Structural, stability and thermoelectric properties for the monoclinic phase of NaSbS2 and NaSbSe2: A theoretical investigation

This study is the first attempt towards establishing computational insight into the structural, electronic, mechanical, dynamical and thermoelectric properties of the monoclinic phases of NaSbS2 and NaSbSe2. The mechanical properties are predicted using the Hill approximation. Dynamical stability was investigated by computing the phonon frequency to check for the absence of imaginary modes. Lattice thermal conductivity was calculated by using a single-mode relaxation-time approximation in the linearized phonon Boltzmann equation from first-principles an-harmonic lattice dynamics calculations. We found that the lattice thermal conductivity of NaSbS2 and NaSbSe2 are anisotropic, with values ranging between 0.753 and 1.173 Wm−1 K−1 at room temperature (300 K). The calculated values of the lattice thermal conductivity are small, especially along the x-axis. The charge transport properties are predicted using Boltzmann transport equations. The highest values attained for the figure of merit are high as 4.22 and 2.88 when the electron concentration is 1018 cm−3 at 600 K for NaSbS2 and NaSbSe2, respectively. This highlights the potential of using NaSbS2 and NaSbSe2 in designing thermoelectric materials since low lattice thermal conductivity and high figure of merit are a requisite for maximizing the efficiency of thermoelectric materials. Graphical abstract