Induced p-type semiconductivity in Mg-doped Nd2Zr2O7 pyrochlore system

Heterovalent B-site MgO substitution in the Nd2Zr2O7-system (Nd2Zr2−xMgxO7−x) has been explored. The pyrochlores were synthesized by a polymeric sol-gel method and characterized by X-ray diffraction (XRD), Raman spectroscopy and Scanning Electron Microscopy to determine structure, phase composition and microstructure. Impedance Spectroscopy (IS) was employed to study the electrical behavior of the ceramics over the ranges 200–800 °C and under pure N2 and O2. The XRD showed that the solid solution limit was x > 0.02 and all the materials show a cubic ̅ structure. The Raman results confirm the structural disorder created by the introduction of Mg2+ and the subsequent generation of oxygen vacancies. The IS data shows a dramatical increase of the oxide-ion conductivity when doping and that the conductivity depends strongly on the atmosphere, leading to p-type semiconductivity under pure O2 atmosphere. The present study highlights the use of heterovalent dopants to drastically increase the oxide-ion conductivity of pyrochlore-like materials

Atmosphere- and Voltage-Dependent Electronic Conductivity of Oxide-Ion-Conducting Zr1-xYxO2-x/2 Ceramics.

Cubic, fluorite-structured solid solutions Zr1-xYxO2-x/2 (YSZ; x = 0.4-0.7) were prepared by sol-gel synthesis. Impedance measurements on pellets of 85% approximate density sintered at 1300 °C for 24 h showed strong evidence of oxide ion conduction with an inclined Warburg spike at low frequencies and capacitance values of ∼10(-6) F cm(-1) at 40 Hz. Arrhenius plots of total pellet conductivities were linear with activation energies of 1.4-1.56 eV. The conductivity decreased with x and was 2-4 orders of magnitude lower than that with optimized YSZ, x = 0.08. When the atmosphere was changed from N2 to O2 during impedance measurements, two reversible effects were seen: the Warburg spike contracted greatly, and the sample resistance decreased. These effects were more noticeable at higher x and are attributed to the introduction of p-type electronic conduction, in parallel with the preexisting oxide ion conduction. A similar reversible result was observed upon application of a direct-current (dc) bias during impedance measurements. When either pO2 is increased or a dc bias is applied, hole creation is believed to arise by the ionization of underbonded oxide ions situated near the Y(3+) dopant ions. The ionized electrons are trapped at surface oxygen species, and the holes that are left on oxygen are responsible for p-type conduction. The electrolytic domain of x = 0.4-0.7 extends up to approximately 10(-2) atm of O2 before p-type conduction is observed. The upper pO2 limit of the electrolytic domain of x = 0.08 is not known but is likely to be close to or slightly above 1 atm of O2

Site-selective symmetries of Eu3+-doped BaTiO3 ceramics: a structural elucidation by optical spectroscopy

Eu3+-Doped BaTiO3 ceramics with dopant contents between 0 and 10 mol% were prepared by sol–gel synthesis based on the nominal compositions (Ba1−3xEu2x)TiO3 and Ba(Ti1−xEux)O3−x/2, where two possible substitution mechanisms are addressed. By means of optical spectroscopy, our study gives a plausible elucidation of Eu3+ site occupation in micron-sized BaTiO3 particles. Time-resolved fluorescence line narrowing (TRFLN) shows the presence of five different crystal field sites for europium ions and possible symmetries are inferred for each one. The solubility limit of the lanthanide ion was found to be about 3 mol%. The experimental results are consistent with the preference of Eu3+ to occupy Ba2+ sites regardless of the nominal compositions and target substitution mechanism. However, TRFLN results showed that the dopant could also occupy Ti4+ sites, highlighting the amphoteric character of Eu3+. The existence of anti-Stokes and Stokes vibronic sidebands in the 5D0 → 7F0,1 transitions of Eu3+ ions is confirmed. This can explain the lack of resolution found in room temperature spectra of these transitions due to vibronic mixing of the excited levels. The existence of non-equivalent europium sites with different spectroscopic properties could have an impact on the optical properties of doped-BaTiO3 ceramics and associated applications

Síntesis de nuevos pigmentos cerámicos de color rojo mediante el método de Pechini

Various garnet solid solutions within the structure of the yttrium aluminium garnet, Y3Al5O12 (YAG) were sinthesized via Pechini and the possibility for the modification of the crystal field intensity acting on the chromophore Cr(III) ion by means of substitution with divalent (Mn(II) and Mg(II)) and tetravalent (Ce(IV), Zr(IV) and Sn(IV)) cations was studied. The results obtained indicate that the crystal field can not be intensifyed through the introduction of divalent ions since they stabilize the Cr(IV) ion which probably expands the structure by inducing distortions in it. The introduction of tetravalent ions makes possible the intensification of the crystal field acting on the chromium and the change of the typical green colour of Cr(III) in a week field to red which is typical for the same in strong field. However, the structural studies carried out by X-ray diffraction show that together with the garnet fase Y0,15Zr0,85O1,93 and Y2Sn2O7 are formed, the latter having pyrochlore structure. It was demonstarted that the introduction of Cr(III) in the mantioned pyrochlore structure results in the formation of a redviolet pigment. Finally, the sinthesized fases were checked as pigments using an industrial transparent boron containing frit. The tin containing pigments demonstrated good pigmenting ability and stability at 1080ºC.<br><br>A través del presente trabajo, se han estudiado la obtención de diferentes disoluciones sólidas en la estructura granate de itrio y aluminio, Y3Al5O12 (YAG) sintetizadas vía Pechini y la posibilidad de modificar la intensidad del campo cristalino, Do, que actúa sobre el ion Cr(III), como agente cromóforo, mediante la introducción de cationes divalentes (Mn(II) y Mg(II)) y tetravalentes (Ce(IV), Zr(IV) y Sn(IV)). Los resultados alcanzados han posibilitado comprender que con la introducción de los iones divalentes, éstos no han sido suficientes para intensificar dicho campo del Cr(III), ya que estabilizan al ion Cr(IV) y esto provoca distorsiones estructurales. Con la introducción de los iones tetravalentes, sí que se consigue intensificar el campo que actúa sobre el ion cromóforo, observándose la transición de la tonalidad verde, típica del Cr(III) en un campo débil, a una tonalidad roja, típica para el mismo ion en un campo fuerte. Sin embargo y a través de los estudios estructurales mediante difracción de rayos X, se ha observado que junto con el desarrollo de la estructura granate, disolución sólida, aparecen fases laterales de Y0,15Zr0,85O1,93 y Y2Sn2O7, esta última como fase pirocloro. Se ha comprobado la obtención de una tonalidad roja-violácea con la introducción de Cr(III) en dicha estructura pirocloro. Por último, se han probado las fases sintetizadas como pigmentos mediante la utilización de un vidriado (frita) transparente de naturaleza borácica, industrial. Los materiales que contienen estaño en su estructura han mostrado un buen poder pigmentante y estabilidad a 1080ºC

Optical response of Ce(III) and Eu(II) doped hybrid materials synthesised by Sol-Gel processing

This study deals with the preparation of two hybrid organic-inorganic matrices via sol–gel, starting from alkylalkoxisilanes Si(CH3)(OCH2CH3)3 (MTEOS)–SiH(CH3)(OCH2CH3)2 (MDES), A system, and SiH(CH3)(OCH2CH3)2 (MDES) SiH(OCH2CH3)3 (TREOS), B system, together with zirconium n-propoxide. Another type-A is carried out by adding acetylacetone, A-acac system. The matrices are characterised by infrared spectroscopy, nuclear magnetic resonance (NMR-MAS), and chemical analysis. Optical characteristics of the matrices have been studied. A-acac and B matrices are doped with an Eu(III) salt, and A and B matrices are doped with a Ce(IV) salt. Absorption and emission studies show the presence of Eu(II) and Ce(III). The transition metal alkoxide that catalysed cleavage of the Si–H bonds was used to reduce in situ at room temperature, the rare earth cations. Depending on chemical strategy, the resulting hybrid materials can be processed as transparent bulks or coatings which exhibit a good transparency in the UV–visible domain. Both the undoped and the rare earth doped matrices exhibit a strong blue emission.<br><br>En este trabajo se aborda la preparación de dos matrices híbridas orgánico-inorgánicas por vía sol-gel, a partir de mezclas de alquilalcoxisilanos Si(CH3)(OCH2CH3)3 (MTEOS)–SiH(CH3)(OCH2CH3)2 (MDES), sistema A, y SiH(CH3)(OCH2CH3)2 (MDES)–SiH(OCH2CH3)3 (TREOS), sistema B, en presencia de n-propóxido de circonio. Se efectúa una variación del sistema A por adición de acetilacetona, sistema A-acac. Las matrices se caracterizan por espectroscopia infrarroja, resonancia magnética nuclear (RMN-MAS) y análisis químico. Se estudian las características ópticas de los materiales obtenidos. Las matrices A-acac y B se dopan con una sal de Eu(III) y las matrices A y B con una sal de Ce(IV). Los estudios de absorción y emisión indican la presencia de Eu(II) y Ce(III), es decir estos estados de oxidación se han generado in situ a temperatura ambiente en los sistemas estudiados, debido a la presencia de hidrógeno generado procedente de la rotura de los enlaces Si–H catalizada por la presencia de n-propóxido de circonio. Dependiendo del método de síntesis, los materiales híbridos resultantes pueden constituirse en forma de monolitos, películas, fibras, etc. transparentes en el intervalo UV–visible. Tanto las matrices sin dopar con las dopadas con iones de tierras raras exhiben una fuerte emisión azul

Laser and electron beam-induced formation of Ag/Cr structures on Ag2CrO4

The interactions of silver chromate (Ag2CrO4) with a femtosecond (fs) laser and electron beam irradiations were investigated. For the first time, the growth and coalescence of metallic Ag nanoparticles (NPs) on an Ag2CrO4 surface via fs laser irradiation can be reported. Furthermore, electron beam irradiation causes a segregation process of Ag NPs in which Ag nanofilaments are obtained. The Ag2CrO4 particles were characterized using X-ray diffraction (XRD), micro-Raman spectroscopy (Raman), field-emission scanning electron microscopy (FE-SEM), high-resolution electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). According to the results, the fs/electron beam irradiations of Ag2CrO4 are environmentally friendly, fast, and effective methods to produce metallic Ag NPs. Both strategies have enormous potential for the synthesis and morphological control of Ag NPs on templates or biotemplates, core-shells, decorations, and composites based on Ag2-xCrO4:Ag materials, which are difficult to synthesize using conventional chemical and physical methods

Towards the scale-up of the formation of nanoparticles on α-Ag2WO4 with bactericidal properties by femtosecond laser irradiation

Abstract In recent years, complex nanocomposites formed by Ag nanoparticles coupled to an α-Ag2WO4 semiconductor network have emerged as promising bactericides, where the semiconductor attracts bacterial agents and Ag nanoparticles neutralize them. However, the production rate of such materials has been limited to transmission electron microscope processing, making it difficult to cross the barrier from basic research to real applications. The interaction between pulsed laser radiation and α-Ag2WO4 has revealed a new processing alternative to scale up the production of the nanocomposite resulting in a 32-fold improvement of bactericidal performance, and at the same time obtaining a new class of spherical AgxWyOz nanoparticles

Synthesis, antifungal evaluation and optical properties of silver molybdate microcrystals in different solvents: a combined experimental and theoretical study

International audienceIn this study, we investigate the structure, antifungal activity, and optical properties of beta-Ag2MoO4 using experimental and theoretical approaches. beta-Ag2MoO4 samples were prepared by a co-precipitation method using different solvents (water, ethanol and ammonia), and their antifungal activity against Candida albicans was investigated. The samples were characterized by X-ray diffraction, micro-Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy with energy dispersive spectroscopy. The optical properties were investigated by UV-Vis spectroscopy and photoluminescence measurements at room temperature. The thermodynamic equilibrium shape of the beta-Ag2MoO4 crystals was determined based on the surface energies calculated using Wulff construction. The (011) orientation was the predominant surface in the morphology. The experimental morphology was obtained by varying the surface energy ratio for each facet. A large decrease in surface energy for the (111) surface provided the experimental morphology for crystals synthesized using water and ethanol as solvents; when the surface energies for both (011) and (001) surfaces increased, the crystal morphology obtained using ammonia as a solvent was reproduced. A correlation between the exposed surfaces and antifungal activity was revealed, and an explanation to this behavior that arises from different morphologies and structural data was provided. Theoretical calculations confirm the rationality of the experimental scheme and elucidate the underlying reason for the fungistatic and fungicidal activity against Candida albicans