Isotropic strain effects on the electronic, lattice dynamics, and thermoelectric properties of half Heusler NbIrSn

We have investigated and reported several novel 18 valence electron semiconducting hH

compounds, such as TiXPb (X = Ni, Pd, Pt), XFeTe (X = Ti, Hf), and NbIrSn, which exhibit

excellent high temperature thermoelectric performance with zT > 1. Strain engineering has been

effectively used to modify the electronic band structure and improve the transport properties of

thermoelectric materials. In this work, we investigated the effect of strain on the charge and phonon

transport, and hence on the thermoelectric performance of NbIrSn, using density functional theory

(DFT) and semiclassical Boltzmann transport theory. NbIrSn remains dynamically stable under

tensile strain up to 6% and compressive strain down to -22%, while its mechanical stability is

maintained between -12% and 6%; strains beyond these limits disrupt its structural integrity. Our

observations show that strain significantly affects the elastic constants, mechanical properties, and

lattice thermal conductivity under both tensile and compressive conditions. Electronic structure

analysis shows that the band gap of NbIrSn depends on strain, reaching its maximum 0.64 eV at -

2% compressive strain. Compressive strain improves the power factor and lowers lattice thermal

conductivity, leading to a maximum zT of 1.86 at 1200 K for hole doping under -12% strain,

compared to zT of 1.33 for the unstrained case. This highlights the effect of strain to improve high

temperature thermoelectric performance over the unstrained material.