First Principles Investigation of Anomalous Pressure-Dependent Thermal Conductivity of Chalcopyrites

The effect of compression on the thermal conductivity of CuGaS(2), CuInS(2), CuInTe(2), and AgInTe(2) chalcopyrites (space group I-42d) was studied at 300 K using phonon Boltzmann transport equation (BTE) calculations. The thermal conductivity was evaluated by solving the BTE with harmonic and third-order interatomic force constants. The thermal conductivity of CuGaS(2) increases with pressure, which is a common behavior. Striking differences occur for the other three compounds. CuInTe(2) and AgInTe(2) exhibit a drop in the thermal conductivity upon increasing pressure, which is anomalous. AgInTe(2) reaches a very low thermal conductivity of 0.2 W·m(−1)·K(−1) at 2.6 GPa, being beneficial for many energy devices, such as thermoelectrics. CuInS(2) is an intermediate case. Based on the phonon dispersion data, the phonon frequencies of the acoustic modes for CuInTe(2) and AgInTe(2) decrease with increasing pressure, thereby driving the anomaly, while there is no significant pressure effect for CuGaS(2). This leads to the negative Grüneisen parameter for CuInTe(2) and AgInTe(2), a decreased phonon relaxation time, and a decreased thermal conductivity. This softening of the acoustic modes upon compression is suggested to be due to a rotational motion of the chalcopyrite building blocks rather than a compressive oscillation. The negative Grüneisen parameters and the anomalous phonon behavior yield a negative thermal expansion coefficient at lower temperatures, based on the Grüneisen vibrational theory.