Charge Delocalization, Oxidation States, and Silver Mobility in the Mixed Silver-Copper Oxide AgCuO2

The electronic structure of AgCuO2, and more specifically the possible charge delocalization and its implications for the transport properties, has been the object of debate. Here the problem is faced by means of first-principles density functional theory calculations of the electron and phonon band structures as well as molecular dynamics simulations for different temperatures. It is found that both Cu and Ag exhibit noninteger oxidation states, in agreement with previous spectroscopic studies. The robust CuO2 chains impose a relatively short contact distance to the silver atoms, which are forced to partially use their dz2 orbitals to build a band. This band is partially emptied through overlap with a band of the CuO2 chain, which should be empty if copper were in a Cu3+ oxidation state. In that way, although structural correlations could roughly be consistent with an Ag+Cu3+O2 formulation, the appropriate oxidation states for the silver and copper atoms become Ag(1+δ)+ and Cu(3−δ)+, and as a consequence, the stoichiometric material should be metallic. The study of the electronic structure suggests that Ag atoms form relatively stable chains that can easily slide despite the linear coordination with oxygen atoms of the CuO2 chains. Phonon dispersion calculations and molecular dynamics simulations confirm the stability of the structure although pointing out that sliding of the silver chains is an easy motion that does not lead to substantial modifications of the electronic structure around the Fermi level and, thus, should not alter the good conductivity of the system. However, this sliding of the silver atoms from the equilibrium position explains the observed large thermal factors

Energy transfer, structural and luminescent properties of the color tunable Phosphor Y2WO6:Sm3+

Inorganic phosphors based on monoclinic Y2WO6 doped with Sm3þ ions were prepared via conventional solid-state reactions at high temperature. A total of six samples were obtained with different Sm3þ concentrations (0e9%). The purity of the as-prepared phases was checked by powder X-ray diffraction (PXRD). The excitation, emission, and time-resolved emission spectra were examined in detail. The experimental decay curves were fitted to the InokutieHirayama model, showing that the Sm3þ ions form clusters at all compositions. Periodic hybrid density functional theory calculations were also carried out on the undoped material and on 144-atom supercells of stoichiometry Y1.9375Sm0.0625WO6 and Y1.875Sm0.125WO6. The different coordination environments at the Y1, Y2 and Y3 sites are analysed in detail. The calculated structure and band gap of Y2WO6 are in good agreement with experiment with one potentially important discrepancy in a Y3eO bond length. The thermodynamically favoured substitution sites for Sm, Y2 and Y3, in the supercell are not those observed under the preparation conditions used here, since the experimental Rietveld analysis suggests occupation of Y1 at all concentrations. Analogous calculations for the Eu- doped system highlight marked differences between Eu and Sm despite their similar ionic sizes. The calculated densities of states show the position of the 4f levels in the band gap depend on the sites occupied by the dopants and thus expect marked differences in the luminescence spectra, opening up possibilities for tuning device performance

Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxy- nitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON2, which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scan- ning transmission electron microscopy indicate that it crystal- izes in the non-centrosymmetric space group P63mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX3 layers. A synergetic second-order Jahn-Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W6+ cations, induce large distortions at two inequiva- lent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity

Estructura Electrónica de Sólidos Inorgánicos

Los sulfuros de metales de transición exhiben una amplia variedad de propiedades eléctricas y magnéticas que los hacen interesantes tanto en el ámbito científico como en el de las aplicaciones tecnológicas. En este trabajo se estudia, utilizando cálculos ab intio, la estructura electrónica y las propiedades de un conjunto de calcogenuros, la mayoría sulfuros de cobre complejos, para intentar relacionar su estructura electrónica con las propiedades de transporte electrónico y magnéticas. En el primer capítulo se presenta una metodología muy simple para asignar estados de oxidación de metales de transición en compuestos cristalinos que consiste en el análisis conjunto de las energías de los orbitales de core y la población de la capa d de valencia obtenidas en un cálculo de bandas. El método se ha aplicado con éxito a los sulfuros de cobre para los cuales es difícil establecer estados de oxidación de manera fiable a partir de datos experimentales y se ha extendido a compuestos de plata, en particular para elucidar los estados de oxidación de los metales en algunos óxidos mixtos de cobre y plata. Paralelamente se han estudiado también las propiedades magnéticas y electrónicas de CuFeS2 y CuS, dos sulfuros de cobre con propiedades eléctricas y magnéticas complejas para los que se encuentran ciertos datos contradictorios en la literatura. En particular, para la covelina, CuS, se han estudiado aspectos de su estructura electrónica con la finalidad de analizar si las interacciones débiles d10-d10 juegan un papel importante o no en la transición estructural que sufre este compuesto a baja temperatura y para CuFeS2 el trabajo se ha centrado en la determinación de las constantes de acoplamiento magnético para intentar explicar la curva compleja de susceptibilidad magnética que se encuentra para este compuesto. En la presente tesis también se incluye un estudio en el que se intenta predecir si los sulfuros cuaternarios laminares AFeCuS2 con estructura tipo ThCr2Si2 son capaces de soportar litio, tal como lo hacen los sulfuros cuaternarios con estructura CaAl2Si2. En este estudio también se explica porqué no es estable la estructura de tipo CaAl2Si2 cuando se elimina todo el litio de la fase laminar LiFeCuS2. Por último, se presenta un estudio de la estructura electrónica de los calcogenuros TaS3, NbSe3 y TaSe3. El compuesto NbSe3 es un conductor metálico para el cual hay cierta controversia en la literatura sobre si en su estructura de bandas hay cuatro o cinco bandas cruzando el nivel de Fermi. A raíz de un experimento reciente mediante la técnica ARPES en el que se muestra, sin lugar a dudas, que este compuesto presenta cuatro bandas que cruzan el nivel de Fermi hemos realizado un estudio pormenorizado de sus estructura electrónica junto a la de TaS, y TaSe3 en el que se pone de manifiesto que realmente solo hay cuatro bandas cruzando el nivel de Fermi pero que, debido a la importancia de las interacciones débiles Se•••Se que no se describen correctamente en la teoría del funcional de la densidad en algunos cálculos puede llegar a aparecer una quinta banda cruzando el nivel de Fermi.In this thesis we have applied first principles electronic structure methods to study the electronic structure of several transition metal chalcogenides in order to establish the relation between their electronic structure and their electrical and magnetic properties. In the first chapter we develop a new strategy based on the simultaneous use of core orbital energies and valence d shell populations to establish oxidation states of transition metal atoms in complex solid state compounds. As a test case we have successfully applied this new method to copper sulfides and mixed copper-silver oxides. The second and third chapters are devoted to the study of the electronic structure of two copper sulphides, CuS and CuFeS2. For the first one, our analysis focuses on the role that weak d10•••d10 interactions may play in the structural phase transition exhibited by this compound, while for the latter our study deals with the evaluation of exchange coupling constants in order to understand its complex magnetic susceptibility curve. The thesis finishes with a chapter devoted to TaS3, NbSe3 y TaSe3, three pseudo 1D transition metal chalcogenides that have been extensively studied both from the theoretical and experimental points of views. Our main motivation has been the presentation of a recent ARPES study for NbSe3 where it is shown that there are only four bands crossing the Fermi level, a question that is unclear in the literature where in some cases the presence of a fifth band is suggested. Our study shows that indeed there are only four bands crossing the Fermi level, but that the weak Se•••Se interactions present in this structure, which are poorly described in DFT lead in some calculations to a fifth band crossing erroneously the Fermi level

Rich Polymorphism of Layered NbS3

Layered group V transition-metal trichalcogenides are paradigmatic low-dimensional materials providing an ever increasing series of unusual properties. They are all based on the same basic building units, one-dimensional MX3 (M = Nb, Ta; X = S, Se) trigonal-prismatic chains that condense into layers, but their electronic structures exhibit significant differences leading to a broad spectrum of transport properties, ranging from metals with one, two, or three charge density wave instabilities to semimetals with potential topological properties or semiconductors. The different physical and chemical properties are shown to be related with subtle structural differences within the layers that result in half-, third-, or quarter-filled quasi-one-dimensional Nb dz2-type bands, providing a clear-cut illustration of the intimate link between structural and electronic features within a family of solids. An interesting yet not sufficiently explored feature of these solids is the polymorphism. Based on both experimental and new theoretical results, we examine this aspect for NbS3 and show that at least seven different polymorphs with a stability compatible with the presently known phases of this compound are possible. We discuss a simple rationale for the physical properties of the presently known polymorphs as well as predictions for those that have still not been characterized or prepared. It is argued that some of the presently unknown polymorphs may have been prepared in an uncontrolled way as mixtures of different phases which could not be structurally characterized. The rich landscape of structures and properties found for this van der Waals material is suggested to represent an ideal platform for the preparation of flakes with fine-tuned properties for applications in new electronic and optoelectronic devices

Intermolecular Resonance Correlates Electron Pairs Down a Supermolecular Chain:Antiferromagnetism in K‐Doped p‐Terphenyl

Recent interest in potassium-doped p-terphenyl has been fueled by reports of superconductivity at Tc values surprisingly high for organic compounds. Despite these interesting properties, studies of the structure−function relationships within these materials have been scarce. Here, we isolate a phase-pure crystal of potassium-doped p-terphenyl: [K(222)]2[p-terphenyl3]. Emerging antiferromagnetism in the anisotropic structure is studied in depth by magnetometry and electron spin resonance. Combining these experimental results with density functional theory calculations, we describe the antiferromagnetic coupling in this system that occurs in all 3 crystallographic directions. The strongest coupling was found along the ends of the terphenyls, where the additional electron on neighboring p-terphenyls antiferromagnetically couple. This delocalized bonding interaction is reminiscent of the doubly degenerate resonance structure depiction of polyacetylene. These findings hint toward magnetic fluctuation-induced super- conductivity in potassium-doped p-terphenyl, which has a close analogy with high Tc cuprate superconductors. The new approach described here is very versatile as shown by the preparation of two additional salts through systematic changing of the building blocks