A large new family of filled skutterudites stabilized by electron count

Based on the interplay of theory and experiment, a large new family of filled group 9 (Co, Rh and Ir) skutterudites is designed and synthesized. The new materials fill the empty cages in the structures of the known binary CoSb3, RhSb3 and IrSb3 skutterudites with alkaline, alkaline earth, and rare earth atoms to create compounds of the type AyB4X12; A atoms fill the cages to a fraction y, B are the group 9 transition metals, and X is a mixture of electronegative main group elements chosen to achieve chemical stability by adjusting the electron counts to electron-precise values. Forty-three new compounds are reported, antimony-tin and phosphorous-silicon based, with 63 compositional variations presented. The new family of compounds is large and general. The results described here can be extended to the synthesis of hundreds of new group 9 filled skutterudites.Comment: A revised version with the title"A large family of filled skutterudites stabilized by electron count"will appear in Nature Communication

Design, crystal growth, and physical properties of low-temperature thermoelectric materials

Thermoelectric materials serve as the foundation for two important modern technologies, namely 1) solid-state cooling, which enables small-area refrigeration without vibrations or moving parts, and 2) thermoelectric power generation, which has important implications for waste heat recovery and improved sources of alternative energy. Although the overall field of thermoelectrics research has been active for decades, and several consumer and industrial products have already been commercialized, the design and synthesis of new thermoelectrics that outperform long-standing state of the art materials has proven extremely challenging. This is particularly true for low-temperature refrigeration applications, which is the focus of this work; however, scientific advances in this area generally support power generation as well. In order to achieve more efficient materials for virtually all thermoelectric applications, improved materials design principles must be developed and synthetic procedures must be better understood. We aim to contribute to these goals by studying two classes of materials, namely 1) the tetradymites Bi2TeSe2 and Bi2Te2Se, which are close relatives of state of the art thermoelectric cooling materials, and 2) Kondo insulating (-like) FeSb2 and FeSi, which possess anomalously enhanced low-temperature thermoelectric properties that arise from exotic electronic and magnetic properties. The organization of this dissertation is as follows: Chapter 1 is a brief perspective on solid-state chemistry. Chapter 2 presents experimental methods for synthesizing and characterizing thermoelectric materials. In Chapter 3, two original research projects are discussed: first, work on the tetradymite Bi2TeSe2 doped with Sb to achieve an n- to p-type transition, and second, the tetradymite Bi2Te2Se with chemical defects through two different methods. Chapter 4 gives the magnetic and transport properties of FeSb2-RuSb2 alloys, a family of compounds exemplifying what we consider to be the next generation of thermoelectric materials for low-temperature cooling due to their anomalously enhanced low-temperature thermoelectric properties, along with an outlook for seeking additional materials with similarly enhanced properties. Lastly, in Chapter 5, a brief outlook on the future of thermoelectrics is discussed, along with our current and future work on FeSi-RuSi alloys

Low Temperature Thermoelectric Properties of Bi₂₋ₓSbₓTeSe₂ Crystals near the n-p Crossover

Seebeck coefficients, electrical resistivities, thermal conductivities and figure of merit ZT of Bi2-xSbxTeSe2 crystals (x=0.8, 0.9, 1.0, 1.1, and 1.2) measured along the hexagonal basal plane are presented. The crystals gradually change from n- to p-type with increasing Sb content, with the crossover lying in the region between x=1.0 and 1.1. The crossover is accounted for by a simple (p-n) electron-hole compensation model, as supported by carrier concentrations determined from Hall measurements. ZT was found to be maximized near the crossover on the p-type side, with the high electrical resistance of the Se-rich crystals apparently the limiting factor in the performance. These materials may serve as a basis for future nanostructuring or doping studies

Correlated evolution of colossal thermoelectric effect and Kondo insulating behavior

APL Materials. Volume 1, Issue 6, December 2013, Article number 062102.We report the magnetic and transport properties of the Ru 1-xFexSb2 solid solution, showing how the colossal thermoelectric performance of FeSb2 evolves due to changes in the amount of 3d vs. 4d electron character. The physical property trends shed light on the physical picture underlying one of the best low-T thermoelectric power factors known to date. Some of the compositions warrant further study as possible n- and p-type thermoelements for Peltier cooling well below 300 K. Our findings enable us to suggest possible new Kondo insulating systems that might behave similarly to FeSb2 as advanced thermoelectrics. © 2013 Author(s)

Quasi One Dimensional Dirac Electrons on the Surface of Ru2Sn3

Scientific Reports. Volume 4, 4 June 2014, Article number 5168.We present an ARPES study of the surface states of Ru 2 Sn 3, a new type of a strong 3D topological insulator (TI). In contrast to currently known 3D TIs, which display two-dimensional Dirac cones with linear isotropic dispersions crossing through one point in the surface Brillouin Zone (SBZ), the surface states on Ru 2 Sn 3 are highly anisotropic, displaying an almost flat dispersion along certain high-symmetry directions. This results in quasi-one dimensional (1D) Dirac electronic states throughout the SBZ that we argue are inherited from features in the bulk electronic structure of Ru2 Sn3 where the bulk conduction bands are highly anisotropic. Unlike previous experimentally characterized TIs, the topological surface states of Ru 2 Sn3 are the result of a d-p band inversion rather than an s-p band inversion. The observed surface states are the topological equivalent to a single 2D Dirac cone at the surface Brillouin zone

Gold–Gold Bonding: The Key to Stabilizing the 19-Electron Ternary Phases <i>Ln</i>AuSb (<i>Ln</i> = La–Nd and Sm)

We report a new family of ternary 111 hexagonal <i>Ln</i>AuSb (<i>Ln</i> = La–Nd, Sm) compounds that, with a 19 valence electron count, has one extra electron compared to all other known <i>Ln</i>AuZ compounds. LaAuSb, CeAuSb, PrAuSb, NdAuSb, and SmAuSb crystallize in the YPtAs-type structure, and have a doubled unit cell compared to other <i>Ln</i>AuZ phases as a result of the buckling of the Au–Sb honeycomb layers to create interlayer Au–Au dimers. The dimers accommodate the one excess electron per Au and thus these new phases can be considered <i>Ln</i>₂³⁺(Au–Au)⁰Sb₂^3–. Band structure, density of states, and crystal orbital calculations confirm this picture, which results in a nearly complete band gap between full and empty electronic states and stable compounds; we can thus present a structural stability phase diagram for the <i>Ln</i>Au<i>Z</i> (Z = Ge, As, Sn, Sb, Pb, Bi) family of phases. Those calculations also show that LaAuSb has a bulk Dirac cone below the Fermi level. The YPtAs-type <i>Ln</i>AuSb family reported here is an example of the uniqueness of gold chemistry applied to a rigidly closed shell system in an unconventional way