Real structure of Ge4Bi2Te7: refinement on diffuse scattering data with the 3D-ΔPDF method

Metastable Ge4Bi2Te7 is highly disordered; the average structure corresponds to the rocksalt type. The diffraction pattern shows diffuse streaks interconnecting Bragg reflections along all cubic 111 directions. These streaks exhibit satellite-like maxima and arise from vacancy ordering in non-periodically spaced defect layers. The atom layers near these vacancy layers are displaced with respect to the average structure: they tend to form -GeTe-type double layers. The three-dimensional difference pair distribution function (3D-PDF) method yields quantitative information on the distribution of defect layer spacings, which peaks at a value corresponding to Ge3Bi2Te6 building blocks. The cation distribution along with the displacement of the atom layers is refined as well, using a least-squares approach. Bi concentrates on cation positions next to the vacancy layers.