fig2

Figure 2. Calculated phonon dispersion curves and projected phonon density of states (PDOS) at T = 300 K for (A) SnSe and (B) NaSn₂SbSe₄. The optical branches are shown in blue in (A) and red in (B), and the acoustic branches are shown in red in (A) and yellow in (B). The grey, purple, red, and green solid lines in subfigures (A) and (B) represent the PDOS of Sn, Se, Na, and Sb atoms, respectively; (C) κ_L calculated from the three-phonon (3ph) and combined three- and four-phonon (3,4ph) scattering models over the temperature range from 300 K to 800 K, and (D) the calculated 3ph and 4ph scattering rates (SRs) as a function of phonon frequency at 300 K for SnSe and NaSn₂SbSe₄.