Proprietes magnetiques d'echantillons polycristallins de CrWO4

The study of the temperature dependence of EPR and Mössbauer spectra on polycristalline samples of CrWO₄, pure and doped with small quantities of ⁵⁷Fe, shows the presence of an antiferro-paramagnetic transition between 40 and 45 K. The magnetic susceptibility displays a broad maxium at a higher temperature ($\text{T}{}_{\max }\text{? 100}\text{K}$). The crystal structure suggests a monodimensional magnetic system but a simple model of linear chains with Heisenberg exchange does not fit the experimental data. Non-negligible interchain exchange and/or some anisotropy in the intrachain magnetic interactions could be responsible for this discrepancy.     

Proprietes magnetiques de l'oxyde de vanadium V3O7

Magnetic susceptibility and magnetization measurements on V₃O₇ from 4,2 to 600 K characterize a paramagnetic behaviour above 18 K : the samples follow a Curie-Weiss law with a Curie constant in good agreement with a spin only value $\text{S}\text{=}\text{1}\text{2}$ per V⁴⁺. At lower temperature the magnetic ordering may be attributed to a metamagnetic behaviour resulting from the layer type structure.     

Proprietes magnetiques de l'oxyde Eu2O3 monocristallin

Single crystals of europium oxide Eu₂O₃ with monoclinic B structure were prepared by the Verneuil process. The magnetic properties of these crystals have been studied. Paramagnetic susceptibility measurements have been performed for various orientations of the crystal with respect to the magnetic field at constant temperatures (5,77 and 300 K) as well as at increasing temperatures (5 – 1000 K). They show an anisotropy. An interpretation of this phenomenon is proposed.     

Contribution a l'etude de la conductivite ionique de l'orthophosphate Na3PO4

The present work is concerned with the ionic conductivity of pure trisodium orthophosphate Na₃PO₄, devoid of any trace of hydroxide NaOH. At the allotropic transition (330°C), we observe a jump of the ionic conductivity and a slight decrease in the activation energy (ΔE = 0,70 ± 0,02 eV for the quadratic variety and ΔE = 0,60 ± 0,04 eV for cubic γ-Na₃PO₄). Na₃PO₄ can be considered to be an electrolytic solid with medium conductivity ($\text{σ = 1.10}{}^{\mathrm{?4}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 370°C).     

Determination structurale et proprietes de conduction d'un nouveau compose de type hollandite: K1,8(Li2,45Sb5,55)O16; mise en evidence d'une surstructure d'ordre 3

The crystal structure of a synthetic hollandite phase K_1,8(Li_2,45Sb_5,55)O₁₆ has been refined by the full matrix least-squares method using 940 three-dimensional reflexions to a final R value of 0.046. The K atoms are randomly distributed on the special positions 2b (0 0 $\text{1}\text{2}$) and 4e (0 0 ±z) with z = 0.340(4).A long-exposure rotation photograph along the c axis shows diffuse X-ray scattering between the layer line. After a thermal treatment the X-ray diffraction pattern shows weak but sharp reflexions that could be indexed by tripling the short axis. The structural studies indicate an ordering of the large K atoms within the channels.The ionic conductivity of this phase has been investigated on a single crystal by hyperfrequences method. The potassium ion conductivity is the ordre of magnitude of 10^?3 (ohm cm^?1) at 30°C along the c axis.     

Etude de la solubilite du phosphate d'aluminium (AlPO4) dans des solutions hydrothermales d'acide orthophosphorique H3PO4

Before investigating the growth of aluminium orthophosphate single crystals (berlinite) under hydrothermal conditions, we have studied AlPO₄ solubility in phosphoric acid solutions, as a function of various parameters: temperature (150° to 550° C), pressure (up to 1400 bars), solvent concentration (5 M to 12 M). Our results are reported here and show that several hydrothermal techniques can be used for growing AlPO₄ crystals: slow increasing the temperature of a saturated solution; growth in a reverse temperature gradient (T < 350° C); growth in a direct temperature gradient (T > 350° C and P > 1000 bars).     

Proprietes electriques de l'oxyde V6O11 pur et dope au titane ou au fer

Electrical conductivity and thermopower have been investigated on V₆O₁₁ single crystals from room temperature down to 50 K.The metal-insulator transition is confirmed, but the behavior of the Seebeek coefficient disagrees with previous results. Discrepancies from sample to sample can be explained by a hopping model taking into account localized V³⁺ and V⁴⁺ levels and a small oxygen non-stoichiometry. Results obtained by Ti⁴⁺ and Fe³⁺ doping are consistent with this hypothesis.     

Structure cristalline du seleniure d'Etain IV et de thallium I : Tl2SnSe3

The cell is orthorhombic, a = 8.051(3); b = 8.169(3); $\text{c}\text{= 21,24(1)}\text{A}\text{?}$; Z = 8; space groupe Pnam, d_calc = 7.2; d_obs = 7.1; μ = 897 cm^?1. R = 0.065 for 1636 reflections.The anion Sn₂Se^4?₃ is formed by two SnSe₄ tetrahedra, which have a common edge. Two kinds of thallium atoms are observed, with different coordinations.     

Conductivite ionique de l'argent dans le compose Ag14Sb12O36F2 etude comparative avec la phase pyrochlore AgSbO3

The compound Ag₁₄Sb₁₂O₃₆F₂, with a desordered KSbO₃ cubic structure, is obtained by an ion exchange method. The conductivity of the silver ions in both Ag₁₄Sb₁₂O₃₆F₂ and AgSbO₃ pyrochlore compounds is investigated by measurement of complex impedance. The e.m.f. measurement and curves i vs. E show that the conduction is essentially assumed by Ag⁺ ions. This study confirms that the cubic KSbO₃ framework structures are good candidates for ionic conductors.     

Diagramme d'equilibre solide-liquide du systeme Ce(PO3)3 — AgPO3

The Ce(PO₃)₃ — AgPO₃ system was investigated by differential thermal analysis (DTA) and X-ray diffraction. Its phase equilibrium diagram was established by recording heating curves on a M5 Setaram μ-DTA aparatus. This diagram shows that the Ce (PO₃)₃—AgPO₃ system forms only one compound, AgCe(PO₃)₄, which melts in a peritectic reaction at 788°C. The chemical preparation, powder diagram and main crystallogrophic characteristics of this compound are given. AgCe(PO₃)₄, a polyphosphate isotypic with NaNd(PO₃)₄, has a monoclinic unit cell with a space group $\text{P2}{}_1\text{n}$ and parameters a = 9.717(4), b = 12.959(5), c = 7.242(2) Å, β = 91.07(2)°, Z = 4, V = 911.79 ų, d_x = 4.11. The different structure types of binary poly and metaphosphates, M₂O.Ln₂O₃.4P₂O₅, are regrouped and a correlation between each crystalline form and the corresponding M³ and Ln³⁺ cation size is dicussed.     

Etude in situ par spectrométrie vibrationnelle de couches minces electrochromes d'oxyde de tungstène WO3 et d'oxyde de molybdene MoO3

Raman spectroscopy, internal and specular I.R. reflection spectroscopies have been applied to WO₃ and MoO₃ thin films (～ 5,000 A). These films are amorphous and transform to crystalline materials at 400°C. Structural similarities between WO₃ films and the new hexagonal form of WO₃ may explain the electrochromic properties of the former.     

Structure cristalline d'une phase cubique desordonnee de type ReO3 excedentaire en anions : ZrF2,67O0,67

The crystal structure of cubic disordered anion-excess ReO₃-related ZrF_2.67O_0.67 (space group Pm3m, $\text{a}\text{= 3.997(2)}\text{A}\text{?}$) has been determined by single crystal X-ray diffraction techniques and refined to a R = 0.038-value. Two different kinds of anions are found, one X(1) slightly displaced (by 0.27 Å) from the ideal ReO₃-anionic site, the other X(2) more considerably (by 1.16 Å). As previously proposed (8,14) the anion-excess over the parent ReO₃-type structure, is accomodated by the formation of interstitial X(2)-X(2) pairs across a vacant X(1) site.The most likely anionic arrangements around zirconium atoms are assessed and on those basis, the structure is described as a disordered three-dimensional framework of edge sharing ZrX(1)₅X(2)₂ pentagonal bipyramids and corner sharing ZrX(1)₅X(2) distorted octahedra.     

Comportement du nitrure de niobium cubique δ en atmosphere d'oxygene

The oxidation of cubic δ NbN_.87 plates and NbN_.90 powder in oxygen at pressures of 25–730 Torr in the temperature range 425–600°C obeys pressure independent sigmoïdal kinetics. There is formation of a low temperature form of Nb₂O₅. The reaction is governed by an inner interface process, the shape of the curves being associated to a chain fragmentation of the nitride substrate.     

Proprietes magnetiques de quelques ferrites mixtes d'alcalinoterreux et de terre rare

Magnetic properties of some members of the series Sr(Ba)Ln₂Fe₂O₇ (Ln = rare earth) have been investigated by measurements of their magnetic susceptibility in the temperature range 4.2 to 1300 K. The iron sublattice orders antiferromagnetically at temperatures close to 550 K. Below this temperature the rare earth ion remains paramagnetic.     

Interaction d'echange Ti3+-Ti3+ au sein des phases Ti1−xTaxO2

The magnetic susceptibility of TiO₂ doped with Ta has been studied in the temperature range 4.2–300 K. The results confirm the presence of Ti³⁺ ions as it was recently suggested by the electrical properties. The interaction of d electrons localized on the Ti³⁺ ions which form isolated clusters along the c axis direction was calculated. The value of the exchange integral $\text{J}\text{k}\text{= ?116}\text{K}$ was determined by fitting the experimental curve for a ratio $\text{Ta}\text{Ti}$ equal to 0.09.     

Anisotropie de la decomposition d'un solide I-Existence d'un ellipsoide de reaction pour la transformation du sulfate de cuiver pentahydrate en trihydrate

Monocrystalline platelets of Copper Sulfate pentahydrate, oriented with respect to the crystal lattice are submitted to temperature (45°C) and water vapor pressure (2,5 torr) conditions such that the pentahydrate is converted into trihydrate. Any area that is converted, developped from a site is elliptical. For similar types of platelets, all the ellipses have an identical orientation and their axes a constant ratio. Studies on platelets with different orientations allow an ellipsoid to be reconstituted and we try to locate it with respect to the structure and ellipsoids characterizing the vectorial physical properties of the initial building up.     

Mise en evidence d'un transfert d'energie de type Gd3+ → Tb3+ dans les phases Sr2Gd1−xTbxGaO5

Energy transfer from optically excited Gd³⁺ ions to Tb³⁺ ions is reported in Sr₂Gd_1?xTb_xGaO₅. It is illustrated by the comparison of the emission and excitation spectra of Sr₂GdGaO₅ and of Sr₂Gd_0,995Tb_0,005GaO₅ between 77 and 300 K.     

Etude du comportement de l'antimoine v et du strontium dans les oxydes a structure en cages du type A3M8O21

New compounds Ba₃(Sb₄Ti₄)O₂₁ and Ba_3?xSr_x(M₄Ti₄)O₂₁ (M = Nb, Ta, Sb) isostructural with the niobates A₃(Nb_8?xM_x)O₂₁ previously described, have been synthesized. The influence of the antimony and strontium ions on the stability of the A₃M₈O₂₁ structure has been discussed in terms of electrostatic repulsion forces, polarization and covalency. Atomic positions, determined from powder diffraction patterns, are given for Ba₃(Sb₄Ti₄)O₂ and Sr₃(M₄Ti₄)O₂₁ (M = Nb, Ta).