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Single crystals of CuWO, are grown by the flux method using Na2W2O7 as a flux. Semiconducting n-type behaviour, with an activation energy for the electrical conductivity of ΔE = 0.28 eV is obtained by heating under argon flow. The analysis of the spectral response gives two indirect interband transitions at 2.15 and 2.34 eV. The choice of appropriate equivalent circuit for the CuWO4/0.1 N H2SO4, + 0.5 M KCl interface and of appropriate frequency range enables the determination of the space charge capacity. The corresponding Mott-Schottky plots are used to determine the donor concentration (ND = 5.5 × 1019 cm−3) and electron affinity (EA = 4.6 eV) Photon absorption responsible for the electronic transition at 2.34 eV is probably due to a Cu2+ W6+ charge transfer, although a competing hypothesis involving a O2- . Cu2+ charge transfer cannot by absolutely ruled out. Des monocristaux de CuWO4, ont ete obtenus par une methode de flux utilisant Na2W2O7 comme solvant. Un traitement thermique sous courant d'argon a permis de leur confbrer un comportement semi-conducteur de type n, caracterise par la variation thermique de la conductivite electrique et du pouvoir thermoelectrique. Les electrodes ont ete etudiees dans une solution 0,1 NH2SO4, + 0,5 M KCl. Le trace des courbes de Mott-Schottky a permis de determiner la concentration en donneurs ND = 5.5 × 1019 cm−3) et l'affinite electronique (EA = 4.6 eV). L'etude de la reponse spectrale met en evidence au moins deux transitions, a 2,15 et 2,34 eV. La deuxieme correspond tres probablement au transfert de charge Cu2+ W6+ bien que le rǒle du transfert O2- Cu2+ ne puisse ětre totalement ecarte.

Large crystals with composition Na5-xAxW3O9F5 (x = 0, 0.5 and A = Li, K) have been grown by the Bridgman-Stockbarger technique. They have ferroelastic and ferroelectric properties. Dielectric, birefringence and thermal expansion measurements have been performed along two directions parallel or perpendicular to the [010] monoclinic axis. They allowed to detect, in addition to the Curie temperature Tc, a lot of transitions for T < Tc. All the transition temperatures decrease when Na+ is replaced by Li+ or K+.

Resume Deux series de phases de structure “bronzes oxygenes de tungstene quadratiques” et de composition BaxLi5−2xT5O15(T = Nb, Ta) ont ete mises en evidence au sein des systemes BaNb2O6LiNbO3 et BaTa2O6LiTaO3. Toutes ces phases possedent des transitions ferroelectriques-paraelectriques. La temperature de Curie TC augmente avec le taux en lithium. La valeur de TC correspondant a la phase Ba2.03Li0.94Nb5O15 est la valeur la plus elevee obtenue jusqu'a present pour le type structural envisage. Ces phases se comportent comme des doubleurs de frequence. Le niobate Ba2.14Li0.71 Nb5O15 presente un rendement harmonique tres eleve, de l'ordre de 250 fois celui du K.D.P., 2.5 fois superieur donc a celui du “banana” Ba2NaNb5O15. Les etudes radiocristallographique et dielectrique du systeme Ba2.14Li0.71Nb5O15Ba2.14Li0.71Ta5O15 ont permis la mise en evidence de trois domaines respectivement antiferroelectrique, ferroelectrique et paraelectrique. La temperature de Curie ferroelectrique et le rendement harmonique en optique non lineaire diminuent lorsque le taux en tantale augmente.

The atomic arrangement in polar SrA1F5 is related to that in an identical structure but opposite polarity by means of atomic displacements no greater than about 0.5A. Such displacements reverse the spontaneous polarization and lead to a predicted ferroelectric-paraelectric phase transition temperature of 685K. A specific heat anomaly is observed at about 695K and the dielectric constant exhibits an anomaly at about 715K. The piezoelectric d33 coefficient varies from sample to sample, in the range 0.1 < d33 < 0.8 pC N−1. The properties of SrA1F5 are hence those of a new ferroelectric material. Nine members of the AIIBIIIF5 family have been identified, with A = Sr and B = Al, Cr, Ga; A = Ba and B = Ti, V, Fe; A = Pb and B = Al, Ga; and EuA1F5. The phase transition temperature is a function of the A2+ and B3+ ionic radii.

Crystals of the title compounds exhibit structural phase transitions from the prototype Fm3m structure to distorted low temperature phases showing ferroelectric-ferroelastic properties. The Raman spectra of these materials are interpreted in terms of MO3F3 isolated octahedra having a C2v symmetry; it is suggested that ferroelectricity is induced by the ordering of the octahedra dipole moments.

Crystals of Na5W3O9F5 exhibit a paraelectric-ferroelectric phase transition at Tc ∼ 800 K. The assignment of the sites occupied by the O and F atoms in the chiolite network has been made possible from Raman scattering experiments. A model is proposed for the ferroelectric transition, in which the driving mechanism is due to reorientations of WX6 octahedra (X = O, F).

The crystal structure of K 6 Ta 6.5 O 14.5 F 9.5 has been determined from 714 automatic diffractometer data, and refined in the space group P6 to an R-factor of 0.061. Its structure can be described as a three-dimensional framework lattice formed by the interpenetration of two distinct sublattices with the formulations [Ta 6 X 21 ] n and [Ta 0.5 X 3 ] n respectively, in which are inserted the K atoms. The three-dimensional skeleton [Ta 6 X 21 ] n is formed entirely of octahedra TaX 6 , while the sublattice [Ta 0.5 X 3 ] n is an infinite chain of trigonal prisms.

La structure cristalline de l′oxyfluorotungstate de cuivre CuWO3F2 a ete determinee par diffraction × sur monocristal (R = 0,019). CuWO3F2 cristallise dans le systeme monoclinique avec le groupe spatial P21/m et les parametres a = 5,223(1) A, b = 9,599(4) A, c = 3,670(1) A, β = 106,26(1)°, Z = 2. Le reseau est constitue d′octaedres WX6 (X = O, F) lies par des sommets en position trans formant une chaine infinie. Les atomes de cuivre divalent sont inseres entre les chaines en coordinence octaedrique deformee (4+2). L′ordre oxygene-fluor au sein de CuWO3F2 a ete etabli a l′aide de la spectroscopie Raman, de calculs d′energie electrostatique et de la methode des bilans des valences de liaisons. La chaine octaedrique peut se formuler (WO2F2O2/2)2−. Crystal Structure of CuWO3F2 The crystal structure of the copper(II) oxyfluoridetungstate CuWO3F2 has been determined from single crystal X-ray data. The symmetry is monoclinic (space group P21/m) with lattice parameters a = 5.223(1) A, b = 9.599(4) A, c = 3.670(1) A, β = 106.26(1)° (Z = 2). The structure has been fixed by the heavy atom method and refined by least square calculations down to a R factor of 0.019. Oxygen-fluorine ordering has been determined on the network built up by corner sharing octahedra constituting infinite chains, by several methods: Raman spectroscopy, electrostatic energy calculations, and bond valence determinations. The structural formulation of the octahedra is (WO2F2O2/2)2−.

Crystal structures of complex oxyfluorides Na 2 WO 2 F 4 , LiW 3 O 9 F, and NaMoO 3 F have been previously determined by X-ray diffraction but without assigning the specific position of oxygen or fluorine. Using various techniques such as 19 F nuclear magnetic resonance, Raman spectroscopy, electrostatic energy and site potential calculations, analysis of the local balance between charge of neighboring ions, clear evidence has been given of an ordered oxygen-fluorine distribution within the anionic sublattices which is proposed. The accuracy of each method used has been discussed for each phase.

CsCdBr 3 : Nd 3+ crystals have been studied by laser selective excitation and time resolved spectroscopy. Due to the charge compensation resulting from the Nd 3+ /Cd 2+ substitution, the proportion of Nd 3+ pairs is very large and does not follow the statistical laws. This proportion has been measured, its variation with concentration has been established and the effect of codoping with a monovalent metal observed. From a comparison between the properties of single ions and pairs, energy transfers have been analysed. De-excitation channels have been determined for some excited levels from the dynamics and the relative intensities of luminescence lines. An unusual behaviour, which can lead to applications, has been observed: the 4 F 3/2 luminescence of pairs if favoured with respect to that of single ions. This behaviour is well explained Etude experimentale par des techniques d'excitation selective par laser pulse avec resolution temporelle, avec une grande proportion de Nd 3+ associes en paires, en suivant par les lois de la statistique, du fait de la substitution de Cd 2+ par Nd 3+ . Mesure de cette proportion et de sa variation avec la concentration; observation d'un effet de codopage par un ion monovalent. Analyse des transferts d'energie. Determination des chemins de desexcitation depuis quelques niveaux excites a partir de la dynamique et des intensites relatives des raies de luminescence. Observation de la luminescence du niveau 4 F 3/2 favorisee pour les paires par rapport aux ions isoles; interpretation

The magnetic properties of RbVCl3, CsVCl3, CsVBr3, and CsVI3 have been investigated in the temperature range 4.2–800°K. In agreement with the 2H structure of these phases, the obtained results have been interpreted on the basis of a monodimensional Heisenberg model. The values of the exchange integrals have been determined by means of Smith and Friedberg [Phys. Rev. 176, 660 (1968)] calculations. They involve strong antiferromagnetic interactions, which decrease from the chloride to the iodide. The nature of the coupling between vanadium atoms is discussed.

The ordering of Cu and W atoms in the distorted wolframite structure of CuWO 4 gives rise to zigzag chains of edge sharing CuO 6 octahedra linked by similar chains of WO 6 octahedra. The existence of two kinds of connections between the CuO 6 octahedra leads to alternating antiferromagnetic interactions along the chains. The magnetic behavior is explained using a Heisenberg type model of Duffy and Barr with the largest exchange integral : J = − 67 K and an alternation parameter : a = 0.8. k B EPR measurements have shown that threedimensional ordering occurs at T N ⋍ 24 K .

Chiolite Na5Al3F14 exhibits a structural phase transition at Tc=150K; this transition is studied by means of Raman scattering, X-ray diffraction and calorimetric measurements. The experimental data are consistent with a ferrodistortional-ferroic transition, from a tetragonal room-temperature phase P4/mnc (Z=2) to a monoclinic low-temperature phase P21/n (Z=2). The Raman spectra show the existence of a soft mode, Raman active in both phases and corresponding to Eg symmetry in the tetragonal phase; it is assigned to a rotational motion of the AlF6 octahedra within the (Al3F14)n5n- complex layers. The behaviour of this soft mode is interpreted as the result of a coupling with the strain of the same symmetry (pseudo-proper ferroelastic transition).

A precise structural determination of KPbCr 2 F 9 was carried out. The symmetry is orthorhombic with a = 9.81(5), b = 5.412(3), c = 13.93(1) A, and Pnma . The structure was refined from 1285 X-ray reflections by full-matrix least squares to an R = 0.041. The lattice is made up of double chains (Cr 2 F 9 ) 3 n − n running along the b axis with Pb and K atoms ensuring its cohesion. The results of the magnetic studies are reported and discussed.

Structural phase transitions are characterized in several elpasolites corresponding to the general formula Rb2KMIIIF6 (M = Al, Cr, Fe, Y, Bi) using X-ray diffraction and calorimetric techniques. The increase of the transition temperature with increasing M3+ size — as observed for most terms of the series — can be related to simultaneous decrease of the Goldschmidt tolerance factor t towards the lower limit of the stability range of the cubic lattice. Evidence is given of the symmetries of the high- and low-temperature forms, together with preliminary transition entropies. Large single crystals of Rb2KFeF6 and Rb2KYF6 are grown using a Bridgman method. Une transition de phase structurale a ete caracterisee dans plusieurs elpasolites de formule generale Rb2KMIIIF6 (M = Al, Cr, Fe, Y, Bi) par diffraction X et calorimetrie. L'augmentation de la temperature de transition avec la taille du cation trivalent, observee pour la plupart des composes, a ete reliee a la diminution simultanee du facteur de tolerance t qui tend vers la limite inferieure du domaine de stabilite de la variete cubique. Les caracteristiques cristallographiques des varietes de haute et de basse temperatures ont ete definies: des valeurs preliminaires sur les entropies de transition ont ete determinees. Des monocristaux de grande taille de Rb2KFeF6 et de Rb2KYF6 ont ete obtenus a l'aide d'une methode de Bridgman.

In the new material Li6Gd(BO3)3 Gd3+ ions belong to parallel chains separated by about 7A for an intrachain distance of about 4A, so that the energy transfer among Gd3+ ions is expected to be mostly one-dimensional. The Gd3+ 6P7/2→8S7/2 fluorescence has been studied from 300K to 1.5K. Above 40K the fast exponential fluorescence decay indicates fast diffusion of the energy among the Gd3+ ions and trapping by impurities. Below 40K the energy transfer takes place predominantly toward perturbed Gd3+ ions and is diffusion-limited. The nonexponential intrinsic fluorescence decay consists in a short-time component corresponding to diffusion and a longtime component coming from thermal back-transfers from,the traps. The cross-over from a fast diffusion regime to a diffusion-limited regime is discussed in relation to the one-dimensional crystal structure.

A structural analysis of lithium tantalum oxyfluoride LiTaO2F2 has been carried out by means of single-crystal diffraction methods. The symmetry is tetragonal, space group I4/mmm, with a cell having the dimensions : a = 3.903 ± 0.003, c = 25.380 ± 0.008 A and containing five formula units. The structure was solved by Patterson and Fourier techniques and was refined by full-matrix least squares to an R value of 0.056. The structure consist of infinite ReO3 type blocks three units-cells thick stacked perpendicularly to the c axis and shifted along the [110] direction with respect to each other. These blocks are connected together by the lithium atoms in a squarepyramidal coordination.

The tungsten bronzes of formulae Co x WO 3 and Ni x WO 3 (0 3 , have a symmetry which increases with rising x. They are semi-conductors. Accurate density measurements made on single crystals prepared by chemical transport show that nickel and cobalt are inserted in the WO 3 network and not substituted to tungsten.

The growth of Ce1−xPrxO2 single crystals (x≃0.05) has been investigated in connection with a systematic research work o n rare earth based red pigments. Ce1−xPrxO2 crystals have been grown by spontaneous nucleation during slow cooling of a high temperature solution using PbF2-Pb3P2O8-NaF or Na2B4O7-NaF as solvents. X-ray diffraction, chemical analysis and optical study were performed on the obtained single crystals. Stabilization of Pr4+ in cerium oxide accounted for the observed red color.

Single crystals of a new oxyfluoromolybdate of formula NaMoO3F have been grown by a Bridgman technique. Based on X-ray diffraction data and optical observations (monoclinic symmetry, parameters: a = 7.138 A , b = 3.715 A , c = 6.511 A , β = 110.08° , with Z = 2 and space group P21), a hypothetical structure has been proposed by comparison with that of the previously reported CuNbO3F phase.

Neodymium oxide chloride single crystals up to 10 x 10 x 0.1 mm3 have been grown from molten neodymium chloride in the temperature range 900-700°C. A slow-cooling technique (3°C/h) with a sealed platinum-10% rhodium crucible was used. The best crystals are transparent, pale purple in color and have well developed (001) faces. They have been characterized by X-ray diffraction and optical measurements.

The structure of potassium tantalum oxyfluoride K₂Ta₂O₃F₆ has been determined from a Patterson synthesis and refined by least squares (R = 0.065). The orthorhombic unit cell has the space group Pnma (a = 10.212 (5) Å ; b = 5.581 (3) Å ; c = 14.499 (7) Å) and contains four motifs. The structure is made up of double infinite zigzag chains along the [010] direction. These chains are formed by the (TaO₃F₃) ⁴⁻ octahedra sharing three oxygen corners of a common face. The K⁺ ions occupy the vacant sites between the files and ensure the cohesion of the lattice., La structure de l'oxyfluorotantalate de potassium K₂Ta₂O₃F₆ a été déterminée à partir d'une synthèse de Patterson et affinée par la méthode des moindres carrés (R = 0,065). La maille élémentaire de symétrie orthorhombique comporte le groupe spatial Pnma (a = 10,212 (5) Å ; b = 5,581 (3) Å ; c = 14,499 (7) Å) et contient quatre motifs formulaires. Le réseau est formé de chaînes doubles se développant en zigzag dans la direction de l'axe b, constituées d'octaèdres (TaO₃F₃) ⁴⁻ mettant en commun trois sommets oxygénés d'une même face. Des ions K⁺ occupent les sites vacants entre les files assurant la cohésion du réseau., Vlasse Marcus, Chaminade Jean-Pierre, Pouchard Michel. La structure cristalline de l'oxyfluorotantalate de potassium K2Ta2O3F6. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 99, 1, 1976. pp. 3-7.

International audience; CsCdBr3 doped with trivalent rare earths is an attractive system to study energy transfer phenomena. Trivalent rare earths substituted to Cd2+ can be associated in pairs close to a Cd2+ vacancy. In these systems, cooperative relaxation (up conversion and cross relaxation) are very efficient. A comparaison between Nd3+ absorption and excitation spectra shows that the great majority of Nd3+ are associated in one kind of pairs even in low doping crystals.

Lithium has been intercalated chemically and electrochemically in LnNb3O9 (Ln = La, Nd) perovskite-type phases and LiW3O9F which can be considered as a hexagonal tungsten bronze derivative.The crystallographic formula of the LnNb3O9 starting material is □3/2 (Ln□1/2 )Nb3O9. In both systems, solid solutions are observed in the fir part of the intercalation reaction. While almost all perovskite cavities are filled in the neodymium phases, the higher ionic character of the La-O bonds prevents practically the Li intercalation in the □' sites.In the LiW3O9F phase two lithium atoms can be electrochemically intercalated via a single phase mechanism. The reaction is completely reversible. For higher amounts of intercalation (x > 3), irreversible structural modifications occur.In both systems the unit cell parameter variation is negligible during intercalation. This behavior results from the blocking up of the framework by Ln3+ ions in LnNb3O9 phases or by the stacking of the three octahedra triangular arrangement in the hexagonal tungsten bronze structure.

Les bronzes de vanadium MxV2O5 sont des oxydes non stœchiométriques qui contiennent des atomes M intercalés dans le réseau de V2O5. Le métal M est ionisé et cède des électrons au réseau. Dans le cas M = Cu, il y a x électrons cédés. Les vanadiums sont également distribués sur les trois sites différents V1, V2, V3 le long de l'axe b du 'réseau monoclinique. Les électrons occupent un site seulement. Ce site est exactement saturé pour x = 2/3. Des mesures de conductivité électrique faites sur des monocristaux parallèlement à l'axe b montrent clairement que les bronzes MxV2O5 sont des conducteurs métalliques pour x > 0,55. Quand on diminue la quantité de cuivre (c'est-à-dire le nombre d'électrons par site), on observe une transition à un état non métallique. On discute les valeurs de la conductivité électrique et de l'effet Seebeck : 1) en termes de la localisation d'Anderson due au désordre entre les ions Cu+ et les lacunes, 2) en termes de polarons comme l'a proposé Mott.

The value of the ferroelectric Curie temperature in compounds with the “tetragonal tungsten bronze structure” is related to the steric effect of large alkali or alkaline-earth cations in the lattice tunnels, to the covalency of the transition element-oxygen bonds, and to the introduction of Li + ions, which due to their small size are able to occupy the nine coordination sites of the structure anisotropically.

Thèse en cotutelle entre L'Université Mohamed KHIDER de Biskra et L'Université de Bordeaux 1; The physical properties of a material are intimately related to its crystalline structure and in the case of doped ions they are dependent on to the sites they occupy. The first part of this work is dedicated to the diphosphate material of sodium and zinc Na2ZnP2O7, glassy and crystallized for doped ions of Co2+, Ni2+, Mn2+ and Eu3+. The crystallized phases were obtained by the Czochralski method whereas the glasses were obtained by quenching from the molten state. A set of physical characteristics were studied (Raman, infrared, EPR, optical absorption, luminescence) to determine the sites occupied by the doped ions and its influence on the optical properties. The second part of this work consists of a better knowledge of lead-free dielectric materials belonging to the family of Perovskite, more particularly to NaNbO3-BaTiO3 solid solution. Single crystals were obtained by the flux method and were characterized using several techniques: X-rays diffraction, microanalysis, thermal evolution of ferroelectric and ferroelastic domains; dielectric, piezoelectric and pyroelectric measurements.; Les propriétés physiques d'un matériau sont intimement liées à sa structure cristalline et dans le cas d'ions dopants aux sites qu'ils occupent. La première partie de ce travail est dédiée au matériau diphosphate de sodium et de zinc Na2ZnP2O7, cristallisé out vitreux et ceci pour les ions dopants Co2+, Ni2+, Mn2+ et Eu3+. Les phases cristallisées ont été obtenues par la méthode Czochralski, les verres par trempe à partir de l'état fondu. Un ensemble de caractéristiques physiques ont été mises en jeu (Raman, infrarouge, RPE, absorption optique, luminescence) pour déterminer les sites occupés par les ions dopants et l'influence sur les propriétés optiques. La deuxième partie de cet travail consiste à une meilleure connaissance des matériaux diélectriques sans plomb appartenant à la famille pérovskite et plus particulièrement à la solution solide NaNbO3-BaTiO3. Des monocistaux ont été obtenus par la méthode des flux et caractérisés en utilisant plusieurs techniques : diffraction X, microanalyse, évolution thermique des domaines ferroélectriques-ferroélastiques, mesures diélectriques, piézoélectriques et pyroélectriques.

Certaines propriétés des cuprates supraconducteurs restent aujourd'hui encore mystérieuses pour le monde scientifique. Ce travail vise a éclaircir l'une d'entre-elles: la nature de leurs porteurs de charge. Une approche théorique a été effectuée sur le composé La2CuO4 puis étendue a des composés de type cuprate supraconducteur exempts de cuivre. Elle consiste en des calculs de potentiel de Madelung et de diagrammes de structure de bandes. Les composes "TlBa2ZnO5" et "TlBa2MgO5" ayant servi de modèles pour ces calculs ont ensuite été préparés puis caractérisés par dosage iodométrique, diffraction des rayons X, spectrométrie Raman, MET et RPE. Les résultats alors obtenus mettent en évidence l'existence de trous d'électrons sur les atomes d'oxygène de ces composés, notes O dans le texte. Nous avons alors tenté d'expliquer le comportement encore inconnu de ces porteurs de charge. Une deuxième étude a été menée en parallèle de la précédente. Elle nous a permis de mettre a jour une nouvelle famille d'oxyhalogenures de tellure et d'alcalino-terreux de formulation A3Te2O6X2. Le composé pour lequel A=Sr et X=Cl a fait l'objet d'une détermination structurale complète. Les composés pour lesquels A=Sr, Ba et X=Br ont été isolés.; non disponible

L' étude d'une technique originale de croissance cristalline, basée sur l'utilisation d'un échangeur thermique et nommée HEM (Heat Exchanger Method), a été entreprise. Un dispositif expérimental a été développé et optimisé. La modélisation des échanges thermiques a conduit a l'élaboration d'un code de calcul numérique. Ce dernier est valide par confrontation des résultats de la simulation aux données expérimentales obtenues lors de la croissance d'un matériau école, le germanium. Le champ d'application de la technique HEM a alors été étendu à l'obtention de céramiques texturées du supraconducteur à haute température critique YBa2Cu3O7-x. Après une étude de la microstructure des textures, un mécanisme de texturation est proposé. Enfin, des cristaux du compose NaMgF3 (perovskite) sont obtenus par HEM. Leur étude en température, par observation des domaines ferroélastiques et par spectroscopie Raman, révèle l'existence d' une seule transition de phase.; non disponible