A Fundamental Study of Structural Effects on Thermal Properties of Quaternary Chalcogenides

Abstract

Abstract Body: A material with a high thermoelectric figure of merit plays an important role in the development of efficient thermoelectric devices. Modification of thermal transport in a crystal lattice is one of the most sought-after methods of developing novel thermoelectric materials. Increasing phonon scattering in a crystalline material to lower thermal transport is one way of achieving this objective. A fundamental understanding of the effects of structural features on the thermal properties is a significant first step towards enhanced thermoelectric properties of a material. Although certain quaternary chalcogenides have been examined for their applications as thermoelectric materials, fundamental research into altering the thermal conductivity of this large class of materials remains to be explored. This work contributes towards this area of research by investigating the thermal conductivity and heat capacity of select quaternary chalcogenides to analyze their structure-property relationships. Phenomenological models were utilized to examine the thermal conductivity, κ, and heat capacity, Cp, of BaCu2SnQ4 (Q = S, Se) and BaCdSnSe4. The Debye-Callaway model was employed to investigate the phonon scattering phenomena that occur in these materials. Our analyses indicate similar anharmonicity in all crystal lattices. Furthermore, highly distorted corner-sharing chalcogen tetrahedra encompassing Cu/Cd atoms give rise to large thermal parameters for Cu/Cd in these materials. Relatively high scattering from soft modes in BaCu2SnS2Se2 was identified, such dynamic disorder resulting in very low κ values. Additional disorder for BaCu2SnS2Se2 is due to alloying, causing the lowest κ among the quaternary chalcogenides. The Debye and Einstein temperatures from Cp data corroborated the results obtained from our analyses of the κ data. The large primitive cells with complex lattice vibrations these materials possess lead to an increase in optical modes that further suppress κ. These results and analyses, as well as further recent results on other quaternary chalcogenides, demonstrate the direct impact of complex structural features on phonon dynamics, enhancing our fundamental understanding of the thermal properties of these materials. With material property developments being a significant aspect of thermoelectrics research, our results will aid investigations of other quaternary chalcogenides of interest for thermoelectric applications, as will be described in this presentation.