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

Loay Elalfy,Ming Hu,Denis Music

doi:10.3390/ma12213491

Abstract

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.

Highlights

Chalcopyrite compounds (AI BIII C2 VI, AI = IB elements (Cu, Ag), BIII = IIIA elements (Al, Ga, In), CVI = VIA elements (S, Se, Te), space group I-42d, as shown in Figure 1) are well-known semiconductors with a band gap in the range of 0.1 to 1 eV [1,2,3,4]
CuInTe2 and AgInTe2 decrease with increasing pressure, thereby driving the anomaly, while there is no significant pressure effect for CuGaS2
Κph is likely the largest contribution for chalcopyrites since they are semiconductors. It can be obtained as follows: κph = 13 cv v2g τ, where vg is the group velocity of phonons, cv designates the heat capacity, and τ is the phonon relaxation time [26]. The former two values were calculated from the phonon dispersion curves using the Phonopy package [27], while τ was obtained by solving the Boltzmann transport equation, as implemented in the ShengBTE package [28]

Summary

Introduction

1.085, 0.364, 0.469, and 0.967 eV for CuGaS2 , CuInS2 , AgInS2 , and AgInTe2 , respectively [1,2,3,4,5], being consistent with common density functional theory deviations [6]. Their structure can be derived from zincblende (ZnS) by alternating the AI and BIII constituents at the Zn site [7]. Atomic vibrations driving anomalous thermal behavior of CuInC2VI compounds are not known and cannot be deduced from other systems

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Results

Conclusion