Origins of the doping asymmetry in oxides : hole doping in NiO versus electron doping in ZnO

ABSTRACT: The doping response of the prototypical transparent oxides NiO (p-type), ZnO (n-type), and MgO (insulating) is caused by spontaneous formation of compensating centers, leading to Fermi-level pinning at critical Fermi energies. We study the doping principles in these oxides by first-principles calculations of carrier-producing or-compensating defects and of the natural band offsets, and identify the dopability trends with the ionization potentials and electron affinities of the oxides. We find that the room-temperature free-hole density of cation-deficient NiO is limited by a too large ionization energy of the Ni vacancy, but it can be strongly increased by extrinsic dopants with shallower acceptor levels

Nonstoichiometry and hole doping in NiO

ABSTRACT: We have study by means of DFT+U and thermodynamic calculations the doping response of the p-type transparent oxide NiO. We have found from the calculated defect formation enthalpies that Ni vacancy, not the O interstitial, is the main source of nonstoichiometry in NiO. On the other hand, the calculated free-hole concentration at room temperature of pure NiO remains very low compared to the concentration of Ni vacancies; this is due to the too large ionization energy of the Ni vacancy. The free-hole concentration can be strongly increased by extrinsic dopants with a more shallow donor as it is illustrated for the case of Li

Theoretical study of the elastic properties of the minerals Cu3TMSe4 (TM = V, Nb, Ta) by means of atomistic first-principles calculations

ABSTRACT: The elastic properties of the family of isostructural minerals Cu3VSe4, Cu3NbSe4 and Cu3TaSe4 have been calculated for the first time using the state of the art in first-principles atomistic calculations, using Density Functional Theory and the Generalized Gradient Approximation for the exchangecorrelation energy functional. The elastic properties calculated are bulk modulus (B), the elastic constants (c11, c12 and c44), the Zener anisotropy factor (A), the isotropic shear modulus (G), the Young modulus (Y ), and the Poisson ratio (v). By means of these quantities we also computed other thermodynamic properties such as the average transversal (st) and longitudinal (sl) sound velocities and the Debye temperature (Teta D). The calculated values of B, c11, c12 and c44, G, Y and lead us to the conclusion that these compounds are compressible, fragile and brittle.RESUMEN: Las propiedades el´asticas de la familia de los minerales isoestructurales Cu3VSe4, Cu3NbSe4 y Cu3TaSe4 han sido calculadas por primera vez usando el estado del arte en cálculos atomísticos de primeros principios, utilizando la Teoría de los Funcionales de la Densidad y la Aproximación del Gradiente Generalizado para el funcional de la energía de intercambio-correlación. Las propiedades elásticas calculadas son el módulo volumétrico (B), las constantes elásticas (c11, c12 y c44), el factor de anisotropía de Zener (A), el módulo de cizalladura isotrópico (G), el módulo de Young (Y ), y la razón de Poisson (v). A través de estas cantidades también hemos calculado otras propiedades termodinámicas tales como la velocidad promedio del sonido transversal (st) y longitudinal (sl) y la temperatura de Debye (Teta D). Los valores calculados de B, c11, c12 y c44, G, Y , y v nos llevan a la conclusión que estos compuestos son compresibles, frágiles y quebradizos

Calculation of surface stress for fcc transition metals

ABSTRACT: Using the density functional theory, formulated within the framework of the exact muffin-tin orbitals method, we have calculated the surface stress for the (111) free surfaces of the fcc 4d and 5d transition metals. Good agreement is obtained with the available ab initio data for Pd, Ir, and Au, while for Pt we predict a surface stress, which is about 33% lower compared to former theoretical results. The present surface stress values for the 4d and 5d fcc metals show the typical trend characteristic for the cohesive or surface energies of d series

Cálculo de primeros principios de las propiedades electrónicas y dieléctricas de λ-Ta2O5

RESUMEN: Ta2O5 es un semiconductor de gap-ancho el cual tiene interesantes aplicaciones en comunicaciones en la región de las micro-ondas, principalmente está relacionado con la fabricación de filtros y resonadores, donde su tamaño es inversamente proporcional a la constante dieléctrica del material. Por este motivo, en este trabajo presentamos un estudio teórico a partir de la teoría de los funcionales de la densidad (usando PBEsol y el híbrido HSE06 para el funcional de intercambio-correlación) de las propiedades electrónicas y dieléctricas del modelo ortorrómbico -Ta2O5. Aquí, encontramos que este modelo tiene un gap directo de 2.09 y 3.7 eV con PBEsol y HSE06, respectivamente. Además, la constante dieléctrica estática calculada es 51, en buen acuerdo con los valores reportados para otras fases de este semiconductor. Palabras clave: Ta2O5, teoría de los funcionales de la densidad, PBEsol, HSE06, constante dieléctrica.ABSTRACT: Ta2O5 is a wide-bandgap semiconductor that offers interesting applications in microwave communications, mainly related to the manufacture of filters and resonators whose size is inversely proportional to the dielectric constant of the material. For that reason, in this work we present a theoretical study, based on density functional theory (using PBEsol and hybrid HSE06 exchange-correlation functionals), of the electronic and dielectric properties of the orthorhombic model -Ta2O5. We found that this model has a direct gap of 2.09 and 3.7 eV with PBEsol and HSE06, respectively. Furthermore, the calculated static dielectric constant, 51, is in good agreement with the reported values of other phases of this semiconductor. Keywords: Ta2O5, density functional theory, PBEsol, HSE06, dielectric constant

Nonstoichiometry as a source of magnetism in otherwise nonmagnetic oxides : magnetically interacting cation vacancies and their percolation

ABSTRACT: Recently, the possibility of inducing ferromagnetism in nonmagnetic insulators1–8 and in C60-based systems9–11 by creating intrinsic point defects, rather than by the more traditional approach of substitution by magnetic ions, has been discussed. Indeed, it appears that such observations of magnetism are invariably occurring in oxide samples having strong structural deviations from crystalline perfection, including heavily epitaxially textured samples and largesurface-area nanostructure grain boundaries, all associated with significant deviation from stoichiometry12,13. Here, we critically examine the conditions that may lead to such defect-induced ferromagnetism, following the general procedure we developed6 for CaO. Our approach is to first find the conditions leading to defect-induced magnetism under equilibrium. This will quantitatively establish the disparity between the defect concentration needed to establish equilibrium magnetism and what one would need to achieve experimentally with deliberate deviations from equilibrium. We outline four steps needed to determine if defect-induced magnetism is possible

Role of titanium in hydrogen desorption in crystalline sodium alanate

ABSTRACT: The role of Ti in improving the thermodynamics of hydrogen desorption in crystalline sodium alanate sNaAlH4d has been investigated by using the density functional theory. The total energy calculations reveal that Ti prefers to occupy the Na site over that of the Al site when the atomic energies are used as the reference. However, the use of the cohesive energies of Al, Na, and Ti leads to the Al site being the least unfavorable one. Irrespective of whether Ti occupies the Na or the Al site, the energy necessary to remove a hydrogen atom from Ti substituted sodium alanate is significantly lowered from that of the pure alanate. The understanding gained here may help in designing hydrogen storage materials suitable for industrial applications

Structural stability of (Ga,M) As from first principles : Random alloys, ordered compounds, and superlattices

ABSTRACT: We calculate by a combination of density functional theory and mixed-basis cluster expansion the structural stability of ordered and disordered zincblende GaAs-MnAs systems. We find that the ground state of this system is phase separating into GaAs+MnAs, even though the strain energy is negligible. The study of short-period superlattices shows that the least-unstable superlattices are along the (111) orientation whereas the most-unstable orientation is the (201). The formation enthalpy of the random alloy has been calculated; combining it with a mean-field approximation, we obtain the temperature-composition phase diagram showing the miscibility-gap temperature below which the alloy phase separates (…)

Un estudio teórico de la estructura electrónica y las propiedades dieléctricas de B-Nb2O5

ABSTRACT: In this work we have investigated theoretically the electronic and optical properties of niobium-oxide Nb2O5 for one of its crystalline phases, namely the B-phase. We have used Density Functional Theory along with the Generalized Gradient Approximation with one of its new modified functional targeted for solids: PBEsol. The calculated band gap is indirect (Eg=2,54 eV), with the valence band maximum located at and the conduction band minimum located at ( 1/3,1/3,1/3). We present also the calculated real and imaginary part of the dielectric tensor, the index of refraction, the reflectivity, the transmittance and the real part of the optical conductivity. The calculated value of the index of refraction is 2,52, which is in good agreement with the experimental value of 2,64.RESUMEN: En este trabajo hemos investigado teóricamente las propiedades electrónicas y ópticas del óxido de niobio Nb2O5 para una de sus fases cristalinas, a saber la fase B. Usamos la Teoría de los Funcionales de la Densidad junto con la Aproximación del Gradiente Generalizado con uno de sus nuevos funcionales diseñado para sólidos: PBEsol. La banda prohibida calculada es indirecta (Eg = 2,54 eV), con el máximo de la banda de valencia localizado en r y el mínimo de la banda de conducción en ( 1/3,1/3,1/3). También calculamos la parte real e imaginaria del tensor dieléctrico, el índice de refracción, la reflectividad, la transmitancia y la parte real de la conductividad óptica. El valor calculado del índice de refracción es 2,52, que está en buena correspondencia con el valor experimental de 2,64

A first-principles study of the piezoelectric properties of Niobium and Tantalum Pentoxides

ABSTRACT: Nb2O5 and Ta2O5 are wide-bandgap semiconductor oxides that have attracted great interest in recent years due to their technological applications, such as in electronics, telecommunications or photocatalysis. Because of this, we present a study based on first-principles calculations of the piezoelectric properties of the Z and β phases of Ta2O5 as well as the Z and P phases of Nb2O5 by using the Density Functional Theory and the Generalized Gradient Approximation with PBEsol parameterization. Once the equilibrium geometry was determined for each of these phases, we made a calculation using the linear response theory to determine the piezoelectric tensor associated with each phase. We discovered that the Z phase of both compounds presents good piezoelectric response. Additionally, β-Ta2O5 does not show such response.RESUMEN: Nb2O5 y Ta2O5 son óxidos semiconductores de brecha ancha, los cuales en los últimos años han despertado gran interés debido a sus múltiples aplicaciones tecnológicas, ya sea en electrónica, telecomunicaciones o fotocatálisis. Por estas razones, en este trabajo presentamos un estudio a partir de cálculos de primeros principios de las propiedades piezoeléctricas de las fases Z y β de Ta2O5, y de las fases Z y P de Nb2O5 utilizado la teoría de los funcionales de la densidad y la aproximación del gradiente, generalizado con la parametrización PBEsol. Una vez determinada la geometría de equilibrio para cada una de estas fases, realizamos un cálculo utilizando respuesta lineal para determinar el tensor piezoeléctrico asociado a cada una de estas fases, encontrando que la fase Z para ambos compuestos presenta una buena respuesta piezoeléctrica. Adicionalmente, encontramos que la fase β-Ta2O5 no presenta respuesta piezoeléctrica