When the number of atoms in a primitive cell (N) increases, the first Brillouin zone shrinks and folds back the high-frequency portion of acoustic vibrations as optical ones with reduced velocities near zone boundaries. Soft bonds lead to flattened phonon dispersion and thus a low sound velocity. This work shows the cutoff frequency of acoustic phonons (ω_m) as an important parameter to include both effects on the lattice thermal conductivity (κ_L). A low ω_m of ∼0.5 THz in Ag₉GaSe₆ due to its large N and weakly bonded Ag atoms leads this material to be one of the least thermally conductive dense solids (κ_L ∼ 0.15 W/m K). This contributes to the peak thermoelectric figure of merit, zT, as high as 1.5. The principle used here not only leads to the exploration of Ag₉GaSe₆ as a promising thermoelectric but also enables a measurable parameter for designing materials with expected κ_L.

当原始单元（N）中的原子数增加时，第一个布里渊区会收缩并折返为声振动的高频部分，这是光学振动，其区域边界附近的速度会降低。软键会导致声子扩散变平，从而降低声速。这项工作表明声子（的截止频率ω_米）作为一个重要的参数，包括在晶格热导率（两种效果κ_大号）。低ω_米中的Ag〜0.5太赫兹的₉ GASE ₆由于其大Ñ和弱键合的银原子引线这种材料是至少导热致密固体中的一个（κ_大号〜0.15W /米K）。这有助于高达1.5的峰值热电品质因数zT。其原理这里使用不仅导致银的勘探₉ GASE ₆作为一个有前途的热电而且使设计与预期的材料可测量的参数κ_大号。