Sb-doped PbTe: An NMR Perspective

In PbTe, Sb as a dopant can occupy either Pb or Te sites. To understand the effect of Sb on the local charge-carrier concentration in both cases, we have studied high-resolution $^{125}$Te and $^{207}$Pb NMR spectra of Pb$_{1-x}$Sb$_{x}$Te, PbSb$_{x}$Te$_{1-x}$, and $n$- and $p$-type PbTe samples. The spectra of Pb$_{0.9975}$Sb$_{0.0025}$Te and PbSb$_{0.0025}$Te$_{0.9975}$ have distinctly different resonance frequencies due to Knight shifts and chemical shifts produced by Sb at Pb or Te sites. Pb$_{0.9975}$Sb$_{0.0025}$Te is $n$-type while in PbSb$_{0.0025}$Te$_{0.9975}$ both $n$- and $p$-type are found. NMR spectra and spin-lattice $T_{1}$ relaxation of $^{207}$Pb nuclei in PbSb$_{0.0025}$Te$_{0.9975}$, which are sensitive to the hyperfine interaction between charge carriers and NMR nuclei, reveal at least four components, which reflect electronic inhomogeneity of the sample. The local carrier concentrations estimated from $T_{1}$ NMR varies from $n<$3x10$^{17}$ to $p\sim $10$^{19}$ cm$^{-3}$. These multiple components help rationalize the complex temperature dependence of the thermopower of PbSb$_{0.0025}$Te$_{0.9975}$. However, comparison with Hall and Seebeck effects data indicates that some NMR signals are due to localized electron states, which do not directly contribute to transport.