| dc.description.abstract | In this dissertation, NMR techniques combined with various experimental and computational techniques were applied to investigate materials systems including advanced thermoelectric ma- terials and topological chalcogenides. Among thermoelectric materials, substituted half-Heuslers Nb1−xTixFeSb (0 ≤ x ≤ 0.3), pure half-Heuslers NbCoSn, ZrCoSb and TaFeSb, as well as CoSb3- based skutterudites are studied. In addition, studies of the topological chalcogenides, ZrTe5 and ZrTe2 are described here. NMR and magnetic measurements can reveal the magnetic properties of the defects states in the samples. NMR is specialized to provide information of band structure close to Fermi level, which gives a better understanding of electronic structures of materials. For topological materials included here, NMR is also able to detect the Dirac Fermions and then show interesting topological features of them.
To investigate the electronic behavior and magnetic properties of NbFeSb, I have performed 93Nb and 121Sb NMR, specific heat and magnetic measurements on NbFeSb samples heat treated at high temperatures. Magnetic measurements, combined with an observed Schottky anomaly and changes in the NMR line width indicate the presence of a 0.2% concentrated native magnetic defect in stoichiometric NbFeSb samples. The origin of these native defects is believed due to Fe antisites on Nb sites. In addition, NMR shift and spin-lattice relaxation results below 200 K reveal a Korringa-like response indicating heavily-doped p-type behavior due to native defects. Above 280 K this goes over to an activated behavior indicating the presence of an impurity band, empty at low temperatures, which is located around 0.03 eV above valence band maximum.
To further understand this system, I have measured 93Nb and 121Sb NMR and 57Fe Mössbauer studies combined with DFT calculations of Nb1−xTixFeSb (0 ≤ x ≤ 0.3). With Ti substitution, this is one of the most promising thermoelectric systems for applications above 1000 K. These studies provide local information about defects and electronic configurations in these heavily p-type ma- terials. The NMR spin-lattice relaxation rate provides a measure of states within the valence band. With increasing x, changes of relaxation rate vs carrier concentration for different substitution fractions indicate the importance of resonant levels which do not contribute to charge transport. The local paramagnetic susceptibility is significantly larger than expected based on DFT calculations, which I discuss in terms of an enhancement of the susceptibility due to a Coulomb enhancement mechanism. I also analyzed Mössbauer spectra of Ti-substituted samples which show small depar- tures from a binomial distribution of substituted atoms, while for unsubstituted p-type NbFeSb, the amplitude of a Mössbauer satellite peak increases vs temperature, a measure of the T -dependent charging of a population of defects residing about 30 meV above the valence band edge, providing further information about an impurity band at this location.
I also investigated 59Co, 93Nb, and 121Sb NMR spectroscopy on an additional series of half- Heusler semiconductors, including NbCoSn, ZrCoSb, TaFeSb and NbFeSb, to better understand their electronic properties and general composition-dependent trends. These materials are of inter- est as potentially high efficiency thermoelectric materials. Compared to the other materials, I find that ZrCoSb tends to have a relatively large amount of local disorder, apparently antisite defects. This contributes to a small excitation gap corresponding to an impurity band near the band edge. In NbCoSn and TaFeSb, Curie-Weiss-type behavior is revealed, which indicates a small density of interacting paramagnetic defects. In general, very large paramagnetic chemical shifts are observed associated with closely spaced d states split between the conduction and valence bands. DFT methods were generally successful in reproducing the chemical shift trend for these half-Heusler materials, however, I identify an enhancement of the larger-magnitude shifts, connected to electron interaction effects. This trend is also connected to changes in d-electron hybridization across the series.
I have additionally applied 59Co NMR and transport measurements to probe the electronic be- havior of n-type filled skutterudites BaxYbyCo4Sb12 and AxCo4Sb12 (A = Ba, Sr), which similar to the half-Heusler materials are also excellent candidate thermoelectric materials. The results demonstrate consistently that a shallow defect level near the conduction band minimum dominates the electronic behavior, in contrast to the behavior of unfilled CoSb3. To analyze the results, I mod- eled the defect as having a single peak in the density of states, occupied at low temperatures due to donated charges from filler atoms. I fitted the NMR shifts and spin-lattice relaxation rates allowing for arbitrary carrier densities and degeneracies. The results provide a consistent picture for the Hall data, explaining the temperature dependence of the carrier concentration. Furthermore, with- out adjusting model parameters, I calculated Seebeck coefficient curves, which also provide good consistency. In agreement with recently reported computational results, it appears that composite native defects induced by the presence of filler atoms can explain this behavior. These results pro- vide a better understanding of the balance of charge carriers, of crucial importance for designing improved thermoelectric materials.
While zirconium tellurides are also of interest as thermoelectric materials, my work on these materials addresses most significantly their topological behavior. Among these topological chalco- genides, I carried out 125Te NMR measurements of the topological quantum material ZrTe5. Spin- lattice relaxation results, well-explained by a theoretical model of Dirac electron systems, reveal that the topological characteristic of ZrTe5 is T-dependent, changing from weak topological in- sulator to strong topological insulator as temperature increases. Electronic structure calculations confirm this ordering, the reverse of what has been proposed. NMR results demonstrate a gapless Dirac semimetal state occurring at a Lifshitz transition temperature, Tc = 85 K in our crystals. I demonstrate that the changes in NMR shift at Tc also provide direct evidence of band inversion when the topological phase transition occurs.
Finally, NMR studies of the transition metal dichalcogenide ZrTe2 were completed. The mea- sured NMR shift anisotropy reveals a quasi-2D behavior connected to a topological nodal line close to the Fermi level. With the magnetic field perpendicular to the ZrTe2 layers, the measured shift can be well-fitted by a combination of enhanced orbital diamagnetism and spin shift due to high mobility Dirac electrons. The spin-lattice relaxation rates with external field both parallel and perpendicular to the layers at low temperatures match the expected behavior associated with extended orbital hyperfine interaction due to quasi-2D Dirac carriers. In addition, calculated band structures also show clear evidence for the existence of nodal line in ZrTe2 between Γ and A. For intermediate temperatures, there is a sharp reduction in spin-lattice relaxation rate which can be explained as due to a reduced lifetime for these carriers, which matches the reported large change in mobility in the same temperature range. Above 200 K, the local orbital contribution starts to dominate in an orbital relaxation mechanism revealing the mixture of atomic functions. | en |
