First-principles study of the electronic, optical, and lattice vibrational properties of ${\text{AgSbTe}}_2$

First-principles calculations of the electronic, optical, and lattice vibrational properties of ${\text{AgSbTe}}_2$ were performed with the generalized gradient approximation (GGA) and the screened-exchange local-density approximation (sx-LDA) method, which successfully corrects the band-gap problem found with GGA. We find a vanishing density of states at the Fermi level, which is consistent with the semiconducting behavior of ${\text{AgSbTe}}_2$. Various optical properties, including the dielectric function, absorption coefficient, and refractive index, as functions of the photon energy are also calculated with the sx-LDA and are found to be in good agreement with experiments. Phonon spectra obtained by the cumulant force constant method show that the optic modes of ${\text{AgSbTe}}_2$ are very low in frequency and should scatter strongly with acoustic modes during heat transport. The calculated specific-heat curve is in general agreement with experiment. Ag/Sb disorder is expected to have a small effect on the electrical transport but may introduce strong phonon scattering due to the large force constant disorder. The scattering of acoustic phonons by optic modes and the possible Ag/Sb disorder may explain the extremely low lattice thermal conductivity of ${\text{AgSbTe}}_2$.

Figure 1

Band structures of ${\text{AgSbTe}}_2$ by GGA (dashed lines) and sx-LDA (solid lines). The effect of spin-orbit coupling is included in both calculations.

Figure 2

(Color online) PDOS of ${\text{AgSbTe}}_2$ by GGA (left) and by sx-LDA (right). The effect of spin-orbit coupling is already included in both calculations.

Figure 3

(Color online) The real and imaginary parts of the dielectric function of ${\text{AgSbTe}}_2$ calculated by sx-LDA.

Figure 4

(a) The absorption coefficient $\alpha$ and (b) the refractive index $n$ as functions of photon energy calculated by sx-LDA. The inset in panel (a) shows that the theoretical absorption edge is located at about $0.70–0.80\text{eV}$.

Figure 5

The phonon dispersions of ${\text{AgSbTe}}_2$ calculated with GGA and the 64-atom supercell.

Figure 6

(Color online) Theoretical and experimental specific heat. Note the theoretical curve is for $C_v$, while the measured data are $C_p$.