Ultralow Thermal Conductivity in Diamond-Like Semiconductors: Selective Scattering of Phonons from Antisite Defects

In this work, we discover anomalously low lattice thermal conductivity (<0.25 W/mK at 300 °C) in the Hg-containing quaternary diamond-like semiconductors within the Cu₂II_BIVTe₄ (II_B: Zn, Cd, Hg) (IV: Si, Ge, Sn) set of compositions. Using high-temperature X-ray diffraction, resonant ultrasound spectroscopy, and transport properties, we uncover the critical role of the antisite defects Hg_Cu and Cu_Hg on phonon transport within the Hg-containing systems. Despite the differences in chemistry between Hg and Cu, the high concentration of these antisite defects emerges from the energetic proximity of the kesterite and stannite cation motifs. Our phonon calculations reveal that heavier group II_B elements not only introduce low-lying optical modes, but the subsequent antisite defects also possess unusually strong point defect phonon scattering power. The scattering strength stems from the fundamentally different vibrational modes supported by the constituent elements (e.g., Hg and Cu). Despite the significant impact on the thermal properties, antisite defects do not negatively impact the mobility (>50 cm²/(Vs) at 300 °C) in Hg-containing systems, leading to predicted zT > 1.5 in Cu₂HgGeTe₄ and Cu₂HgSnTe₄ under optimized doping. In addition to introducing a potentially new p-type thermoelectric material, this work provides (1) a strategy to use the proximity of phase transitions to increase point defect phonon scattering, and (2) a means to quantify the power of a given point defect through inexpensive phonon calculations.