Influence of Strain on Thermoelectric Properties of NaYX (X=C,Ge) Half-Heusler Compounds.

By performing the density functional theory-based first principle calculations, we investigated electronic and transport properties of pure NaYX (X=C,Ge) and by applying uniaxial strain of NaYX (X=C,Ge) HH compounds. NaYX (X=C,Ge) HH compounds obey Born-Huang stability criteria. The formation energy and phonon dispersion support their chemical and dynamical stability. Our electronic band structure calculations show that NaYC,NaYGe is a direct band gap semiconductor having an energy band gap of 0.52eV and 0.57eV. After applying strain in NaYX (X=C,Ge) also has direct band gap but position of VBM and CBM is changed. We have calculated the transport properties of NaYX (X=C,Ge) Seebeck coefficient, electrical conductivity, electronic thermal conductivity by using constant relaxation time (10fs). Lattice thermal conductivity k l of NaYX (X=C,Ge) compounds were obtained with Slack's equation designed to compute k l within the experimental range. At temperature 1200 K, the k l for NaYC and NaYGe are 9.20 W m - 1 K - 1 , 5.97 W m - 1 K - 1 respectively We find that both n-type and p-type NaYX (X=C,Ge) are suitable for thermoelectric applications. Application of uniaxial strain has a significant impact on the electronic structure and transport properties. Among the investigated compounds, p-type NaYGe emerges as the better material in view of thermoelectric performance. [ABSTRACT FROM AUTHOR]