     

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.     

Sur l'existence des oxydes rhomboe driques A(III) B(II) B′(III)O4

The symmetry group of the rhombohedral structure of the mixed oxides A(III)B(II)B′(III)O₄ is R3&#x0304;m: A is in an octahedral coordination, B and B′ occupy the same crystallographic sites and are in a pyramidal coordination with a triangular basis.The existence of this structural type with Ni²⁺ and Cr³⁺ ions is incompatible with this coordination close to tetrahedral.The geometric conditions necessary for adoption of the rhombohedral structure have been clearly defined by comparing the cation-oxygen and oxygen-oxygen distances determined on 4 monocrystals (InCuAlO₄, InFe₁, ₇₅Si_O, ₂₅O₄, YbFe₂O₄ and Yb_O,₅Eu_O,₅Fe₂O₄).The shortest cation-oxygen distance in the pyramidal site is designated by d′: it can be calculated through the relation: by using the ionic radii of ions in tetrahedral coordination, given by R.D. Shannon.The oxygen - A³⁺ cation distance of the octahedron, d, is close to the sum of the ionic radii of the ions in coordinence 6.Mixed oxides ABB′O₄ can crystallize in the rhombohedral system if the ratio d′/d is between 0.842 and 0.888 and these two limiting values can be refined for each couple B, B′.     

Rate law versus oxygen pressure in spinel oxidation to γ lacunar spinels

In the controlled oxidation of spinels (Fe²⁺Fe³⁺_2?xM³⁺_x)O^2?₄ (M³⁺=Al³⁺, Cr³⁺ ; 0 < x < 2) and (Fe²⁺Al³⁺_2?xCr³⁺_x)O^2?₄ to the metastable phases γ(Fe³⁺_1?yM³⁺_y)₂O^2?₃ and $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{O}{}^{\mathrm{2?}}{}_{\mathrm{3?}}\text{(0 <}\text{y}\text{<}\text{2}\text{3}\text{)}$, over the temperature range 200–450°C, the rate law is usually v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{2}$. This law is well interpreted if we consider a total association vacancy-positive charge. However, for low oxidation extents a (a < 0.3) the law written as is in better agreement with a partial association of defects. For a total dissociation of defects exponent n of the pressure law is always smaller.     

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).     

Fracture of thin sections containing surface cracks

Surface-cracked specimens of several thicknesses of 7075-T651 and 7075-T6 aluminum were tested in uniaxial tension. For thicknesses t less than 0.25 in., the gross fracture stress σ_f of 7075-T651 Al was empirically related to flaw size by the following expression:

$\text{δ}{}_{\mathrm{f}}\text{σ}{}_{\mathrm{ult}}\text{= 1 + S(}\text{a}\text{φ}{}^2\text{.t}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}$

where σ_ult is the ultimate strength, a the crack depth, φ a function of crack shape, and S a proportionality constant equal to ?1.7 in.^{$\text{?1}\text{2}$}. For 0.25-in. thick 7075-T651 aluminum, σ_f was found to obey this relationship only when $\text{a}\text{φ}{}^2$ is less than 0.065 in.; for larger flaws, such that $\text{0.065 <}\text{a}\text{φ}{}^2\text{< 0.11, σ}{}_{\mathrm{f}}$ is better predicted by Irwin's surface-crack equation with an apparent K_IC value of $\text{32.2}\text{ksi-in.}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$.Fracture data for thin sections of 2014-T6 and 2014-T651 Al tested at ?423°F are analyzed in terms of the empirical relationship above and are found to be in good agreement. For these alloys, S has a value of ?2.6 in.^{$\text{?1}\text{2}$}.Applicability of the empirical relationship and Irwin's surface-crack analysis to the fracture of thin sections is discussed in terms of crack size, section thickness, and plastic zone size.     

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.     

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.     

Carburization and decarburization kinetics of iron in CH4 — H2 mixtures between 1000 – 1100 °C

In this study the rate constants of the methane decomposition reaction on iron surfaces were determined in the 1000–1100°C temperature range, by grav? metric methods. Earlier works showed that the reaction velocity was given by The results indicate that the constant values vary from 2.72 × 10^?6 to $\text{16.74 × 10}{}^{\mathrm{?6}}\text{mol C/cm}{}^2\text{/sec/atm}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for k and 2.61 × 10^?8 to $\text{8.62 × 10}{}^{\mathrm{?8}}\text{mol C/cm}{}^2\text{/sec/atm}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ for k′ between 1000 and 1100°C.