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

The effect of compression on the thermal conductivity of CuGaS₂, CuInS₂, CuInTe₂, and AgInTe₂ 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 CuGaS₂ increases with pressure, which is a common behavior. Striking differences occur for the other three compounds. CuInTe₂ and AgInTe₂ exhibit a drop in the thermal conductivity upon increasing pressure, which is anomalous. AgInTe₂ 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. CuInS₂ is an intermediate case. Based on the phonon dispersion data, the phonon frequencies of the acoustic modes for CuInTe₂ and AgInTe₂ decrease with increasing pressure, thereby driving the anomaly, while there is no significant pressure effect for CuGaS₂. This leads to the negative Grüneisen parameter for CuInTe₂ and AgInTe₂, 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.