Les disilicates de terres rares Ln2Si2O7: Precisions sur leur nouvelle forme cristalline X et leur polymorphisme sous pression

The new X form of rare earth disilicates Ln₂Si₂O₇ (Ln=Lu, Yb, Tm) is described by means of X-ray powder data. The X-ray indexation is made using single microcristal electronic diffraction. The investigation of structural relation between Ln₂Si₂O₇ and M₂A₂O₇ compounds as also the X-ray powder data of X-Ln₂Si₂O₇ show that this X form is isostructural with tetragonal Er₂Ge₂O₇. The rare earth coordination polyhedra and the (Si₂O₇)^6? configuration are compared with those actually known in neighbouring phases and discussed in terms of pressure effect.     

Etude cristallochimique d'une nouvelle serie d'oxydes doubles de rhenium et de terres rares

The preparation of a new series of heptavalent rhenium compounds of Ln₃ReO₈ type (Ln = rare earths) is described. The crystals are orthorombic for Ln = La (space groups Ccmm, Ccm2₁ or Cc2m) and Ln = Pr, …, Gd (space group B22₁2), face centered cubic of fluorite type for Ln = Tb, …, Lu and Y.     

Verres formes par les oxysulfures de terres rares Ln2O2S

Glass formation in the Ln₂O₂S-Ga₂S₃ systems (Ln = La to Nd). Preparations. Extent of the glassy regions as a fonction of the R.E. content and of the nature of the R.E. : Vitreous transition temperatures and crystallization temperatures. Microhardness. Optical properties.     

Etude par microscopie electronique des couches minces des sesquioxydes mixtes de terres rares Nd2O3 90%  Y2O3 10%

Thin films of materials belonging to the binary rare earth oxides system Nd₂O₃-Y₂O₃ at the composition 10 moles % of Y₂O₃ were investigated by electron microscopy and diffraction. After several thermal treatments using the electron beam we observed remarkable configurations of interfacial dislocations between the layers in the monoclinic B type oxide, with different chemical composition (i.e. different ratio of Nd to Y) superimposed on one another. Two modes of epitaxy between the hexagonal and cubic phases of the mixed oxides were observed : (001)_hex parallel to (111)_cub or to (112)_cub although the binary diagram does not show any coexistence domain of those two phases.     

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 cristallochimique et dielectrique des oxyfluorures de terres rares Na4Ln(WNb2)O9F5

Six new compounds with formula Na₄Ln(WNb₂)O₉F₅ (Ln = Y, Nd, Eu, Gd, Dy, Lu) have been synthetized. The corresponding room temperature phases have a tetragonal symmetry and a chiolite-type structure. At low temperature a ferroelectric-paraelectric transition is detected for each compound. The Curie temperature increases with the size of the Ln³⁺ ion.     

Etude des conditions de formation par voie hydrothermale des borates de terres rares TBO3 pseudo-vaterite

The pseudo-vaterite structure of the rare-earths-metals (Lu to Sm) is unique with no analogy among nitrates and carbonates. A careful study by hydrothermal synthesis of the conditions of formation of these compounds gives useful informations on the crystallographic structure and allows to synthetize three new pseudo-vaterite compounds, NdBO₃, PrBO₃, CeBO₃.     

Vingt nouveaux germaniures ternaires TR5T4Ge10 de metaux tr des terres rares et T = Co,Rh, Ir. Supraconductivite de Lu5Rh4Ge10 et Lu5Ir4Ge10

Twenty ternary germanides, which crystallize in the Sc₅Co₄Si₁₀ type, have been prepared in the RE-T-Ge systems where RE is a rare earth and T = Co, Rh, Ir: with Co, RE = Tm-Lu; with Rh, RE = Y, Gd-Lu; with Ir, RE = Gd-Lu. Lu₅Rh₄Ge₁₀ and Lu₅Ir₄Ge₁₀ show superconducting transitions at 2.2?1.6 K and 2.01?1.94 K respectively.     

Verres formes par les sulfures L2S3 des terres rares avec le sulfure de gallium Ga2S3

Formation of glasses in the systems L₂S₃Ga₂S₃ (L = Y and La to Er, and La to Er, except Eu). Conditions of preparation. Extent of the glassy regions as a fonction of the quenching temperatures and of the rare earth element. Crystallisation by heating: vitreous transition temperature Tg, and crystallisation temperature Tc. Mechanism of crystallisation. Preliminary study of the absorption spectra.     

Proprietes physiques et structurales de la phase CrxV1−xO2

The Cr_xV_1−xO₂ solid solution has been studied by D.T.A., X-ray diffraction, magnetic susceptibility and electrical conductivity measurements. Three phases with related structures are made in evidence at any composition; they are respectively monoclinic, orthorhombic and tetragonal with the rutile structure. The extension of the homogeneity ranges with composition and temperature and the evolution of the conduction mechanisms are explained.     

Sur quelques nouveaux oxydes mixtes de formule BaLn2Fe2O7 et SrLn2Fe2O7

Several new compounds have been prepared, with the general formula : BaLn₂Fe₂O₇ (Ln³⁺ = La to Gd) and SrLn₂Fe₂O₇ (Ln³⁺ = Nd to Tb). The experimental conditions are given, along with some crystal data. The diffraction patterns show a close similarity with that of Sr₃Ti₂O₇.     

Etude des phases ferroelastiques de Pb3V2O8 : Determination de la maille cristalline de la phase α ferroelectrique basse temperature

The cell parameters of the ferroelastic and ferroelectric phase of Pb₃V₂O₈ (T < 262 K) have been determined by very accurate X-ray powder diffractometry ; at 77 K, $\text{a}\text{= 7.460 (1)}\text{A}\text{?}$, $\text{b}\text{= 6.191 (1)}\text{A}\text{?}$, $\text{c}\text{= 9.348 (1)}\text{A}\text{?}$ and $\text{β = 116.63 (1)}\text{A}\text{?}$. This monoclinic cell is characterized by Z = 2 and the space group is A2. Thermal expansion tensors have been measured versus temperature for the three phases α (T < 262K), β (262K < T < 360 K) and γ (T > 360 K). The intermediate phase β is characterized by an important thermal expansion anisotropy and its molecular volume is higher than that of the two other phases α and γ.     

Structure cristalline d'une nouvelle variete de pyrophosphate de nickel: Ni2P2O7

Three forms of nickel pyrophosphate are already known. A new form was prepared and structure determined by Patterson method. σ Ni₂P₂O₇ is monoclinic with a = 5.212(3), b = 9.913(5), $\text{c}\text{= 4.475(3)}\text{A}\text{?}$, β = 97°46(10). The space group is $\text{P}\text{2}{}_1\text{a}$. The structure was refined until a R value of 0.041 for 1374 independent reflexions. Framework is built with P₂O₇ groups and NiO₆ octaedra. The P-O-P bonding is linear. σ Ni₂P₂O₇ is isotypic with the pyrosilicate Er₂Si₂O₇.     

Influence de la pression partielle d'oxygene sur la nature des defauts ponctuels dans Ca2LaFe3O8+x

The type of point defects in non-stoichiometric Ca₂LaFe₃O_8+x ferrite has been studied by measuring the variation of electrical conductivity with oxygen partial pressure at various temperatures. Several ordering possibilities occur. The oxygen atoms inserted, mostly 1-charged, have a strong tendency to couple at sufficient oxygen pressure and at low temperature. Interstitial oxygen atoms in the lattice vacancies induce structural evolution as non-stoichiometry becomes significant enough.     

Les phases SrLa2Al2O7 et SrGd2Al2O7

SrLa₂Al₂O₇ and SrGd₂Al₂O₇ belong to the structural type Sr₃Ti₂O₇. In SrGd₂Al₂O₇ strontium and gadolinium occupy respectively the 12 and 9 coordinated sites, whereas in SrLa₂Al₂O₇ the homologous cations are statistically distributed. The fluorescence spectra of both phases activated by Eu³⁺ ions show that the only possible position of Eu³⁺ is the 9 coordinated site.     

Mode de formation et stabilite des phases AnBnO3n+2 mises en evidence dans les systemes La2Ti2O7 - CaTiO3, Nd2Ti2O7 - CaTiO3 et Ca2Nb2O7 - CaTiO3

The stability of the A_nB_nO_3n+2 phases, whose structure is derived from the perovskite structure, was investigated through X-Ray diffraction, in the following systems: La₂Ti₂O₇ - CaTiO₃, Nd₂Ti₂O₇ - CaTiO₃ and Ca₂Nb₂O₇ - CaTiO₃. Some of the phases quenched from the liquid are metastable and include vacancies overA and B sites. A model of formation of these phases is proposed.     

Etude par diffraction X et microscopie electronique des composes inedits de formule AnBnO3n+2 dans les systems La2Ti2O7CaTiO3, Nd2Ti2O7CaTiO3 et Ca2Nb2O7CaTiO3

New compounds corresponding to the formula A_nB_nO_3n+2 with n = 4,5; 5 and 6 have been found in the two following systems Nd₂Ti₂O₇CaTiO₃ and Ca₂Nb₂O₇CaTiO₃. These compounds have been investigated by electron microscopy. The results obtained are in good agreement with our structural hypothesis.     

Elaboration et caracterisation de monocristaux de V2O3 par transport en phase vapeur

Single crystals of V₂O₃ are obtained by HCl or TeCl₄ chemical transport method in sealed quartz tubes with temperature gradients of 1050–930°C and 990–900°C respectively. The hexagonal prims are 5 mm long and 1 or 2 mm wide. Both kinds of crystals are characterized by sharp insulator-metal transition at 156 and 137 K respectively, they present a large hysteresis. The sharp decrease in resistivity with increasing temperature is of the order of 10⁸ at the transition.     

Intercalation de solvants organiques polaires dans la structure lamellaire des gels de V2O5

The swelling of vanadium pentoxide gels has been measured in three polar organic solvents: sulfolane, dimethylsulfoxide and propylene carbonate. These solvents take the place of the interfoliar water in the V₂O₅ xerogel. Propylene carbonate forms double-layer intercalates corresponding to a basal spacing of 21.5 Å. DMSO leads to a single-layer compound ($\text{d}\text{= 16.5}\text{A}\text{?}$); the same conclusion could probably apply for the sulfolane compound ($\text{d}\text{= 17.6}\text{A}\text{?}$).