Lattice stability and formation energies of intrinsic defects in Mg2Si and Mg2Ge via first principles simulations

We report an ab initio study of the semiconducting Mg2X (with X = Si, Ge) compounds and in particular we analyze the formation energy of the different point defects with the aim to understand the intrinsic doping mechanisms. We find that the formation energy of Mg2Ge is 50 % larger than the one of Mg2Si, in agreement with the experimental tendency. From the study of the stability and the electronic properties of the most stable defects taking into account the growth conditions, we show that the main reason for the n-doping in these materials comes from interstitial magnesium defects. Conversely, since other defects acting like acceptors such as Mg vacancies or multivacancies are more stable in Mg2Ge than in Mg2Si, this explains why Mg2Ge can be of n or p type, contrary to Mg2Si. The finding that the most stable defects are different in Mg2Si and Mg2Ge and depend on the growth conditions is important and must be taken into account in the search of the optimal doping to improve the thermoelectric properties of these materials.Comment: 25 pages, 6 Table

Synthèse et caractérisation de ZnO nanométrique par la méthode chimie douce

Nous avons synthétisé des matériaux nanostructurés de ZnO par différentes voies de chimie douce, c est-à-dire, par les méthodes sol-gel et pyrolsol. Cette recherche a consisté à étudier et mettre au point les paramètres expérimentaux qui influencent les propriétés structurales de ZnO tel que la nature du précurseur, du complexant ou du stabilisateur, leurs concentrations et, quand c est le cas, la concentration en dopant. Les paramètres thermiques comme la température de réaction et la température de traitement thermique ont aussi été optimisés. Les matériaux de ZnO contenant du chrome et de l aluminium ont également été étudiés. Les couches minces de ZnO non dopé préparées par le méthode dip coating sur substrat de verre ont donné dans les meilleurs cas des tailles d environ 40 nm. Pour le matériau dopé, ZnO :Al, 10at% Al, les particules atteigne des dimensions d environ 5 nm. Pour les couches minces préparées par la méthode pyrosol, la plus petite taille obtenue de ZnO non dopé est d environ 70 nm et pour ZnO :Al, 5at% Al, les tailles sont d environ 90 nm. Enfin, les nanoparticules de ZnO obtenues par précipitation des solutions présentent des taille d environ 10 nm. Des poudres de CrxZn1-xO nanométriques où x est 0.05, 0.10 et 0.15, présentent un comportement ferromagnétique faible avec la température par les méthodes de DXR, MET et MEB de couplage de ces différents méthodes physiques à permis d apporter de conclusion sur les méthode d élaboration par chimie douceWe have synthesized nanostructured ZnO materials by various ways of soft chemistry techniques, i.e. sol-gel and pyrosol methods. This research has consisted to study and develop the experiment parameters which influence the structural properties of nanostructured ZnO such as nature of precursor, complexing agent or stabilizer, precursor and dopant concentration. The thermal parameters such as reaction temperature and heat treatment temperature have been optimized as well. The ZnO materials containing of chromium and aluminium have also been studied. The undoped ZnO thin films prepared by dip coating on glass substrate give, in the best case, the approximate size 40 nm. In case of doped ZnO material, ZnO:Al, 10 at.% Al, the sizes reach approximate 5 nm. The ZnO thin films are prepared by pyrosol method giving the smallest size of undoped ZnO about 70 nm and ZnO:Al, 5 at.% Al about 90 nm. Lastly, the ZnO nanoparticles are prepared by precipitation giving the smallest size about 10 nm. The nanocrystalline CrxZn1-xO powders where x is 0.05, 0.10 and 0.15, have exhibited a weak ferromagnetic behavior with Curie temperature of 0.6, 2.6 and 7.1 K, respectivelyMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Structural stability of ternary C22-Zr6X2Co (X=Al, Ga, Sn, As, Sb, Bi, Te) and C22-Zr6Sn2T′ (T′=Fe, Co, Ni, Cu) compounds

International audienceThe crystal and electronic structures, and the thermodynamic properties of Zr6X2Co (X=Al, Ga, Sn, As, Sb, Bi, Te) and Zr6Sn2T′ (T′=Fe, Co, Ni, Cu) ternary compounds in the Fe2P-type structure have been investigated by means of first principle calculations. The calculated structural parameters are in good agreement with the experimental data. The total electronic densities of states as well as the Bader charges of the atoms have been computed. Both electronic and size effects allow to explain the stability of the ternary Zr6X2Co (X=Al, Ga, Sn, As, Sb, Bi, Te) and Zr6Sn2T′ (T′=Fe, Co, Ni, Cu) compounds

Thermodynamic modeling of the Cd–Sb–Zn ternary system

International audienceThe thermodynamic modeling and optimization of Cd–Sb, Sb–Zn binary systems and the Cd–Sb–Zn ternary system were carried out by means of the CALPHAD (CALculation of the PHAse Diagram) technique. The solution phases (liquid, hexagonal closed packed) were modeled with the Redlich–Kister equation. The Sb3Zn4 phases at high temperature ( and ), and Sb2Zn3 ones (two modifications: η and ξ) were treated as stoichiometric compounds. Two two-sublattice models, (Sb)(Cd,Zn,V a) and , were applied to describe the compounds and in the Cd–Sb–Zn system, respectively. A set of self-consistent thermodynamic descriptions of the Cd–Sb–Zn system were obtained

Enthalpies of formation of TM–X compounds (X=Al, Ga, Si, Ge, Sn). Comparison of ab-initio values and experimental data

International audienceNowadays first-principles calculations have achieved considerable reliability for the prediction of the properties of materials. These calculations are based on the density functional theory. In the present work, we have obtained a large number of enthalpies of formation of binary intermetallic compounds of transition metals with the elements Al, Ga, Si, Ge or Sn. The ab-initio values have been calculated with VASP in the generalized gradient approximation. We have compared the calculated enthalpies of formation of compounds to the values obtained with calorimetric methods. An excellent agreement is observed in the cases of aluminum and silicon based compounds. We also observe that in the case of highly negative values of enthalpies of formation of early transition metal alloys the experimental values are often more negative than the calculated values but it is not the case in Pt based compounds. In the cases of the Pt, Mo, Ru, Rh compounds with Al, Ga, Si, Ge or Sn, a very nice agreement between calculated and experimental values is observed. (C) 2016 Elsevier Ltd. All rights reserved.Notes: ISI Document Delivery No.: DV9Z

Enthalpies of formation and electronic densities of states of vanadium bodies

International audienceThe structural and electronic properties of vanadium borides have been obtained from DFT calculations. The calculated value of the enthalpy of formation of C32-VB2 compound is in good agreement with the experimental one. The calculated values of the lattice parameters at T = 0 K agree well with the experimental values. The total and partial electronic densities of states have been computed. Special features of the V electronic partial density of states have been evidenced and correlated to the local environment of the atoms and their charge

Structural stability of intermetallic phases in the Ga-Ti system

International audienceThe total energies of intermetallic compounds in the Ga-Ti system are calculated employing electronic density functional theory (DFT) using pseudopotentials constructed by the projector augmented waves (PAW) method in the generalized gradient (GGA) approximation for the exchange and correlation energy. The calculations are performed for the experimentally observed compounds at their ideal stoichiometry as well as for structures which are stable in systems of early transition metals or rare earth elements with p elements of columns IIIB, IVB, and VB. The calculated formation enthalpy of the hexagonal B82-GaTi2 compound is in excellent agreement with the value obtained by direct reaction calorimetry. The composition dependence of the enthalpies of formation is slightly asymmetric, the values of the enthalpies of formation being slightly more negative in the Ga-rich side. For the stable intermetallic compounds, the calculated zero-temperature lattice parameters agree well with those obtained experimentally at ambient temperature. Furthermore, for stable phases with unit-cell-internal degree(s) of freedom, the results of ab initio calculations show good agreement when compared with data obtained by structural analysis of X-ray diffraction

First-principles calculations of phase stability in the Ti-Zr-Si ternary system

International audienceFirst-principles calculations of the enthalpies of formation of ternary compounds with Fe2P-type (C22, hP9, , N°189) and Mn5Si3-type (D88, hP16, P63/mcm, N°193) structures have been performed in the Ti-Zr-Si system. At low temperature, two ternary compounds C22- Ti3Zr3Si3 and D88-Ti4Zr6Si3 are stable. The existence at high temperature of two solid solutions, (TiZr)5Si3 with the Mn5Si3-type structure and (TiZr)2Si with the Fe2P-type structure, is emphasized. The calculated isothermal section at 1323 K agrees well with the experimental results in the Ti-rich region

Etude du système ternaire Cd-Sb-Zn ; élaboration et caractérisation du composé thermoélectrique Zn4Sb3

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF