半Heusler Nb $_{1-x}$ Ti $_x$ FeSb中的缺陷充电和共振能级
Defect charging and resonant levels in half-Heusler Nb$_{1-x}$Ti$_x$FeSb
摘要：
本文报道了在1000 K以上应用的最有前途的热电系统之一的Nb $_{1-x}$ Ti $_x$ FeSb（$0\leqslant x \leqslant0.3$）的$^{93}$ Nb和$^{121}$ Sb NMR和$^{57}$ Fe穆斯堡尔谱研究以及DFT计算。这些研究提供了关于这些重$p$型材料中的缺陷和电子构型的局部信息。NMR自旋-晶格弛豫速率提供了对价带内状态的测量。随着$x$的增加，不同取代分数的弛豫速率与载流子浓度的变化表明共振能级的重要性，这并不有助于电荷传输。局部顺磁磁化率是显着大于预期的DFT计算的基础上，我们讨论的增强的磁化率由于库仑增强机制。Ti取代样品的穆斯堡尔谱显示出与取代原子的二项分布的微小偏离，而对于未取代的$p$型NbFeSb，穆斯堡尔卫星峰的振幅随温度增加，测量位于价带边缘以上约30 meV的缺陷群的$T$依赖性充电，指示该位置处的杂质带。
英文摘要：
We report $^{93}$Nb and $^{121}$Sb NMR and $^{57}$Fe Mössbauer studies combined with DFT calculations of Nb$_{1-x}$Ti$_x$FeSb ($0\leqslant x \leqslant0.3$), 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 materials. 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 we discuss in terms of an enhancement of the susceptibility due to a Coulomb enhancement mechanism. The Mössbauer spectra of Ti-substituted samples 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, a measure of the $T$-dependent charging of a population of defects residing about 30 meV above the valence band edge, indicative of an impurity band at this location.
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