A synthesis and characterization of lanthanum potassium niobate oxide with tetragonal tungsten bronze (TTB) type structure

The main aim of this study is to lower the reaction and sintering temperatures of K2LaNb5O15 in order to improve the quality of this material. The K2LaNb5O15 compound has been synthetized by soft chemistry (coprecipitation) and its physico-chemical properties were studied.This method permits the synthesis of chemical purty, fine powder materials with good crystallization at low temperatures.The main aim of this study is to lower the reaction and sintering temperatures of K2LaNb5O15 in order to improve the quality of this material. The K2LaNb5O15 compound has been synthetized by soft chemistry (coprecipitation) and its physico-chemical properties were studied.This method permits the synthesis of chemical purty, fine powder materials with good crystallization at low temperatures

Phase diagram of TTB ferroelectric compounds Pb1−xK2xNb2O6

Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1−xK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions.Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1−xK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions

Synthesis and single crystal X-ray structure of the tetragonal tungsten bronze Pb0.91K1.72Li1.46Nb5O15

Crystals of Pb0.91K1.72Li1.46Nb5O15, belonging to tetragonal tungsten bronze materials, were grown by the slow cooling technique, and characterized by means of single crystal X-ray diffraction: the structure was solved in the P4bm tetragonal space group, with the following unit cell parameters: a = 12.548(5), c = 4.042(5) Å, V = 636.4(9) Å3. The three-dimensional framework can be described as a layered structure down crystallographic axis c, with arrays of NbO6 octahedra, whose corner sharing makes up the formation of tunnels filled up by Li, Pb and K displaying complex cation-oxygen coordinations

Élaboration et caractérisation de cristaux dérivés de K3Li2Nb5O15

Des cristaux de bonne qualité de type K3Li2Nb5O15 et de structure “bronze” ont été élaborés par fusion et refroidissement lent en présence d'un pseudo-flux (K2CO3 et Nb2O5 en excès). L'analyse chimique montre que leur composition correspond à la formulation K2,82(Nb–O)0,08Li1,54Nb5O15. Ces matériaux cristallisent dans le système quadratique à 295 K avec le groupe spatial P4bm. Les mesures diélectriques ont été effectuées selon deux directions [001] et [100], perpendiculaires aux grandes faces des cristaux élaborés. Les variations thermiques de la permittivité et de la biréfringence ont permis de mettre en évidence deux maxima l'un à $T_{1}\cong 100$ K selon [100], l'autre à $T_{\rm C}\cong 680$ K selon [001] ; ce dernier correspond à al température de Curie ferroélectrique

Influence de la non-stoechiométrie sur les propriétés ferroélectriques, de conduction et de relaxation de céramiques de type Sr6_{6}Ti2_{2}Nb8_{8}O30_{30}

High densified Sr$_{6}$Ti$_{2}$Nb$_{8}$O$_{30}$ ceramics have been annealed under either oxidizing or reducing atmosphere. The materials crystallize with a tetragonal tungsten bronze structure and show a ferroelectric-paraelectric transition. A space charge dielectric relaxation occurs at low temperature for air sintered samples. Such a behavior disappears after long annealing under either oxygen (grains and grain boundaries are thus insulators) or hydrogen-argon (the ceramics become extrinsic n-type semiconductor).Des céramiques de Sr$_{6}$Ti$_{2}$Nb$_{8}$O$_{30}$ ont subi des traitements de recuit sous atmosphère oxydante ou réductrice. Les matériaux cristallisent avec la structure bronze quadratique de tungstène et présentent une transition ferroélectrique-paraélectrique. Il existe une relaxation de charges d'espace à basse température pour les échantillons frittés à l'air. Un tel comportement disparaît après un long recuit soit sous oxygène (les grains et les joints de grains sont alors isolants), soit sous hydrogène-argon (les céramiques deviennent alors semi-conductrices extrinsèques de type n)

Élaboration et caractérisation de cristaux dérivés de K3_{3}Li2_{2}Nb5_{5}O15_{15}

Good quality crystals of K$_{3}$Li$_{2}$Nb$_{5}$O$_{15}$ type were grown from the melt followed by method slow cooling using a pseudo-flux method (an excess of K$_{2}$CO$_{3}$ and Nb$_{2}$O$_{5}$ is present). The crystals were identified by X-ray diffraction, they crystallized with the tungsten bronze-type structure in the tetragonal system at room temperature. The space group is P4bm. Their composition (K$_{2.82}$(Nb–O)$_{0.08}$Li$_{1.54}$Nb$_{5}$O$_{15}$) was determined by chemical analysis. Dielectric measurements were realized along the [001] and [100] directions in the frequency range from 10$^{3}$ to $3 \times 10^{5}$ Hz. The temperature dependences of dielectric constants $\varepsilon^{\prime}_{\rm r33}$ and $\varepsilon^{\prime}_{\rm r11}$ and of birefringence $\Delta n$ show two maximums at $T_{1}\cong 100$ K and $T_{\rm C}\cong 680$ K, the latter being the ferroelectric Curie temperature.Des cristaux de bonne qualité de type K$_{3}$Li$_{2}$Nb$_{5}$O$_{15}$ et de structure “bronze” ont été élaborés par fusion et refroidissement lent en présence d'un pseudo-flux (K$_{2}$CO$_{3}$ et Nb$_{2}$O$_{5}$ en excès). L'analyse chimique montre que leur composition correspond à la formulation K$_{2,82}$(Nb–O)$_{0,08}$Li$_{1,54}$Nb$_{5}$O$_{15}$. Ces matériaux cristallisent dans le système quadratique à 295 K avec le groupe spatial P4bm. Les mesures diélectriques ont été effectuées selon deux directions [001] et [100], perpendiculaires aux grandes faces des cristaux élaborés. Les variations thermiques de la permittivité et de la biréfringence ont permis de mettre en évidence deux maxima l'un à $T_{1}\cong 100$ K selon [100], l'autre à $T_{\rm C}\cong 680$ K selon [001] ; ce dernier correspond à al température de Curie ferroélectrique

Study of structural short order and surface changes of SiO2 compounds

In this work dependence between structure, surface and reactivity of SiO2 is investigated. Different samples are prepared with and without the presence of calcium cations and are characterized by different techniques. The FTIR frequency shifting of the bridging oxygen stretching vibration Si–O–Si and the binding energy shiftingof the Si2p and O1s photoelectron peaks have been observed and discussed. Results show the effect of presence of calcium on the structural change, surface and reactivity of SiO2 aggregate. After reaction, a structural and SiO2 surface changes occur, as the FTIR spectra illustrate depending on the presence of the calcium cations

Study of structural short order and surface changes of SiO2 compounds

In this work dependence between structure, surface and reactivity of SiO2 is investigated. Different samples are prepared with and without the presence of calcium cations and are characterized by different techniques. The FTIR frequency shifting of the bridging oxygen stretching vibration Si–O–Si and the binding energy shiftingof the Si2p and O1s photoelectron peaks have been observed and discussed. Results show the effect of presence of calcium on the structural change, surface and reactivity of SiO2 aggregate. After reaction, a structural and SiO2 surface changes occur, as the FTIR spectra illustrate depending on the presence of the calcium cations