DENSITY FUNCTIONAL THEORY STUDY OF STRUCTURAL, MECHANICAL, ELASTIC ANISOTROPY, LATTICE DYNAMICS, AND ELECTRONIC PROPERTIES OF NICKEL AND PALLADIUM BASED HEUSLER ALLOYS

Abstract

This study explores the structural, mechanical, elastic anisotropy, lattice dynamics, and electronic properties of nickel and palladium based Heusler compounds for thermoelectric (TE) applications. Current energy sources such as fossil fuels have been crucial in supplying the world's needs, however, they face certain difficulties such as greenhouse gas emissions. The availability of thermal energy in the form of waste heat has drawn attention to the TE energy conversion process. Low efficiency and thermal stability are still problems for several energy-conversion materials that exploit different varieties and compositions in their alloys. These challenges are linked to the physical properties of energy-conversion materials and the kind of contact electrode that enhances the performance of TE devices. The compounds investigated in this study belong to Heusler alloys with designations including half and full Heusler. Computations were performed using density functional theory (DFT) as implemented in the Quantum ESPRESSO (QE) code. For the exchange-correlation functional, the generalized gradient approximation (GGA) was used, while the projected augmented wave method (PAW) was used for electron-ion interaction. Optimized lattice constants for all compounds investigated agree with available experimental and theoretical values in the literature. The lattice parameter mismatch for HfNiSn/HfNi2Sn, ZrNiSn/ZrNi2Sn, and TiNiSn/TiNi2Sn compounds was minimal, ranging between 2.5 % and 3 %. Mechanical properties indicated that nickel-based half Heusler compounds were brittle except for ZrNiSi and TiNiSn, which were ductile. Additionally, ductile behavior was noted in palladium based half Heusler and nickel based full Heusler compounds. Elastic constants were found to satisfy the general conditions for stability for cubic symmetry, except for TiNi2Ge. Based on anisotropies in sound velocities in the [111], [110], and [100] crystal directions, anisotropy indices, and directional surfaces, all compounds exhibited anisotropic character. Lattice dynamics properties showed that, with the exception of TiPdGe, half-Heusler alloys and full Heusler compounds of TiNi2Sn, TiNi2Si, ZrNi2Sn, and HfNi2Sn were dynamically stable. The electronic properties for nickel-based compounds showed that HfNiSn, ZrNiSn, and TiNiSn compounds have the least indirect energy gaps of 0.3847 eV, 0.5019 eV, and 0.4508 eV respectively. Among the palladium-based compounds, TiPdSn, ZrPdSn, and HfPdSn recorded narrow energy band gaps of 0.4804 eV, 0.4747 eV, and 0.3815 eV, respectively. The findings of this work offer theoretical guidance for improving the performance of TE materials and devices that can significantly contribute to clean energy solutions, hence aligning with the global shift towards sustainable energy practices.