Room-Temperature Thermal Conductivity and Grüneisen Parameter of the I–III–VI2 Chalcopyrite Compounds

Hidden Local Symmetry Breaking in Silver Diamondoid Compounds is Root Cause of Ultralow Thermal Conductivity

Typically, conventional structure transitions occur from a low symmetry state to a higher symmetry state upon warming. In this work, an unexpected local symmetry breaking in the tetragonal diamondoid compound AgGaTe₂ is reported, which, upon warming, evolves continuously from an undistorted ground state to a locally distorted state while retaining average crystallographic symmetry. This is a rare phenomenon previously referred to as emphanisis. This distorted state, caused by the weak sd³ orbital hybridization of tetrahedral Ag atoms, causes their displacement off the tetrahedron center and promotes a global distortion of the crystal structure resulting in strong acoustic-optical phonon scattering and an ultralow lattice thermal conductivity of 0.26 W m^-1 K^-1 at 850 K in AgGaTe₂ . The findings explain the underlying reason for the unexpectedly low thermal conductivities of silver-based compounds compared to copper-based analogs and provide a guideline to suppressing heat transport in diamondoid and other materials.

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.