Defect charging and resonant levels in half-Heusler Nb$_{1-x}$Ti$_x$FeSb

Abstract

We report NMR and Mössbauer studies along with DFT calculations of Nb1−xTixFeSb (0 x 0.3), one of the most promising thermoelectric systems. These studies provide local information about defect changes and atomic configurations in these heavily p-type materials. The NMR spinlattice relaxation rate is dominated by an orbital contribution rather than spin contribution, which provides a good measure of states within the valence band. The changes of effective mass and carrier concentration for different substitution fractions indicate the importance of deep resonant states. Both the measured Knight shift and chemical shift are significantly larger than expected, which we discuss in terms of an enhancement of Pauli susceptibility and Van Vleck susceptibility due to a Coulomb enhancement mechanism. The Mössbauer spectra of Ti-substituted samples are analyzed to show small departures from a binomial distribution of substituted atoms, while for unsubstituted p-type NbFeSb, the amplitude of a Mössbauer satellite peak increases vs temperature, showing a T -dependent charging behavior of defects and the corresponding impurity band located around 30 meV above the valence band.

Half-Heusler thermoelectric materials: NMR studies

“…However, since other mechanisms can contribute to 1/T 1 and generally increase the rate, here we can consider that 1/T 1 T serves to provide an upper limit estimate for K c . For example, in substituted NbFeSb a significant orbital contribution was shown to contribute to the relaxation rate [18]; however, this term does not contribute to K c [18], and as noted above, paramagnetic contributions due to local moments can in some cases also contribute to 1/T 1 .…”

“…5 as simple vertical lines. For the case of as described above measurements of Ti-substituted materials were previously used to extract the chemical shifts [18], and the corresponding statistical error bars from this fit are displayed in Fig. 5.…”

“…Both 93 Nb and 121 Sb relaxation peaks have very similar magnitudes further confirming the mechanism of long-range dipolar interaction with dilute local magnetic spins in NbFeSb and validating the previous results.NbFeSb has a significant Knight shift for both nuclei. In Ref [18],. both 93 Nb and 121 Sb chemical shifts were extracted by studying a series of Ti-substituted samples (Nb 1−x Ti x FeSb).…”