Comparison of the polarizability of periodic systems computed by using the length and velocity operators.

International audienceThe theorem relating the length (L) and velocity (V) operators, that permits to compute in two alternative ways the polarizabilities of finite systems, is generalized to periodic infinite cases. The two alternative strategies have been implemented in the CRYSTAL code, that uses Gaussian type basis sets, within the CPHF and CPKS formalisms. The dielec. const. of diamond, SiC, silicon and MgO has been obtained with four different hamiltonians (HF, LDA, PBE, B3LYP). The effect of basis set and other computational parameters are discussed. It turns out that when a relatively extended basis set is used, LDA and PBE results obtained with the L and V operators nearly coincide, whereas HF and B3LYP schemes provide different results, as expected on the basis of the non-commutability of the HF-exchange and length operators. [on SciFinder(R)

Structural and Electronic Transport Properties in Sr-Doped BiCuSeO

International audienc

First-principles study of the optical properties of BeO in its ambient and high-pressure phases

Optical properties such as the dynamic dielectric function, reflectance, and energy-loss function of beryllium oxide (BeO) in its ambient and high-pressure phases are reported for a wide energy range of 0-50 eV. The calculations of optical properties employ first-principles methods based on all-electron density functional theory together with sum over states and finite-field methods. Our results show subtle differences in the calculated optical properties of the wurtzite, zincblende, rocksalt and CsCl phases of BeO, which may be attributed to the higher symmetry and packing density of these phases. For the wurtzite phase, the calculated band gap of 10.4 eV corresponds well with the experimental value of 10.6 eV and the calculated (average) index of refraction of 1.70 shows excellent agreement with the experimental value of 1.72. © 2009 Elsevier Ltd. All rights reserved

Structure and Transport Properties of the BiCuSeO-BiCuSO Solid Solution

In this paper, we report on the crystal structure and the electrical and thermal transport properties of the BiCuSe1−xSxO series. From the evolution of the structural parameters with the substitution rate, we can confidently conclude that a complete solid solution exists between the BiCuSeO and BiCuSO end members, without any miscibility gap. However, the decrease of the stability of the materials when increasing the sulfur fraction, with a simultaneous volatilization, makes it difficult to obtain S-rich samples in a single phase. The band gap of the materials linearly increases between 0.8 eV for BiCuSeO and 1.1 eV in BiCuSO, and the covalent character of the Cu-Ch (Ch = chalcogen element, namely S or Se here) bond slightly decreases when increasing the sulfur fraction. The thermal conductivity of the end members is nearly the same, but a significant decrease is observed for the samples belonging to the solid solution, which can be explained by point defect scattering due to atomic mass and radii fluctuations between Se and S. When increasing the sulfur fraction, the electrical resistivity of the samples strongly increases, which could be linked to an evolution of the energy of formation of copper vacancies, which act as acceptor dopants in these materials

First-principles study of the structural, electronic, and optical properties of Ga2O3 in its monoclinic and hexagonal phases.

International audienceThe authors report the results of a comprehensive study on the structural, electronic, and optical properties of Ga2O3 in its ambient, monoclinic (β) and high-pressure, hexagonal (α) phases in the framework of all-electron d. functional theory. In both phases, the conduction band min. is at the zone center while the valance band max. is rather flat in the k space. The calcd. electron effective mass m*e/m0 comes out to be 0.342 and 0.276 for β-Ga2O3 and α-Ga2O3, resp. The dynamic dielec. function, reflectance, and energy-loss function for both phases are reported for a wide energy range of 0-50 eV. The subtle differences in electronic and optical properties can be attributed to the higher symmetry, coordination no. of Ga atoms, and packing d. in α-Ga2O3 relative to that in β-Ga2O3. [on SciFinder(R)

Room-temperature soft mode and ferroelectric like polarization in SrTiO3 ultrathin films: Infrared and ab initio study

International audienceDue to the remarkable possibilities of epitaxially growing strontium titanate (SrTiO 3 or STO) on silicon, this oxide is widely used as a buffer layer for integrating other perovskite oxides which allows for the development of various functional electronic devices on silicon. Moreover, STO is known to be an incipient ferroelectric in bulk but may become ferroelectric when in the form of strained ultrathin films. Given the importance of the potential applications for electronics if this property is demonstrated, we performed a spectroscopic study of STO on Si(001) templates coupling experimental and ab initio investigations. We selected six samples of ultrathin films: three strained samples (of thickness 4, 9 and 48 nm) and three relaxed samples (of equivalent thickness). Their infrared spectra show that both the mechanical stress and the thickness play major roles: higher energy modes evolve as soft modes in thinner strained films. In order to support these observations, the dynamical ab initio calculations allowed deriving the conditions for STO films to become ferroelectric at room temperature as shown by the development of a soft mode and the divergence of the in-plane dielectric constant

Structural and Electronic Transport Properties in Sr-Doped BiCuSeO

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi³⁺ by Sr²⁺ induces a strong decrease of the electrical resistivity up to the solubility limit reached for <i>x</i> = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu_1–<i>x</i>OS. We show that the thermoelectric properties of Bi_1–<i>x</i>Sr_<i>x</i>CuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials

Structural and Electronic Transport Properties in Sr-Doped BiCuSeO

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi³⁺ by Sr²⁺ induces a strong decrease of the electrical resistivity up to the solubility limit reached for <i>x</i> = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu_1–<i>x</i>OS. We show that the thermoelectric properties of Bi_1–<i>x</i>Sr_<i>x</i>CuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials

Structural and Electronic Transport Properties in Sr-Doped BiCuSeO

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi³⁺ by Sr²⁺ induces a strong decrease of the electrical resistivity up to the solubility limit reached for <i>x</i> = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu_1–<i>x</i>OS. We show that the thermoelectric properties of Bi_1–<i>x</i>Sr_<i>x</i>CuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials