On the condensation mechanism of the dioxides of sixth- group elements: an ab initio approach

International audienceA quantum mechanical ab initio simulation of XO2 molecules (X = O, S, Se, Te and Po) and the relevant dimers has been performed. The X - O bond polarity and the bent configuration of the XO2 species are discussed as factors governing their condensation mechanisms. Three points are emphasized: (i) the fundamental difference between the constitution of the condensed phases of OO2 and SO2, on the one hand, and those of SeO2 and TeO2, on the other; (ii) the occurrence of polymorphism in crystalline TeO2 and its absence in crystalline SeO2; (iii) the driving forces of the gamma-alpha polymorph transformation in crystalline TeO2

Role of sintering time, crystalline phases and symmetry in the piezoelectric properties of lead-free KNN-modified ceramics

International audienceLead-free KNN-modified piezoceramics of the system (Li,Na,K)(Nb,Ta,Sb)O3 were prepared by conventional solid-state sintering. The X-ray diffraction patterns revealed a perovskite phase, together with some minor secondary phase, which was assigned to K3LiNb6O17, tetragonal tungsten–bronze (TTB). A structural evolution toward a pure tetragonal structure with the increasing sintering time was observed, associated with the decrease of TTB phase. A correlation between higher tetragonality and higher piezoelectric response was clearly evidenced. Contrary to the case of the LiTaO3 modified KNN, very large abnormal grains with TTB structure were not detected. As a consequence, the simultaneous modification by tantalum and antimony seems to induce during sintering a different behaviour from the one of LiTaO3 modified KNN

Raman spectroscopy study of the Na0.5Bi0.5TiO3–PbTiO3 system

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Ab initio study of non-linear optical susceptibility of TeO2-based glasses

International audienceTo gain a better insight into the origin of the outstanding nonlinear optic susceptibilities of TeO2-based glasses whose numerical characteristics are almost two orders of magnitude higher than those of SiO2-based glasses, a comparative computer simulation of their dielectric properties was performed using ab initio studies of a series of (SiO2)p and (TeO2)p polymer molecules. This comparison showed that these properties are reproducible only in a TeO2 glass simulated as an ensemble of chainlike (TeO2)p polymer molecules with p-->[infinity], which was interpreted as evidence for the essential nonlocality of the electronic polarizability mechanism in that glass. The relevant model calculations showed the reasonableness of this hypothesis, which was subsequently used to explain the influence of modifier content on the nonlinear optic susceptibility of the tellurite glasses

Structural, electrical and optical properties of carbon-doped CdS thin films prepared by pulsed-laser deposition

International audienceCdS compounds have attracted interest due to their applications in optoelectronic devices such as solar cells and optical switches. The efficiency of these devices strongly depends on homogeneity, structural quality and morphology of the layers. The purpose of this work is to develop pure and carbon-doped CdS thin films on glass substrates, using pulsed laser deposition (PLD) process at relatively low deposition temperature. A computer controlled multi-target system allows the deposition of ITO conductive films as well as the one of pure or doped CdS films. In this case, predefined precise sequences of laser shots (KrF, wavelength 248 nm, pulse duration 20 ns) on the CdS and vitreous carbon targets, respectively lead to a well controlled carbon doping of deposited CdS films. Structural and chemical characteristics of the films and the multilayers investigated using XRD and Raman spectroscopy as well as optical and electrical properties are studied as a function of PLD deposition parameters and carbon doping

A thorough FT-IR spectroscopy study on micrometric silicon oxide films deposited by atmospheric pressure microwave plasma torch

International audienceSiOxHyCz micrometer thick films are deposited from an argon/hexamethyldisiloxane mixture on Si (100) substrate by plasma enhanced chemical vapour deposition process using an axial injection torch at atmospheric pressure. Results highlight a similar effect of low and high substrate temperatures both on the deposition process and on the microstructure of the deposited films. Mesoscopically, scanning electron microscopy analyses reveal that particles are promptly produced in the gas phase and incorporated to the film. Microscopically, a detailed infrared analysis in transmission mode demonstrates a high carbon contamination in the low and high temperature intervals resulting in a lower stoichiometry. This work allows to define an optimum growth window for the substrate temperature, leading to smooth, particle-free and carbon-free films: [60 ◦ C; 90 ◦ C]