High pressure phase stability, elastic anisotropy and electronic properties of GePd

Abstract

Palladium germanide (GePd) is one of the most promising materials for use in nano-electronic ohmic contacts to active areas of germanium-based devices. It is important, for device integrity, to identify the various characteristics of this germinide as these can influence the stability of the device at various temperatures and pressures. We have used Particle Swarm Optimization (PSO) crystal lattice structure searching and first-principles structural optimization to explore three low Gibbs free energy polymorphs of GePd under a varying applied hydrostatic pressure from 0 to 1200 kbars. The first phase had a cubic zinc blende structure with Space Group, F4‾3m. The second had a cubic sodium chloride structure (Fm3‾m) and thirdly a structure with the primitive orthorhombic (Pnma). Dynamical and mechanical stabilities were only established for the orthorhombic GePd (o-GePd) structure using its phonon dispersions and elastic constants. This material was found to be ductile with a B/G value of 2.71. The Universal Elastic Anisotropy Index of o-GePd was 17.82% which indicates a notable susceptibility to micro-cracks under elevated temperature conditions. However, the material is essentially isotropic in terms of its response to hydrostatic compression without shearing. Our electronic structure calculations indicate that o-GePd presents metallicity with an electron-type (as opposed to hole-type) conductivity that is likely to increase with pressure.