Intercalation processes and diffusion paths of lithium ions in spinel-type structured Li1+xTi2O4: density functional theory study

Intercalation processes and corresponding diffusion paths of Li ions into spinel-type structured Li1+xTi2O4 0 x 0.375 are systematically studied by means of periodic density functional theory calculations for different compositions and arrangements. An analysis of the site preference for intercalation processes is carried out, while energy barriers for the diffusion paths have been computed in detail. Our results indicate that the Li insertion is thermodynamically favorable at octahedral sites 16c in the studied composition range, and Li migration from tetrahedral sites 8a to octahedral sites 16c stabilizes the structure and becomes favorable for compositions x 0.25. Diffusion paths from less stable arrangements involving Li migrations between tetrahedral and octahedral sites exhibit the lowest energy barrier since the corresponding trajectories and energy profiles take place across a triangle made by three neighboring oxygen anions without structural modification. Theoretical and experimental diffusion coefficients are in reasonable agreemen

Visible PL phenomenon at room temperature in disordered structure of SrWO 4 powder

Abstract. The SrWO 4 (SWO) powders were synthesized by the polymeric precursor method and annealed at different temperatures. The SWO structure was obtained by X-ray diffraction and the corresponding photoluminescence (PL) spectra was measured. The PL results reveal that the structural order-disorder degree in the SWO lattice influences in the PL emission intensity. Only the structurally order-disordered samples present broad and intense PL band in the visible range. To understand the origin of this phenomenon, we performed quantum-mechanical calculations with crystalline and order-disordered SWO periodic models. Their electronic structures were analyzed in terms of band structure. The appearance of localized levels in the band gap of the order-disordered structure was evidenced and is a favorable condition for the intense PL to occur

Estudo mecânico-quântico ab initio da propriedade fotoluminescente em compostos PbWO4, BaWO4, SrWO4 e dos processos de intercalação e difusão de Li no composto Li1+xTi2O4

Computational simulation by means of quantum-mechanical calculations is becoming an increasingly important tool in scientific research of materiaIs due the great advances in the performance of computers and in the development of more efficient algorithms, providing a deeper understanding of nanoscale mechanisms which often cannot be detected directly by the experimental measures. The development of materiaIs with efficient optical properties is of great industrial interest. The photoluminescence (PL), one important optical property, has been intensively explored in scientific optical research and in technological applications. The literature presents that the most intense PL emissions of the compounds PbWO4, Ba WO4 and SrWO4 is favored by structural disorder in its lattice. Results of computeI simulations of structurally disordered compounds PbWO4, BaWO4 and SrWO4 by means of quantum-mechanical ab initio calculations were presented in this thesis, in arder to interpret why these disordered structures favor the most intense PL emissions. The calculations indicated that these structurally disordered compounds favor intense PL emissions because contain non-homogeneous distribution of charges in its lattice causing the generation of trapped electronic boles, thus, providing electron-hole recombinations to intense and broad PL emissions. The progress of research in batteries based on lithium-ion (Li batteries) has aIso great technological importance due the commercial demand of portable devices. In particular the compound Li1+xTi2O4 (O ≤X ≤ 1) is a perspective material for application in Li batteries due to its high fatia energy storagejweight. Results of computational simulations using quantum-mechanical ab initio calculations in processes of Li intercalation and diffusion in the structure Li1+xTi2O4 (0 ≤ X ≤ 0.375) were also presented in this work, in arder to identify the most favorable migration paths of considered different concentrations, and that such identification is not experimentally possible. The calculations of the processes of Li intercalation and diffusion in the structure Li1+xTi2O4 indicate that the Li insertion is energetically favorable in alI range of concentration x studied and the Li migration are favorable only for x 2: 0.25. Quantum-mechanical calculations aIso indicate the more favorable migration paths for each concentration x considered, and that migration of lithium is more favorable in less stable local Li arrangements because the energy cost for lithium migrating in more stable arrangements is greater than the energy cost for lithium migrating in less stable arrangements.Universidade Federal de Sao CarlosCom os grandes avanços no desempenho de computadores e no desenvolvimento de algoritmos cada vez mais eficientes, a simulação computacional por meio de cálculos mecânico-quânticos vem se tornando uma ferramenta cada vez mais importante na pesquisa científica de materiais, proporcionando um entendimento mais profundo de mecanismos microscópicos, que muitas vezes não podem ser detectados diretamente por medidas experimentais. O desenvolvimento de materiais com eficientes propriedades óticas é de grande interesse industrial. A fotoluminescência (FL), uma das principais propriedades óticas, vem sendo explorada intensivamente tanto em pesquisas científicas quanto em aplicações tecnológicas. São evidenciados na literatura que a desordem estrutural nos compostos PbW04, BaW04 e SrW04 favorecem as mais intensas emissões FL. Nesta tese foram apresentados resultados de simulações computacionais das estruturas desordenadas dos compostos PbWO4, BaW04 e SrWO4 por meio de cálculos mecânicoquânticos ab initio, com o objetivo de interpretar o porquê estas estruturas favorecem as mais intensas emissões FL. Os cálculos apontaram que estes compostos desordenados estruturalmente favorecem intensas emissões FL porque suas redes contêm distribuição não homogênea de cargas que provocam a geração de buracos eletrônicos armadilhados, proporcionando assim, recombinações elétron-buraco para a emissão FL. O avanço da pesquisa em baterias à base de íon de lítio (baterias de Li) também é de grande importância tecnológica devido principalmente o crescimento na demanda de dispositivos portáteis. Em particular o composto Lil+x Ti2O4 (O ≤ X ≤ 1) é um perspectivo material para aplicação em baterias de Li devido a sua alta razão de armazenagem energia/peso. Neste trabalho também foram apresentados resultados de simulações computacionais por meio de cálculos mecânico-quânticos ab initio de processos de intercalação e difusão de Li na estrutura Lil+xTi2O4 (O ≤ X ≤ 0.375), com o objetivo de identificar os caminhos de migrações mais favoráveis nas diferentes concentrações consideradas, sendo que atualmente tal identificação não é possível experimentalmente. Os cálculos dos processos de intercalação e difusão de Li na estrutura Lil+x TbO4 apontaram que a inserção de Li é favorável energeticamente para todo o intervalo de concentração x estudado e as migrações de Li são favoráveis somente a partir de x 2: 0.25. Cálculos mecânico-quânticos também indicaram os caminhos de migrações mais favoráveis para cada concentração .T considerada, e que migrações de lítios são mais favoráveis em arranjos locais de Li menos estáveis, porque o custo energético de migrar lítios de arranjos mais estáveis é maior do que migrar lítios de arranjos menos estáveis

Visible PL phenomenon at room temperature in disordered structure of SrWO4 powder

International audienceThe SrWO4 (SWO) powders were synthesized by the polymeric precursor method and annealed at different temperatures. The SWO structure was obtained by X-ray diffraction and the corresponding photoluminescence (PL) spectra was measured. The PL results reveal that the structural order-disorder degree in the SWO lattice influences in the PL emission intensity. Only the structurally order-disordered samples present broad and intense PL band in the visible range. To understand the origin of this phenomenon, we performed quantum-mechanical calculations with crystalline and order-disordered SWO periodic models. Their electronic structures were analyzed in terms of band structure. The appearance of localized levels in the band gap of the order-disordered structure was evidenced and is a favorable condition for the intense PL to occur

Contribution of structural order–disorder to the room-temperature photoluminescence of lead zirconate titanate powders

International audienceIntense and broad visible photoluminescent (PL) band was observed at room temperature in structurally disordered PbZr0:53Ti0:47O3 powders. The lead zirconate titanate PbZr0:53Ti0:47O3 powders prepared by the polymeric precursor method and heat treated at different temperatures were structurally characterized at long range by means of X-ray diffraction. The PL was measured at room temperature samples heat treated at different temperatures. Experimental measurements and quantum-mechanical calculations showed that the high structural order and the high structural disorder in PbZr0:53Ti0:47O3 lattice are not favorable to the intense PL emission. Only samples containing simultaneous structural order and disorder in their lattice present the intense visible PL emission at room temperature

Photoluminescent property of mechanically milled Ba Wo4 powder

International audienceCrystalline BaWO4 (BWO) powder obtained by the polymeric precursor method was structurally disordered by means of high-energy mechanical milling. For the first time a strong and broad photoluminescence (PL) has been measured at room temperature for mechanically milled BWO powder and interpreted by ground-state quantum mechanical calculations in the density functional theory framework. Two periodic models have been studied; one representing the crystalline form and the other one representing the disordered BWO powder. These models allowed the calculation of electronic properties, which are consistent with the experimental results, showing that structural disorder in the lattice is an important condition to generate an intense and broad PL band

The Role of Short-Range Disorder in BaWO4 Crystals in the Intense Green Photoluminescence

An intense and broad visible photoluminescence (PL) band was observed at room temperature in short-range disordered BaWO4 crystals. The scheelite crystalline BaWO4 powders prepared by the polymeric precursor method and annealed at different temperatures were structurally characterized by means of X-ray diffraction and Fourier transform (FT) Raman spectroscopy measurements. Quantum-mechanical calculations indicated that the disorder at short range only of Ba atoms in the BaWO4 lattice has a very important role in the charge transfer involved in the intense green PL emission. This indication was detected and confirmed by means of FT Raman spectroscopy measurements. The experimental and theoretical results are in good agreement, both showing the specific type of structural disorder that is the most favorable condition for generating the most intense green PL emission in the scheelite BaWO4 lattice.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação Amazônia Paraense de Amparo à Pesquisa (FAPESPA

Intense and broad photoluminescence at room temperature in structurally disordered Ba[Zr0.25Ti0.75]O-3 powders: An experimental/theoretical correlation

Intense and broad photoluminescence (PL) emission at room temperature was observed on structurally disordered Ba[Zr0.25Ti0.75]O-3 (BZT) powders synthesized by the polymeric precursor method. BZT powders were annealed at 573 K for different times and at 973 K for 2 h in oxygen atmosphere. The single-phase cubic perovskite structure of the powder annealed at 973 K for 2 It was identified by X-ray diffraction and Fourier transform Raman techniques. PL emission increased with the increase of annealing time, which reached its maximum value in the powder annealed at 573 K for 192 h. First principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and disordered models. The theoretical calculations and experimental measurements of Ultraviolet-visible absorption spectroscopy indicate that the presence of intermediary energy levels in the band gap is favorable for the intense and broad PL emission at room temperature in disordered BZT powders. The PL behavior is probably due the existence of a charge gradient on the disordered structure, denoted by means of a charge transfer process from [TiO5]-[ZrO6] or [TiO6]-[ZrO5] clusters to [TiO6]-[ZrO6] clusters. (C) 2008 Elsevier Ltd. All rights reserved

Contribution of structural order-disorder to the green photoluminescence of PbWO4

International audienceAn intense and broad visible photoluminescence PL band was observed at room temperature in structurally disordered PbWO4 thin films. The scheelite lead tungstate PbWO4 films prepared by the polymeric precursor method and annealed at different temperatures were structurally characterized by means of x-ray diffraction and atomic force microscopy analysis. Quantum-mechanical calculations showed that the local disorder of the network modifier Pb has a very important role in the charge transfer involved in the green PL emission. The experimental and theoretical results are in good agreement, both indicating that the generation of the intense visible PL band is related to simultaneous structural order and disorder in the scheelite PbWO4 lattice