Novel Methods for Electronic Structure Calculations.

 

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Susana Ramos (1,2)

N. V. González Lemus(1,2) G. F. Cabezac(3,5), A. Fernández Guillermet(4,5)

1. Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, (8300) Neuquén, Argentina
2. Instituto de Investigación y Desarrollo en Ingeniería de Procesos, Biotecnología y Energías Alternativas – CONICET-UNCo
3. Dpto. de Física. Universidad Nacional del Sur, Bahía Blanca. Argentina
4. Centro Atómico Bariloche e Instituto Balseiro, Avda. Bustillo 9500, (8400) Bariloche, Argentina
5. CONICET

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Ab initio study of Cu-In-Sn and Ni-In-Sn intermetallics: Systematics and correlations in the cohesive and electronic properties

The thermophysical properties of intermetallic phases (IPs) of the Cu-In-Sn and Ni-In-Sn systems are of current interest in relation to the design of lead-free soldering (LFS) technologies. Considerable efforts have been devoted to the development and application of predictive approaches to describe the thermodynamics of binary and higher order IPs of actual and potential technological relevance for the lead-free soldering methods. In this work our research advances towards the characterization and understanding of cohesion and bonding properties in these type of systems are described. A systematic and comparative ab initio density functional theory (DFT) study of a large group of stable, metastable and hypothetical IPs of the binary Me-X (Me = Cu,Ni; X = In,Sn) subsystems, is presented. Hypothetical compounds, for which experimental data is usually not available, are involved in the thermodynamic CALPHAD type modeling of the phase diagrams of these systems; the ab initio methods can be of great use to provide useful input data for these modeling methods. For all the set of Me-X IPs considered, the composition and volume dependence of their bulk modulus (Bo) and cohesive energies (Ecoh) are discussed, as well as the systematic effects of replacing Cu by Ni in these IPs. New correlations between cohesion-related quantities such as (Bo/Vo)½ and (Ecoh½/Vo) with the number of valence electrons per unit volume, are established and compared with similar Miedema´s empirical relations. For the Ni3In-hP8 and Ni3In-cP4 phases, which compete in stability, also their elastic constants and averaged elastic moduli are calculated; and based on these results predictions are made for the mechanical properties of this compound. The composition and structure dependence of the cohesion for this class of compounds is analyzed in terms of a theoretical view that involves the hybridization of the d-states of Cu or Ni with the s and p-states of In or Sn. A comparative analysis is performed of the density of states (DOS) of various representative, iso-structural Me-X compounds. Various effects of relevance to understand the consequences of replacing Cu by Ni in LFS alloys are highlighted and explained microscopically. In addition to the reported thermodynamic information, which can be of direct use as input in the CALPHAD optimizations, the picture of the variations in cohesive properties emerging from the present work should be useful in systematizing the thermophysical and structural database for this class of compounds. Finally, by considering specific ternary IPs of the Cu-In-Sn and the Cu-In-Sb related system, the problem of the design of structural models based on sublattice schemes is discussed.