1,273 research outputs found
Small Polarons in Transition Metal Oxide
Get PDFThe formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation, the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular focus on the archetypal polaron material TiO2. After an introductory part, surveying the fundamental theoretical and experimental aspects of the physics of polarons, the chapter examines how to model small polarons using first-principles schemes in order to predict, understand, and interpret a variety of polaron properties in bulk phases and surfaces. Following the spirit of this handbook, different types of computational procedures and prescriptions are presented with specific instructions on the setup required to model polaron effects
Assessing the performance of self-consistent hybrid functional for band gap calculation in oxide semiconductors
No full textIn this paper we assess the predictive power of the self-consistent hybrid functional scPBE0 in calculating the band gap of oxide semiconductors. The computational procedure is based on the self-consistent evaluation of the mixing parameter μ by means of an iterative calculation of the static dielectric constant using the perturbation expansion after discretization method and making use of the relation . Our materials dataset is formed by 30 compounds covering a wide range of band gaps and dielectric properties, and includes materials with a wide spectrum of applications such as thermoelectrics, photocatalysis, photovoltaics, transparent conducting oxides, and refractory materials. Our results show that the scPBE0 functional provides better band gaps than the non self-consistent hybrids PBE0 and HSE06, but scPBE0 does not show significant improvement on the description of the static dielectric constants. Overall, the scPBE0 data exhibit a mean absolute percentage error of 14% (band gaps) and 10% (ε ∞). For materials with weak dielectric screening and large excitonic biding energies scPBE0, unlike PBE0 and HSE06, overestimates the band gaps, but the value of the gap becomes very close to the experimental value when excitonic effects are included (e.g. for SiO2). However, special caution must be given to the compounds with small band gaps due to the tendency of scPBE0 to overestimate the dielectric constant in the proximity of the metallic limit
Archivio digitale "Cesare Lombroso"
No full textclicca sull'immagine per ingrandire La Biblioteca "L. Bergamini" dell'Università di Torino ha ereditato molti testi risalenti al periodo in cui Cesare Lombroso era direttore della Clinica Psichiatrica della Regia Università di Torino. Si tratta sia di scritti di Lombroso sia di libri, opuscoli ed estratti da lui utilizzati nel proprio lavoro scientifico e clinico. Ecco l'elenco dei testi digitalizzati (file PDF): Cesare Lombroso, Sulla medicina legale del cadavere secondo gli ultimi studi di..
Low-dimensionality and epitaxial stabilization in metal supported oxide nanostructures: MnxOy on Pd(100)
No full textThis chapter presents a survey of the growth and structure of manganese oxide nanolayers on a Pd(100) substrate, investigated in two different thickness regimes through a plethora of surface science techniques (scanning tunneling microscopy, atomic force microscopy (AFM), low energy electron diffraction (LEED), SPA-LEED, X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS), and high-resolution electron energy loss spectroscopy) and state-of-the-art theoretical tools based on density functional theory and hybrid functionals. The electronic and structural properties of the films are analyzed as a function of film thickness and growth conditions. Epitaxial (geometric) relationships that favor the growth of the different oxide phases are investigated, with special attention to the stability of the Mn3O4 (001)/MnO(001) interface and the phase stability diagram of Mn x O y /Pd(100) phases at a Mn coverage of about one monolayer. A rich variety of two-dimensional (2D) nanophases, which are novel in terms of their structural and electronic properties, have been identified, which could play an important role in mediating the epitaxial growth of MnO thicker films on Pd(100). Furthermore, the formation of O or Mn vacancies drives the transition between 2D phases with similar structural units but different lattice periodicity, indicating that ion vacancies, mixed valence states, and substoichiometry lie on the basis of the architectural flexibility in the monolayer regime. Interestingly, the latter concepts play a major role in the more complex class of functional oxides such as the manganites, of which binary manganese oxides are the simplest parent compounds
Challenges and Opportunities in Modeling Oxides for Energy and Information Devices
No full textThe growth of computational resources has enabled investigations of large-scale and highly correlated problems by using first principles computational techniques such as density functional theory (DFT). In context of oxide materials, these problems include oxide surface reconstructions (Diebold et al. 2010), diffusion and reaction barriers in heterogeneous systems (Chizallet and Raybaud 2014; Aksyonov et al. 2018), phase diagrams for transition metal oxides (Park et al. 2014; Leonov 2015), and point defects as well as extended defects (Youssef and Yildiz 2012; Sun et al. 2015). These developments have opened up new opportunities for predicting not only the bulk crystal properties of oxides, but also the effect of complex microstructures such as associated point defects (Hu et al. 2013; Liu et al. 2012; Zhang et al. 2014; T-Thienprasert et al. 2012), grain boundaries (Polfus et al. 2012; McKenna and Shluger 2009; Hojo et al. 2010), dislocations (Sun et al. 2015; Hojo et al. 2011; McKenna 2013), and surfaces (Lee and Morgan 2015; Freysoldt and Neugebauer 2018; Bajdich et al. 2015) under thermodynamic drivers. These developments can ultimately allow for ab initio prediction of realistic device performance. Yet, challenges remain on both the theoretical and algorithmic level to accurately predict oxide materials properties on a complex potential energy surface. Here we summarize several growing fields in addressing these challenges and present our perspectives on future directions that these methods will enable
Ab initio prediction of the high-pressure phase diagram of BaBiO3
Get PDFBaBiO3 is a well-known example of a 3D charge density wave (CDW) compound, in which the CDW behavior is induced by charge disproportionation at the Bi site. At ambient pressure, this compound is a charge-ordered insulator, but little is known about its high-pressure behavior. In this work, we study from first principles the high-pressure phase diagram of BaBiO3
using phonon mode analysis and evolutionary crystal structure prediction. We show that charge disproportionation is very robust in this compound and persists up to 100 GPa. This causes the system to remain insulating up to the highest pressure we studied
Dimensionality-strain phase diagram of strontium iridates
No full textThe competition between spin-orbit coupling, bandwidth (W), and electron-electron interaction (U) makes iridates highly susceptible to small external perturbations, which can trigger the onset of novel types of electronic and magnetic states. Here we employ first principles calculations based on density functional theory and on the constrained random phase approximation to study how dimensionality and strain affect the strength of U and W in (SrIrO3)m/(SrTiO3) superlattices. The result is a phase diagram explaining two different types of controllable magnetic and electronic transitions, spin-flop and insulator-to-metal, connected with the disruption of the Jeff=1/2 state which cannot be understood within a simplified local picture
Ab initio study of ABiO3 (A=Ba, Sr, Ca) under high pressure
Get PDFUsing ab initio crystal structure prediction we study the high-pressure phase diagram of ABiO(3) bismuthates (A = Ba, Sr, Ca) in a pressure range up to 100 GPa. All compounds show a transition from the low-pressure perovskite structure to highly distorted, low-symmetry phases at high pressures (PD transition), and remain charge-disproportionated and insulating up to the highest pressure studied. The PD transition at high pressures in bismuthates can be understood as a combined effect of steric arguments and of the strong tendency of bismuth to charge-disproportionation. In fact, distorted structures permit to achieve a very efficient atomic packing, and at the same time, to have Bi-O bonds of different lengths. The shift of the PD transition to higher pressures with increasing cation size within the ABiO(3) series can be explained in terms of chemical pressure
Polaronic Hole Trapping in Doped BaBiO3
No full textThe present ab initio study shows that in BaBiO3, Bi3+ sites can trap two holes from the valence band to form Bi5+ cations. The trapping is accompanied by large local lattice distortions; therefore the composite particle consisting of the electronic hole and the local lattice phonon field forms a polaron. Our study clearly shows that even sp elements can trap carriers at lattice sites, if local lattice relaxations are sufficiently large to screen the localized hole. The derived model describes all relevant experimental results, and settles the issue of why hole-doped BaBiO3 remains semiconducting upon moderate hole doping
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