Computacional modelling of structural, electronic and thermodynamic properties of Cu-In intermetallic phases: ab-initio and phase diagram calculations

S. RAMOS DE DEBIAGGI; G. F. CABEZA; C. DELUQUE TORO; M. RUDA; A. M. MONTI; S. SOMMADOSSI; A. FERN¨¢NDEZ GUILLERMET

R¨ªo de Janeiro

Congreso; 11th. Internacional Conference on Advanced Materiales - VIII Encontro SBPMat.; 2009

SBPMat

This work is part of an investigation of the phase equilibria  of the Cu-In-Sn system, in connection to the application of In-Sn alloys as a lead free micro-soldering alloy. The aim is to understand aspects of the phase formation between the common Cu contact element and the Sn-In solder alloy involved in the process of diffusion soldering [1]. In the present study we focus on the phase equilibria of one of the three constituent binaries, Cu-In. This system contains five distinct binary phases, the two high temperature phases: the  ¦Â Cu4In with BCC structure and the ¦Ã Cu9In4 phase with Cu9Al4 type structure; the ¦Ä Cu7In3 phase with a unique structure type connected to NiAs and  ¦Ã-brass structures; the ¦Ç-phase field and the compound Cu11In9 with CuAl type structure. The ¦Ç-phase field is rather complex and has not yet been resolved. This phase field is composed of at least two different but closely related superstructures of the NiAs/Ni2In (hP4/hP6) type: the high temperature (¦Ç?) phase and the low temperature (¦Ç) phase. The  ¦Ç-¦Ç? transition temperature is composition dependent and lies between 310-389¡ãC [2]. The crystalline structure of ¦Ç? corresponds to an hP6 lattice with random parcial occupation of Cu sites at symmetry Wyckoff positions 2d. The ¦Ç crystalline structure would correspond to modulated superstructures based on the hP6 structure  with ordered distribution of Cu vacancies at sites 2d [3]. The phase field has a range of stability extended in both composition (33-37 at.% In) and temperature (T < 667 ¡ãC). During an investigation of this ¦Ç-phase field, Piao et. al [4] has recently found a new binary Cu10In7 phase. This compound crystallizes in the monoclinic spacegroup C2/m, with a structure closely related to the Cu11In9 structure type.  In the present work we perform ab-initio calculations to characterize structural, electronic and thermodynamic properties of some intermetallic phases of the Cu-In system: the recently proposed binary Cu10In7 phase as well as the ideal hexagonal B8 NiAs/InNi2  type structures which represent  the basic lattice underlying the more complex superstructures proposed for the ¦Ç-phase field. We calculate equilibrium lattice parameters, cohesive and formation energies of the phases considered. The calculations performed here are based on the electronic density-functional theory (DFT) carried out using the Viena ab-initio simulation package (VASP) [5] employing electronic density-funcional,  PAW (projector augmented wave) potentials and the generalized gradient approximation (GGA) for the exchange-correlation energy. Using the CALPHAD method we postulate the new phase as a soichiometric one and we analize the range of enthalpy an entropy of formation in order to assess its stabilty within the equilibrium phase diagram.  References   [1] S. Sommadossi, A. Fern¨¢ndez Guillermet, Intermetallics 15, (2007) 912-917. [2] H. Okamoto, J. Ph. Equil. 5, (1994) 226. [3] S. Lidin, Acta Cryst. B54, (1998) 97.  [4] S. Piao and Sven Lidin, Z. Anorg. Allg. Chem. 634, (2008), 2589-2593.  [5] G. Kresse and J. Furthmuller, Phys. Rev. B 54, (1996) 11169, G. Kresse and J. Furthmuller, Comp. Mater. Sci. 6 (1996).