Lattice Dynamics and thermodynamic Responses of XNbSn Half-Heusler Semiconductors: A First-Principles Approach

In this work, we have evaluated how CoNbSn, IrNbSn, and RhNbSn half Heusler alloys respond to temperature change and the accompanying lattice vibrations as a cubic crystal. There are reports in the literature for CoNbSn with which we compared our result; there are, however, no reports for the other two alloys except for their Debye temperature obtained via machine learning, and our results compare well. Considering that results in the literature for IrNbSn and RhNbSn are scanty, we first computed the alloys' structural and electronic properties to establish their structural stability using the density functional theory and generalised gradient approximation as implemented in the quantum espresso computational suite. We confirmed the equilibrium lattice structure by exploring the three possibilities for a half Heusler alloy and fitting the results to the state's Murnaghan equation. The negative formation energies obtained supports experimental simulation of the alloys. Results from the lattice dynamics and thermodynamic evaluation show that the alloys favour ionic bonding and are ductile. The Debye temperature positions IrNbSn to be the most promising material for thermoelectric application because it has the least Debye temperature; hence it is supposed to have the lowest thermal conductivity. The Dulong-Petit law is obeyed at high temperature as expected. The phonon dispersion and density of states show that the d orbitals of Co and Nb are the significant contributors to the dispersions at both the acoustic and optical modes of the alloys