ELECTRONIC, THERMOELECTRIC, ELASTIC, THERMODYNAMIC PROPERTIES AND PHONON DISPERSION OF Sb-BASED HEUSLER ALLOYS USING FIRST PRINCIPLES CALCULATIONS

Abstract

ABSTRACT Antimony-based Heusler alloys exhibit enhanced thermoelectric performance. These materialsoffer a promising route for efficient waste heat recovery and renewable energy harvesting,providing a sustainable energy conversion. This study investigated the structural, electronic,thermoelectric, elastic,thermodynamic properties, and phonon dispersion of four (4) Sb-basedHeusler alloys: CoHfSb, Li2NaSb, RhHfSb, and RuTaSb. Density Functional Theorycalculationswere performed using the Quantum Espresso package and Thermo_pw code. Theground state energies were calculated by optimizing Self Consistent Field (SCF) parameters. TheBand gaps were obtained from Non-Self Consistent Field (NSCF) band structures while thefigure of merit (ZT) was calculated using BoltzTraP software. Thermo_Pw employed the quasiharmonic model to calculate Poisson ratio, anisotropic values, elastic constants andthermodynamic properties. Phonon calculations were performed using the Debye model. Theminimum ground state energies were obtained at lattice constants 6.0674, 6.8603, 6.2995, and 6.1608 Å for CoHfSb, Li2NaSb, RhHfSb, and RuTaSb respectively.The band gapenergies were calculated to be 0.78, 0.49, 0.81, and 0.46 eV for CoHfSb, Li2NaSb,RhHfSb, and RuTaSb, respectively, indicating semiconducting behaviour. The Figure of Merit (ZT) values were remarkably high for all the alloys, with CoHfSb exhibiting ZT ≈ 0.74 (n- type) and ≈ 0.99 (p-type); Li2NaSb exhibiting ZT ≈ 0.99 (n-type) and ≈ 0.98 (p-type); RhHfSbexhibiting ZT ≈ 0.78 (n-type) and ≈ 0.99 (p-type); and RuTaSb exhibiting ZT ≈ 0.74 (n-type) and ≈ 0.98 (p-type). These highZT values showed a testament to the potential of these alloysin the field ofthermoelectricity and power generation. The Poisson ratio (υ) was calculated tobe0.25, 0.16, 0.30 and 0.32 for CoHfSb, Li2NaSb, RhHfSb and RuTaSb, respectively.Theanisotropic values of the alloys were calculated to be 0.74, 2.29, 1.16 and 1.04 for CoHfSb, Li2NaSb, RhHfSb and RuTaSb, respectively. Negative frequency was absent in the materials’phonon dispersion curves. The Valence Band maximums and Conduction Bandminimums do not align, indicating indirect band gaps. The high ZT values suggestedapplicability for thermoelectricity and powergeneration. Materials with υ > 0.26 were ductile, while the ones with υ < 0.26 were brittle. Thepositive values of the elastic constants confirmedthat the alloys are mechanically stable. The materials were said to beanisotropic, with the anisotropic values not equal to 1 (A ≠ 1). Their vibrational energyincreased with temperature whilevibrational free energy decreased monotonically.Theentropy increased with temperature due to thermal agitation which enhanced configurationaldisorder while the heat capacity became constant above 650 K. Phonon calculations revealed dynamic stability with separated optical and acoustic branches in the phonon dispersion curve in all the materials, providing confidence in their potential applications. This study foundvaluable insights into the properties of these Sb-based Heusler alloys, highlighting theirpotential for innovative applications in thermoelectricity and energy harvesting.