Bournonite PbCuSbS 3 : Stereochemically Active Lone-Pair Electrons that Induce Low Thermal Conductivity Yongkwan Dong, [a] Artem R. Khabibullin, [a] Kaya Wei, [a] James R. Salvador, [b] George S. Nolas,* [a] and Lilia M. Woods* [a] 1. Introduction Transition- and main-group-metal chalcogenides exhibit useful physical and chemical properties that are of interest for tech- nological applications. The low thermal conductivity, k, pos- sessed by many such chalcogenide materials, plays a key role in areas related to rewriteable data storage [1–3] and thermal barrier coatings, [4–6] for example. A low k is essential in rewrite- able devices based on phase-change systems in order to reduce heating during operation, due to rapid local melt quenching processes. [1–3] Thermal barrier materials naturally re- quire low thermal conductivities, due to the desire to protect the underlying substrate. [5, 6] Another related application is for thermoelectricity, [6–10] which is a renewable, solid-state, and ecologically benign method of energy conversion. Materials with a low k are imperative for both power generation and cooling applications. This is clearly evident from the definition of the dimensionless figure of merit (ZT = S 2 T/1k, where S is the Seebeck coefficient, T is the absolute temperature, and 1 is the electrical resistivity, the value through which thermoelectric (TE) materials are evaluated. [8–10] Targeting materials with thermal conductivities in a specific range constitutes major efforts that extend in various direc- tions, including developing novel ab initio simulations tech- niques, [11] and utilizing phase transitions [12] and topological phases in materials. [13] To obtain materials with a low k, many different approaches, such as nanostructuring in bulk, [6, 14–16] al- loying solid solutions with local anisotropic structural disor- der, [17–20] and phonon-mode softening through cage-structure filling, [21, 22] have been employed. In all cases the structure- bonding relationships within a given material class is crucial for understanding the underlying mechanism(s) associated with low k. Recently, Skoug and Morelli [23] have shown a corre- lation between lone-pair electrons (s 2 pair in group 15) and k min (the minimal thermal conductivity) in ordered crystalline chalcogenides. The distortion that lone-pair electrons of group 15 elements and neighboring chalcogen atoms is direct- ly related to low k by inducing unusually high lattice anharmo- nicity. Natural minerals such as tetrahedrites, as well as modi- fied tetrahedrites, possess intrinsically low k, due to Sb lone- pair s 2 electrons [24, 25] and have been considered to be promis- ing thermoelectric materials. Doping, impurities, and other fac- tors are not expected to have an important effect on anhar- monic processes as they are intrinsic to the particular material. This feature is especially useful for thermoelectricity as it pres- ents pathways to optimize the electronic properties by meth- ods that do not typically affect this inherently low lattice k. Nevertheless, significant effort is still needed to obtain a fun- damental microscopic understanding of the particular process- es that contribute to anharmonicity. The lone-pair electrons/ low thermal conductivity correlation is an important step in identifying systems for the potential applications described above. A microscopic understanding of the lone-pair s 2 elec- trons and the mechanisms responsible for enhanced anharmo- nicity, however, is missing. To advance in this important direc- tion, suitable systems must be investigated in detail to deter- mine underlying relationships between the lattice structure An understanding of the structural features and bonding of a particular material, and the properties these features impart on its physical characteristics, is essential in the search for new systems that are of technological interest. For several relevant applications, the design or discovery of low thermal conductiv- ity materials is of great importance. We report on the synthe- sis, crystal structure, thermal conductivity, and electronic-struc- ture calculations of one such material, PbCuSbS 3 . Our analysis is presented in terms of a comparative study with Sb 2 S 3 , from which PbCuSbS 3 can be derived through cation substitution. The measured low thermal conductivity of PbCuSbS 3 is ex- plained by the distortive environment of the Pb and Sb atoms from the stereochemically active lone-pair s 2 electrons and their pronounced repulsive interaction. Our investigation sug- gests a general approach for the design of materials for phase- change-memory, thermal-barrier, thermal-rectification and ther- moelectric applications, as well as other functions for which low thermal conductivity is purposefully sought. [a] Dr. Y. Dong, + A. R. Khabibullin, + K. Wei, Prof. G. S. Nolas, Prof. L. M. Woods Department of Physics, University of South Florida Tampa, FL 33620 (USA) E-mail : gnolas@usf.edu lmwoods@usf.edu [b] Dr. J. R. Salvador Chemical and Materials Systems Laboratory, GM R&D Center Warren, MI 48090 (USA) [ + ] Y.D. and A.R.K. have contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500476. ChemPhysChem 2015, 16, 3264 – 3270 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3264 Articles DOI: 10.1002/cphc.201500476