Impressive Electronic and Thermal Transports in CsK₂Sb: A Thermoelectric Perspective

Abstract

Over the past 2 decades, there has been active research into the hunt for innovative thermoelectric materials with a high value of figure of merit. To investigate the potential of CsK 2 Sb as a thermoelectric material, we analyzed its electrical and thermal transport properties using density functional theory and Boltzmann transport theory. We found that CsK 2 Sb is a direct band gap material with a band gap of 1.44 eV using the Gaussian-attenuating Perdew−Burke− Ernzerhof functional with spin−orbit coupling. Using electron−phonon Wannier calculations, we computed the electron−phonon lifetimes for electrons and holes, which are leveraged to estimate absolute values of the electronic transport coefficients by solving the Boltzmann transport equation for electrons. At 300 K, we have found that CsK 2 Sb exhibits a maximum value of power factor of 5.7 mW K −2 m −1 with electron doping, which is comparable to those of well-known thermoelectric materials. By solving the Boltzmann transport equation for phonons, we demonstrate that CsK 2 Sb has significantly lower phonon group velocity and phonon−phonon lifetimes than other well-known thermoelectric materials, resulting in an ultralow lattice thermal conductivity of 0.25 W m −1 K −1 at 300 K. At 500 K, CsK 2 Sb showcases an exceptional figure of merit of 4.69 (2.48) with electron (hole) doping, surpassing all other full Heusler alloys. These findings reveal that CsK 2 Sb is a "phonon glass electron crystal", a property of an ideal thermoelectric material.

Thermodynamic stability and vibrational properties of multi-alkali antimonides

“…An interpolated value of 8.88 Å is reported in [73]: although it is not far from our prediction (see table 2), we do not consider it a reliable point of comparison. The structural parameters obtained with SCAN for the ternary alkali antimonides considered in this work are in better agreement with the available experimental references than earlier DFT studies adopting semi-local functionals [17,25,27,28,37,38,46,73].…”

“…A clear separation between acoustic and optical branches occurs along the Γ-X, Γ-K, and Γ-L directions. This characteristic, termed 'avoided crossing' , flattens the corresponding phonon bands and reduces the group velocity of the acoustic phonons and, consequently, the lattice thermal conductivity, thereby enhancing the electronic and thermal transport properties of the material [37,39]. In CsK 2 Sb, a distinct separation between optical and acoustic modes is observed, with an evident LO/TO splitting at the Γ point.…”

“…This choice, however, considerably limits the predictive power of the calculations. Notable exceptions are given by some recent first-principle works on the thermoelectric and vibrational properties of multi-alkali antimonides [36,37] even beyond the harmonic approximation [38,39]. Yet, the validity of the chosen approximations on a variety of structures and compositions needs to be carefully checked.…”