Strong anisotropic influence of local-field effects on the dielectric response of {\alpha}-MoO3

Dielectric properties of {\alpha}-MoO3 are investigated by a combination of valence electron-energy loss spectroscopy and ab initio calculation at the random phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO3 has been obtained along the three axes and the importance of LFE has been shown. In particular, taking into account LFE is shown to be essential to describe properly the intensity and position of the Mo-N2,3 edges as well as the low energy part of the spectrum. A detailed study of the energy-loss function in connection with the dielectric response function also shows that the strong anisotropy of the energy-loss function of {\alpha}-MoO3 is driven by an anisotropic influence of LFE. These LFE significantly dampen a large peak in {\epsilon}2, but only along the [010] direction. Thanks to a detailed analysis at specific k-points of the orbitals involved in this transition, the origin of this peak has not only been evidenced but a connection between the inhomogeneity of the electron density and the anisotropic influence of local-field effects has also been established. More specifically, this anisotropy is governed by a strongly inhomogeneous spatial distribution of the empty states. This depletion of the empty states is localized around the terminal oxygens and accentuates the electron inhomogeneity.Comment: Supplemental Material include

Vers de nouveaux complexes thiométalliques (réactivité de tétraoxothiométallates [MO4-nSn]x- (n=1-4) (M=Mo, V, Re) vis à vis du disulfure de carbone et d'alcynes substituése)

Les @travaux présentés dans ce manuscrit s'inscrivent dans la continuité des études intéressant la réactivité des tétraoxothiométallates [MO4-nSn] (M=Mo, V, Re, W) vis à vis de composés électrophiles organiques et inorganiques dans le but de synthétiser de nouveaux complexes thiométallique . ces composés sont impliqués dans des domaines aussi variés que la chimie bioinorganique ou la catalyse hétérogène...VERSAILLES-BU Sciences et IUT (786462101) / SudocSudocFranceF

Synthèse, caractérisation et photo-réactivité de polyoxométallates inorganiques et hybrides organique-inorganique

Les polyoxométallates (POMs) sont des oxydes moléculaires anioniques de métaux de transition à haut degré d oxydation (exclusivement Mo et W dans notre étude) et présentent une grande diversité de composition chimique, de structure et de dimensionnalité. A l état solide, les POMs développent des propriétés optiques photo-induites remarquables qui couplent la photo-génération de paires excitoniques dans le POM à des transferts électroniques, et parfois atomiques, à l interface POM/contre-cation. Dans cette thèse, les transferts électroniques photo-activés des POMs ont été mis à profit pour élaborer de nouveaux matériaux hybrides organique-inorganique photochromes à l état solide ainsi que de nouveaux photocatalyseurs plasmoniques dans le visible. Dans la première partie de ce travail, une nouvelle famille de matériaux photochromes a été synthétisée en combinant des POMs avec des cations sulfonium. Les propriétés optiques de ces systèmes hybrides ont été caractérisées et un nouveau mécanisme de photochromisme a été proposé. Parallèlement à ces travaux, l assemblage de POMs avec des molécules organiques elles-mêmes photo-actives (spiropyrane, spironaphtoxazine) a ouvert la voie à une seconde famille de matériaux aux propriétés photochromiques exaltées. La deuxième partie de cette thèse est consacrée à l étude de la nanostructure Ag@Ag2Mo3O10 2H2O obtenue selon une nouvelle méthode de photo-déposition tout solide à partir du molybdate d argent Ag2Mo3O10 2H2O, les nanofils Ag2Mo3O10 2H2O étant obtenus par une voie de synthèse inédite en conditions douces. La nanostructure Ag@Ag2Mo3O10 2H2O s avère être un photocatalyseur plasmonique efficace sous lumière visible.Polyoxometalates (POMs) are anionic molecular oxide building blocks of high oxidation state transition metals (only Mo and W in our study) and have a wide variety of chemical composition, structure and dimensionality. In the solid state, POMs develop remarkable photo-induced optical properties coupling the photo-generation of excitonic pairs in the POM with electronic, and sometimes atomic, transfers at the POM/counter-cation interface. In this thesis, the photo-activated electronic transfers of the POMs have been used to develop new photochromic organic-inorganic hybrid materials in the solid state as well as new plasmonic photocatalysts in the visible. In the first part of this work, a new family of photochromic materials was synthesized combining POMs with sulfonium cations. The optical properties of these hybrid systems have been characterized and a new mechanism of photochromism was proposed. Alongside this work, the assembly of POMs with photoactive organic molecules (spiropyran and spironaphthoxazine) constitutes a second family of materials with exalted photochromic properties. The second part of this thesis is devoted to the study of the nanostructure Ag@Ag2Mo3O10 2H2O obtained by a new "all solid state" photo-deposition method from the silver molybdate Ag2Mo3O10 2H2O obtained by an unprecedented route under mild conditions. The Ag@Ag2Mo3O10 2H2O nanostructure appears to be an efficient visible-light-driven plasmonic photocatalyst.NANTES-BU Sciences (441092104) / SudocSudocFranceF

Na7 [SbW6O24 ]: a new type of turn-off luminescence humidity sensor based on a lanthanide-free polyoxometalate

International audienceA new type of turn-off photoluminescence (PL) humidity sensor based on a water-responsive anhydrous alkali lanthanide-free [SbW6O24](7-) polyoxometalate (POM) is reported. Na-7[SbW6O24] (NaSbW6) was recently obtained by heating its parent hydrate Na-7[SbW6O24]16H(2)O (NaSbW616H2O) at 200 degrees C. Upon exposure to a moisture-laden atmosphere at room temperature, the anhydrous phase converted back to the hydrated one, leading to a complete quenching of the intense PL intensity. The present work thoroughly investigates the dehydration/rehydration processes of these materials and their precise impact on their PL responses. First, the complete thermal dehydration of NaSbW616H2O involves the progressive formation of two intermediary hydrates, Na-7[SbW6O24]10H(2)O (NaSbW610H2O) and Na-7[SbW6O24]4H(2)O (NaSbW64H2O) which has been characterised by a combination of powder X-ray diffraction (PXRD) analysis, optical microscopy and FT-Raman spectroscopy at variable temperatures. The crystal structure of NaSbW610H2O has been solved from ab initio powder X-ray diffraction determination, revealing structural similiarities with NaSbW616H2O. The humidity-dependent stability of the (an)hydrated phases has been also investigated by dynamic vapor sorption (DVS) and PXRD analyses under variable relative humidity (RH). The room-temperature PL properties of NaSbW610H2O and NaSbW64H2O have been characterised, and their photophysical parameters determined and compared with those of NaSbW616H2O and NaSbW6 recently reported. The PL efficiency of the four materials gradually increases with decreasing their degree of hydration, due to the progressive deletion of non-radiative deactivation pathways caused by O-H vibrators connecting the emitting POM unit. NaSbW6 exhibits humidity-mediated transformations into the three hydrated phases, and its PL response to RH is linear in three distinct ranges. The best sensitivity is observed within the RH range of 0-23%, with a limit of detection of 2.2% RH

Structure–property relationships in normal and mixed dithienylethenes – polyoxometalates supramolecular assemblies with fast solid-state photochromic properties

International audienceFive new highly photochromic hybrid organic-inorganic materials were successfully prepared by supramolecular assembly of normal (1+) and mixed (2+) cationic dithienylethenes (DTEs) and polyoxometalates (POMs) units. Single-crystal X-ray diffraction studies reveal that (1)3[PM12O40]∙5ACN (M = W, Mo) (1-PW12 and 1-PMo12) and (2)3[PM12O40]∙5ACN (M = W, Mo) (2-PW12 and 2-PMo12) are isostructural, while (2)4[Mo8O26]4DMF (2-Mo8) is an isotype of (1)4[Mo8O26]4DMF (1-Mo8) i.e., the first POM-DTE ionic assembly, recently reported. The solid-state photochromic properties of these materials at room temperature are highly tunable with the nature of the DTEs and POMs used, which has been interpreted taking into account steric and electronic factors. Especially, while 1-PMo12 and 2-PMo12 exhibit negligible photoresponses which have not been considered herein, thorough investigations of the photocoloration and fading kinetics in ambient conditions have highlighted the precise impact of both organic and inorganic components on the photoswitching abilities of 1-Mo8, 2-Mo8, 1-PW12 and 2-PW12. Upon UV-light irradiation, the absorption of the normal and mixed DTEs in their closed-ring form exhibits a hypsochromic shift which is more pronounced in hybrid systems integrating -[Mo8O26]4- rather than -[PW12O40]3-, due to the higher negative charge density of the octamolybdate unit. The photocyclization rates of DTEs are also systematically increased when combined with the -[Mo8O26]4- unit, while in contrast, their back cycloreversion rates are not affected whatever the nature of the POMs used. In addition, within isostructural series, hybrids integrating mixed DTEs exhibit faster fading processes than their normal DTE counterparts when exposed to visible light. Finally, among this new series, 2-Mo8 shows the best photochromic performances with a fast photocoloration rate associated with a high coloration contrast, a fast and complete fading process and a high cyclability

Stabilization of β-octamolybdate with large counterions

International audienc

Incorporation of Jahn-Teller Cu(2+) Ions into Magnetoelectric Multiferroic MnWO4: Structural, Magnetic, and Dielectric Permittivity Properties of Mn1-xCuxWO4 (x ≤ 0.25).

Polycrystalline samples of Mn1-xCuxWO4 (x ≤ 0.5) have been prepared by a solid-state synthesis as well as from a citrate synthesis at moderate temperature (850 °C). The goal is to study changes in the structural, magnetic, and dielectric properties of magnetoelectric type-II multiferroic MnWO4 caused by replacing Jahn-Teller-inactive Mn(2+) (d(5), S = 5/2) ions with Jahn-Teller-active Cu(2+) (d(9), S = 1/2) ions. Combination of techniques including scanning electron microscopy, powder X-ray and neutron diffraction, and Raman spectroscopy demonstrates that the polycrystalline samples with low copper content 0 ≤ x ≤ 0.25 are solid solution that forms in the monoclinic P2/c space group. Rietveld analyses indicate that Cu atoms substitutes for Mn atoms at the Mn crystallographic site of the MnWO4 structure and suggest random distributions of Jahn-Teller-distorted CuO6 octahedra in the solid solution. Magnetic susceptibility reveals that only 5% of Cu substitution suppresses the nonpolar collinear AF1 antiferromagnetic structure observed in pure MnWO4. Type-II multiferroicity survives a weak Cu substitution rate (x < 0.15). Multiferroic transition temperature and Néel temperature increase as the amount of Cu increases. New trends in some of the magnetic properties and in dielectric behaviors are observed for x = 0.20 and 0.25. Careful analysis of the magnetic susceptibility reveals that the incorporation of Cu into MnWO4 strengthens the overall antiferromagnetic interaction and reduces the magnetic frustration

Persistent type-II multiferroicity in nanostructured MnWO4 ceramics

In archetypal ferroelectrics, the polarization and related properties result from structural lattice distortions.1 For example, in BaTiO3, it is the cationic off-centering versus the oxygen octahedra that is at the origin of the exceptional dielectric properties. Keeping these properties when downsizing such compounds is thus a challenge because the lattice elastic energy is disturbed by the surface energy even at relatively large grain size of several hundreds of nm..