The effects of point defects on thermal-mechanical properties of BiCuOTe: a first-principles study

Recently, BiCuOTe as a promising thermoelectric material has attracted extensive interest due to its lower thermal conductivity and higher electrical conductivity. However, little is known about the role of point defects in the growth, processing, and device degradation of this material. Moreover, the elastic properties which provide valuable information about the bonding characteristics, heat conductivity, and their anisotropic characters are investigated for effective design and characterization of new devices. Motivated by these considerations, a first-principles study about the stability of point defects and their effects on the thermal-mechanical properties of BiCuOTe was performed. The vacancies are found to be more stable than the interstitials. X_O (here X occupying the O lattice site, with X = Cu, Bi or Te) are generally unfavorable among the considered point defects. Point defects generally have negative effects on elastic constants (except C₆₆), suggesting that the resistance of defective systems to uniaxial and shear deformation is usually weaker than the that of ideal BiCuOTe. Similarly, point defects could deteriorate the ability to resist external compression. However, the introduction of point defects may improve the elastic compliances and depress the Debye temperature, which may increase the thermal expansion efficient of BiCuOTe. As compared with the ideal system, the point defects such as Cu_Bi, Te_Bi, Bi_Te, O_Cu, Cu_Te and Te_Cu may generally reduce the phonon thermal conductivity. This study would provide insights into the effect of point defects on the elastic and thermal properties of BiCuOTe and has important implications in the rational design of superior thermoelectric materials.