Combined EXAFS and first-principles theory study ofPb1−xGexTe

The narrow band-gap semiconductor ${\mathrm{Pb}}_{1-x}{\mathrm{Ge}}_x\mathrm{Te}$ has a low-temperature ferroelectric rhombohedral phase whose average structure is a distorted rocksalt structure. We have measured the extended x-ray-absorption-fine-structure (EXAFS) spectra of ${\mathrm{Pb}}_{1-x}{\mathrm{Ge}}_x\mathrm{Te}$ with $x\approx 0.3$ at the Ge and Te K edges and at the Pb $L_{\mathrm{III}}$ edge. Guided by first-principles calculations, we create a model for the local structure as a distortion from the ideal rocksalt structure. By corefining the spectra from these three edges, we demonstrate that the data are consistent with our fitting model and we directly measure several secondary structural distortions predicted by the theory. This work demonstrates a powerful approach to the determination of local structures in complex materials by using first-principles calculations in conjunction with EXAFS measurements.