NMR and Mössbauer study of <i>p</i>-type half-Heusler thermoelectrics Nb_1−xTi_xFeSb

To investigate the local behavior of defects and mixed compositions in NbFeSb-based semiconductors, for improved thermoelectric efficiency, we have performed ⁹³Nb and ¹²¹Sb NMR, as well as Mössbauer measurements, on pure NbFeSb and a series of p-type Ti-substituted (Nb,Ti)FeSb samples with different substitution levels. A small but consistently increasing paramagnetic defect density is observed with the increase of Ti substitution level revealing the existence of additional Ti-induced paramagnetic defects. NMR line shapes show a clear difference between effect of intrinsic and extrinsic defects in NbFeSb. The NMR shifts can be well understood by a model combining a Knight shift and composition-dependent chemical shift. The results indicate a nearly rigid-band behavior for the valence band with a small enhacement of effective mass vs substitution. For pure NbFeSb samples, the Mössbauer spectra include an additional T-dependent singlet. The increase of its area can be explained based on carriers activated into a shallow acceptor-like defect level above the valence band, consistent with the defect activation results obtained with NMR. In samples with Ti substitution, the Mössbauer spectra are consistent with a random neighbor distribution, indicating no preferential local ordering.