Strain induced high thermoelectric performance of ScAgX (X=Si, Ge, Sn) half- Heusler alloys: A first principles approach

Here we have studied the effect of strain (Tensile & Compressive) on the stability, electronic, mechanical and thermoelectric properties of ScAgX (X = Si, Ge, Sn) Half- Heusler (HH) materials. Firstly, stability analysis of these compounds has done by computing frequency curve and calculating energies. Phonon dispersion curve indicates that these compounds are stable up to −24 % compressive strain. Whereas on the application of tensile strain, ScAgGe is found stable to a high degree of strain. It is stretchable up to 12 % tensile strain, on the other hand ScAgSi and ScAgSn are stretchable up to 8 %. Calculated value of formation energy and cohesive energy, delivers idea about the strength of bonding between the atoms. These energies approach positive values on imposing the higher degree of strain which indicates the less stability of compounds on higher value of strain. After the confirmation of bearable value of strain from the stability criteria, we have explored the electronic properties under the respective value of strain. Electronic band structure shows that band gap gets reduced under the compressive strain and after a certain value these compounds shows the metallic nature. Whereas the band gap gets broadened under the tensile strain. For the application purpose, thermoelectric performance has been evaluated under the strained conditions. Lattice thermal conductivity gets reduced under the tensile strain whereas it increases under the compressive strain. ScAgSn shows the minimum lattice thermal conductivity among these compounds at 4 % strain. Figure of merit (ZT) for these compounds gets enhanced under the strained conditions. For ScAgSi ZT value increases from 0.12 to 0.18, ScAgGe shows enhancement in ZT value from 0.08 to 0.14 and ScAgSn has increased ZT value from 0.16 to 0.40.