First-Principles Calculations of Electronic Structure and Structural Properties for MoV, MoNb, and MoTa
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First-Principles Calculations of Electronic Structure and Structural Properties for MoV, MoNb, and MoTa
R. de Coss, A. Aguayo, and G. Murrieta Departamento de Física Aplicada, CINVESTAV-Mérida, A.P. 73 Cordemex 97310, Mérida, MEXICO.
ABSTRACT First-principles total-energy electronic structure calculations based on the full-potential linearized augmented plane wave (LAPW) method have been used to study the electronic and elastic properties of MoV, MoNb, and MoTa with the B2 (CsCl) estructure. From the calculated values for the bulk modulus we have determined the melting temperature using an empirical correlation. The chemical bond and the electronic structure around the Fermi level are analyzed. In particular, we found that MoTa which have the experimental determined highest melting point of the studied materials, present the largest bulk modulus and the highest degree of covalence bonding of these intermetallic compounds.
INTRODUCTION Intermetallic compounds of bcc refractory transition metals (V, Cr, Nb, Mo, Ta and W) are of great technological interest because of their high hardness, high melting point and high corrosion resistance. The low ductility of the alloys with compositions in the central portion of the binary phase diagram, limits the application of these materials in the bulk [1]. However, recent studies show that also thin films of the molybdenum based alloys Mo-Cr [2], Mo-Ta [3], Mo-Nb [4] containing ~50% Mo give a high protection against corrosion. This discovery open up the possibility of applying this type of materials in a thin film configuration, were the low ductility is less of a problem. Structural characterization of the Mo-(Nb, Ta) alloys using x-ray diffraction shows that forms a bcc solid solution [3,4]. Purely experimental research, with the aim to understand and consequently improve the above mentioned technological properties, is however time consuming and expensive. It is possible to shorten this research with the aid of theoretical tools. Current ab initio densityfunctional calculations cannot adequately determine material properties at high temperature, but it is possible to compute the static lattice equation of state and elastic moduli of ordered binary compounds [5,6]. Known correlations between equilibrium properties and high-temperature properties such as the melting temperature (Tm) can then be used as a guide to steer the experimental research. For example, Friedel has shown that Tm of metals is strongly correlated with the cohesive energy and thus on the bonding properties in the ground state [7], while Fine et N5.33.1 1
al. reported a correlation with the elastic moduli [8]. More recently, Tm of cubic metals was found to be correlated with the band energy [9]. As a first step to study the molybdenum based alloys, in this work we present a firstprinciples total-energy study of the electronic and structural properties for MoV, MoNb, and MoTa in the B2 phase (CsCl structure). From the results of our calculations we have obtained the heat of formation, the melting temperatur
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