| dc.description.abstract | Thermoelectric (TE) materials offer interconversion of thermal and electrical energy through the presence of a heat gradient and thus constitute an attractive option for the recovery of waste heat. Low conversion efficiency and high cost have been major limiting factors in the industrial success of TE materials, and, therefore, a race toward practical materials with enhanced physical properties has emerged to meet the increasing demands. The tetrahedrite-tennantite series is an important family of copper natural ore mineral and also one of the most widespread sulfosalts on the earth. Very low lattice thermal conductivities, less than 0.5 W/m K, were observed in this system, attributed to the unique crystal structure of the tetrahedrite Cu12Sb4S13. In these materials, half of the Cu atoms occupy four-coordinate sites Cu-I and half occupy three-coordinate sites Cu-II. Due to the an-harmonic force introduced by the Sb lone pairs, the equivalent isotropic atomic displacement parameter for the Cu-II atoms is found to be very large, an indication that the Cu-II atoms vibrate with low energy in the stiff framework. Thus, they may strongly interact with heat-carrying phonons similar to other caged TE compounds, e.g. skutterudites and clathrates.
In this work, the origin of the intrinsic low thermal conductivity and electronic behavior is investigated by both theoretical and experimental NMR studies. The effects of additional atom substitution on phase transition and magnetic properties of tetrahedrite are studied with the conclusion that electronic and magnetic characteristics of this compound are not as sensitive to impurities as is typically the case for TE materials. We use NMR technique as a flexible tool to investigate chemical structures and determine symmetries, atomic motion, along with hyperfine interactions. We also applied NMR technique to better understand the anharmonic phonon behavior of undoped Cu12Sb4S13.
We found spin-lattice relaxation rates (1/T1) are dominated by a quadrupolar process indicating anharmonic vibrational dynamics both above and below Tc. We used a quasiharmonic approximation for localized anharmonic oscillators to analyze the impact of Cu rattling. The results demonstrate that Cu-atom rattling dynamics extend unimpeded in the distorted structural configuration below Tc and provide a direct measure of the anharmonic potential well.
We further report 63Cu NMR measurements for the Cu-rich phase of Cu12+xSb4S13(x≤2) and compared to Cu12Sb4S13. We identify the NMR signatures of phase segregation into Cu-poor (x ≈ 0) and Cu-rich (x ≈ 2) phases, with the metal-insulator transition observed in Cu12Sb4S13 suppressed in the Cu-rich phase. Based on NMR T1 and T2 measurements, the results demonstrate Cu-ion hopping below room temperature with an activation energy of ∼150 meV for the Cu-rich phase, consistent with superionic behavior. The NMR results also demonstrate the effects of Cu- ion mobility in the Cu12Sb4S13 phase, but with a larger activation barrier. We identify a small difference in NMR Knight shift for the metallic phase of Cu12Sb4S13 compared to the Cu-rich phase, and when compared to density functional theory calculations, the results indicate a mix of hyperfine contributions to the metallic shift.
We carried out material characterization as well as 63Cu NMR measurement of various substituted tetrahedrites including Cu10.6Zn0.5Ni0.9Sb4S13, Cu10ZnNiSb4S13, Cu10Ni2Sb4S13, Cu10Zn2Sb4S13, Cu11MnSb4S13, Cu10Mn2Sb4S13, and Cu12Sb4–xTexS13 compositions. DFT calculations were also used to model chemical shifts and explore changes in partial DOS for analyzing the results. By modeling the spectra of Zn and Ni substituted materials we found information about the changes in symmetry and electronic behavior in these materials. We observed that all substituted materials have a similar chemical shifts for the Cu-I site which indicates distinct behavior from undoped Cu12Sb4S13. We discussed the results in terms of random local distortions. Lack of Knight shift for most of the substituted tetrahedrites observed which indicates the importance of native defects or generation of a pseudogap structure.
We observed that most of the lineshapes exhibit magnetic broadening at low temperatures. Magnetic moment analysis based on the NMR lineshapes agrees with the previously proposed local moment for Ni substitution in low concentrations. On the other hand, Ni2 spectra analysis indicates more strongly interacting moments with distinct electronic behavior. We also found that rattling is the main effect controlling the scattering of phonons in Zn-Ni substituted materials and this was confirmed by the largest rattling behavior observed in Zn, Ni co-doped sample. Our results indicate that this is not simply tied to the expansion of lattice. A lack of Cu-ionic motion was observed in all substituted materials except in Cu12Sb3TeS13, and the MST is suppressed in almost all of the doped materials. | en |
