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.     

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.     

Un nouveau fluorure de platine : PtIIPtIVF6

From a temperature dependent study of the platinum-flourine system, a new fluoride Pt^IIPt^IVF₆ has been isolated, which contains platinum in an oxidation state lower than that found in the already known platinum fluorides PtF₄, PtF₅ and PtF₆. The material has been characterized by various analytical methods. The fluoride Pt^IIPt^IVF₆ crystallizes in the rhombohedral LiSbF₆ - type structure (R3) with the lattice parameters $\text{a}\text{= 5.56}{}_5\text{A}\text{?}$, α = 53.85°. Magnetic studies on polycristalline samples have shown that Pt^IIPt^IVF₆ orders ferromagnetically below 16 K. This is the first example of platinum (+II) in a fluorine environment and with a high spin structure in an octahedral surrounding.     

New phase with K2NiF4 structure in the LaLiFeO system

The existence has been proved, for the first time, in the LaLiFeO system of the phase (La_1.75Li_.25) (Fe_.5Li_.5)O_4?x with tetragonal symmetry ($\text{a}{}_{\mathrm{o}}\text{=3.765±0.001}\text{A}\text{?}$; $\text{c}{}_{\mathrm{o}}\text{=12.918±0.01}\text{A}\text{?}$; $\text{c}{}_{\mathrm{o}}\text{a}{}_{\mathrm{o}}\text{=3.43}$) and K₂NiF₄-type structure.This phase gives probably the first example of lithium distribution in both A and B sites of A₂BO₄-type mixed oxides. The high $\text{c}{}_{\mathrm{o}}\text{a}{}_{\mathrm{o}}$ ratio suggests a strong elongation of FeO₆ octahedra induced by a high spin Fe(IV) d⁴ configuration. The results of structural studies and of Mössbauer analysis, confirming the above conclusions, are reported.     

Sur une variete haute pression de PdF2 de type fluorine

A fluorite-type form of PdF₂ has been obtained from the rutile-type variety at 25°C under 50 kb and quenched to room conditions. It is the first d-element fluoride presenting this structure so far isolated under normal conditions. The transition corresponds to a volume decrease $\text{δ}\text{V}\text{V}{}_{\mathrm{rutile}}$ of 11 %. The cell parameter $\text{a}\text{= 5.322 ± 0.002}\text{A}\text{?}$ supposes a PdF distance of 2.16 Å. The thermal variation of the magnetic susceptibility suggests an antiferromagnetic behavior with an ordering temperature close to 200 K. The rutile → fluorite transition has been followed by resistivity vs. pressure measurements. Fluorite-type PdF₂ presents at room conditions a resistivity of 10⁵Ω.cm, 10⁵ lower than the rutile form.     

Crystal structures and crystal chemistry in the system Na1+xZr2SixP3−xO12

As part of a search for skeleton structures for fast alkali-ion transport, the system Na_1+xZr₂Si_xP_3?xO₁₂ has been prepared, analyzed structurally and ion exchanged reversibly with Li⁺, Ag⁺, and K⁺ ions. Single-crystal x-ray analysis was used to identify the composition NaZr₂P₃O₁₂ and to refine its structure, which has rhombohedral space group R3?c with cell parameters $\text{a}{}_{\mathrm{<mtext>r</mtext>}}\text{= 8.815(1)}\text{A}\text{?}$ and $\text{c}{}_{\mathrm{<mtext>r</mtext>}}\text{= 22.746(7)}\text{A}\text{?}$. A small distortion to monoclinic symmetry occurs in the interval 1.8 ≤x≤ 2.2. The structure for Na₃Zr₂Si₂PO₁₂, proposed from powder data, has space group $\text{C}\text{2}\text{c}$ with $\text{a}{}_{\mathrm{<mtext>m</mtext>}}\text{= 15.586(9)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{<mtext>m</mtext>}}\text{= 9.029(4)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{<mtext>m</mtext>}}\text{= 9.205(5)}\text{A}\text{?}$, and β = 123.70(5)° Both structures contain a rigid, three-dimensional network of PO₄ or (SiO₄) tetrahedra sharing corners with ZrO₆ octahedra and a three-dimensionally linked interstitial space. Of the two distinguishable alkali-ion sites in the rhombohedral structure, one is completely occupied in both end members, the occupancy of the other varies across the system from 0 to 100 percent. Several properties are compared with the fast Na⁺-ion conductor β-alumina.     

Refinement of the structure of K2Cr2O7

The parameters for the structure of K₂Cr₂O₇ have been refined from 7511 observed reflections; $\text{a}{}_{\mathrm{o}}\text{= 7.4200(6)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 13.399(3)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 7.3845(9)}\text{A}\text{?}$, cosα = ?0.1396(2) (α = 98°2'), cosβ = ?0.0154(2) (β = 90°53'), cosγ = ?0.1078(2) (γ = 96°11'). The discrepancy factor R(F_o²) = 0.0702 with a type 2 extinction correction. The average CrO distances are 1.609Å (unshared) and 1.783Å (shared).     

Strontium metaniobate: Its crystallography,polymorphism, and isomerism

Three polymorphic forms of SrNb₂O₆ are recognized. The monoclinic form has been prepared and studied by x-ray powder diffraction techniques and differential thermal analysis. The results of this study show that SrNb₂O₆ crystallizes in a pseudo-orthorhombic unit cell with $\text{a}{}_{\mathrm{o}}\text{= 10.99}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 7.70}\text{A}\text{?}$ and $\text{c}{}_{\mathrm{o}}\text{= 5.66}\text{A}\text{?}$. Its crystal chemical relations to the other meta-niobates and its thermodynamic stability are discussed.     

Correlation of the thermal expansion coefficients of rare earth and transition metal oxides and fluorides

The product of the average linear thermal expansion coefficient (α_?) times melting point in °C (M_p) tends to be $\text{α}{}_{\mathrm{?}}\text{M}{}_{\mathrm{p}}\text{? 0.020}$ for the oxides of the 4f-rare earths and the comparable fluorides that have the tysonite structure. Correlation of rutile-like structures are based on deviations from α_?M_p = 0.027 caused by the cations moving the anions into sites with planar symmetry. The deviation tends to be proportional to the average cation to nearest neighbor anion distance. Additional effects are also discussed.     

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{?}$     

On new K2LiMF6 phases (M = d- element,Al, Ga,In): Crystal structure of the rhombohedral high-temperature from of K2LiAlF6

The high-temperature form of K₂LiAlF₆ crystallizes in the rhombohedral R3?m space group. This result differs from that previously obtained by Winkler. The lattice constants are $\text{a}\text{= 5.62 ± 0.01}\text{A}\text{?}$, $\text{c}\text{= 27.62 ± 0.01}\text{A}\text{?}$ (hexagonal cell). The structure is of 12R-type (Cs₂NaCrF₆-prototype); it is characterized by units of three octahedra linked by faces and connected to each other by unique AlF₆ octahedra sharing corners only. Within a trimeric group, Al is located in the central site and Li in the two external ones. The lowtemperature phase ($\text{T}{}_{\mathrm{<mtext>trans</mtext><mtext>.</mtext>}}\text{?650°}\text{C}$) is hexagonal (6H-type). Due to tolerance factors smaller than 1, the other K₂LiM^IIIF₆ phases (M = d-element, Ga, In) show a cubic elpasolite structure. d-transition elements with unusual oxidation state (Co (+III), Cu (+III), Pd (+III)) have been stabilized in this structure and their magnetic behavior has been investigated.     

The onset of fracture at the tip of a three-dimensional crack in a solid subjected to bending loads

The condition for the extension of a 3-dimensional crack in a solid subjected to bending loads are examined for the case where the ligament has fully yielded. The main conclusion, for this 3-dimensional problem, is that caution should be exercised when applying the well-known J integral relation:

$\text{J=}\text{?M}{}_{\mathrm{p}}\text{?A}\text{θ}{}_{\mathrm{p}}$

where M_p is the plastic limit moment and φ_p is the plastic rotation.     

Ordering of interstitial nitrogen and oxygen in vanadium near to V8N and V8O

The ordered structure of the V₈N subnitride was studied by X-rays, electron diffraction and electron microscopy. V₈N exists in two different modifications (α′ and α″). The vanadium sublattice of both phases is pseudo-tetragonal, in reality triclinic, with lattice parameters: $\text{α}{}_{\mathrm{<mtext>o</mtext>}}\text{=}\text{b}{}_{\mathrm{o}}\text{= 3.114}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 2.994}\text{A}\text{?}$, α_o = β_o = 90.5^o ± 0.1^o, γ_o = 90^o. The proposed unit cell of α′-V₈N has dimensions: a = 2√2 a_o, c = 2c_o and corresponds to V₃₂N₄. Doublets of nitrogen atoms occupy two sets of octahedral cavities whose shortest axes are aligned along the x and y directions of the sublattice in an ordered fashion. The α″-V₈N phase is a periodically twinned modification of the α′-V₈N, the twin plane being of the (001) type. The V₈O suboxide has the same structure as the α″-V₈N, the sublattice parameters being: $\text{a}{}_{\mathrm{o}}\text{=}\text{b}{}_{\mathrm{o}}\text{= 3.11}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 2.994}\text{A}\text{?}$, α_o = β_o = 90.3^o ± 0.1^o, γ_o = 90^o.     

Temperature dependence of the surface diffusion distance of bismuth atoms adsorbed on mica,carbon and silicon monoxide surfaces

The mean distance of surface diffusion of bismuth adatoms on mica, carbon and silicon monoxide surfaces has been determined at different temperatures by measurement of the instantaneous sticking coefficient and the nucleus density.The surface diffusion distance has been found to increase with decreasing temperature in accordance with the formula

$\text{X=}\text{1}\text{2}\text{a}{}_0\text{v}{}_{\mathrm{0d}}\text{v}{}_{\mathrm{0a}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{exp}\text{E}{}_{\mathrm{a}}\text{? E}{}_{\mathrm{d}}\text{2RT}$

at temperatures above 413, 373 and 383 K for mica, carbon and silicon monoxide respectively. Here X is one-half of the diffusion distance, E_a is the adsorption energy, E_d the activation energy for surface diffusion, a₀ the diffusion jump distance and v_0a and v_0d the vibrational frequencies associated with re-evaporation and with surface diffusion respectively. Below these temperatures it has been found that the temperature dependence of the diffusion distance deviates from the above formula; this can be explained by the presence of residual gas molecules adsorbed on the surfaces.From the temperature dependence of the diffusion distance, the respective values of the pre-exponential term $\text{a}{}_0\text{(}\text{v}{}_{\mathrm{0d}}\text{v}{}_{\mathrm{0a}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and the difference of energies E_a?E_d have been estimated as 7.6 Å and 5.8 kcal mol^?1 for mica, 17 Å and 3.2 kcal mol^?1 for carbon and 58 Å and 1.3 kcal mol^?1 for silicon monoxide.     

Syntheses and magnetic properties of Eu3B2O6 and Sr3B2O6

Europium orthoborate and strontium orthoborate crystallize in the rhombohedral system with two formula units in a cell of dimensions $\text{a}{}_{\mathrm{R}}\text{=6.697}\text{A}\text{?}$, α_R=85.17° for Eu₃B₂O₆, and $\text{a}{}_{\mathrm{R}}\text{=6.695}\text{A}\text{?}$, α_R=85.00° for Sr₃B₂O₆. The equivalent hexagonal lattice parameters are $\text{a}{}_{\mathrm{H}}\text{=9.069}\text{A}\text{?}$, $\text{c}{}_{\mathrm{H}}\text{=12.542}\text{A}\text{?}$, and $\text{a}{}_{\mathrm{H}}\text{=9.046}\text{A}\text{?}$, $\text{c}{}_{\mathrm{H}}\text{=12.566}\text{A}\text{?}$ respectively. Eu₃B₂O₆ appears to be ferromagnetic below 7.5K.     

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).     

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.     

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.     

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^?.     

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.