Physics Department, University of California, Santa Cruz, California 95064, USA.
Phys Rev Lett. 2013 Aug 30;111(9):095504. doi: 10.1103/PhysRevLett.111.095504. Epub 2013 Aug 29.
PbTe is a well-known thermoelectric material. Recent x-ray total scattering studies suggest that Pb moves off center along 100 in PbTe, by ∼0.2 Å at 300 K, producing a split Pb-Te pair distribution. We present an extended x-ray absorption fine structure spectroscopy (EXAFS) study of PbTe (and Tl doped PbTe) to determine if Pb or Te is off center. EXAFS provides sensitive r- or k-space phase information which can differentiate between a split peak for the Pb-Te distribution (indicative of off-center Pb) and a thermally broadened peak. We find no evidence for a split peak for Pb-Te or Te-Pb. At 300 K, the vibration amplitude for Pb-Te (or Te-Pb) is large; this thermally induced disorder is indicative of weak bonds, and the large disorder is consistent with the low thermal conductivity at 300 K. We also find evidence of an anharmonic potential for the nearest Pb-Te bonds, consistent with the overall anharmonicity found for the phonon modes. This effect is modeled by a "skew" factor (C3) which significantly improves the fit of the Pb-Te and Te-Pb peaks for the high temperature EXAFS data; C3 becomes significant above approximately 150-200 K. The consequences of these results will be discussed.
碲化铅是一种众所周知的热电材料。最近的 X 射线全散射研究表明，在 300K 时，铅沿 100 方向偏离中心约 0.2Å，产生分裂的铅-碲配分。我们对碲化铅（和掺铊的碲化铅）进行了扩展 X 射线吸收精细结构光谱（EXAFS）研究，以确定是铅还是碲偏离中心。EXAFS 提供了敏感的 r 或 k 空间相位信息，可以区分铅-碲分布的分裂峰（表明铅偏离中心）和热宽化峰。我们没有发现铅-碲或碲-铅存在分裂峰的证据。在 300K 时，铅-碲（或碲-铅）的振动幅度很大；这种热诱导的无序表明键较弱，而大的无序与 300K 时的低热导率一致。我们还发现了最近的铅-碲键的非谐势的证据，这与声子模式的整体非谐性一致。这种效应可以通过“倾斜”因子（C3）来建模，该因子显著改善了高温 EXAFS 数据中铅-碲和碲-铅峰的拟合；C3 在大约 150-200K 以上变得显著。这些结果的影响将被讨论。
