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

Abstract

Metastable Ge 4 Bi 2 Te 7 is highly disordered; the average structure corresponds to the rocksalt type. The diffraction pattern shows diffuse streaks interconnecting Bragg reflections along all cubic h111i directions. These streaks exhibit satellitelike 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 threedimensional difference pair distribution function (3D-ÁPDF) method yields quantitative information on the distribution of defect layer spacings, which peaks at a value corresponding to Ge 3 Bi 2 Te 6 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.

Increasing Seebeck Coefficients and Thermoelectric Performance of Sn/Sb/Te and Ge/Sb/Te Materials by Cd Doping

“…Figure and Tables S5 and S6 in the Supporting Information) clearly reveal NaCl‐type average structures. As discussed earlier, such a symmetrical model is appropriate although quenching the NaCl‐type HT phase of GST materials yields samples with multiply twinned layer‐like domains, because their size is in the range of the X‐rays' length of coherence and the Te‐atom substructure retains its cubic ABC stacking sequence. No significant deviations from cubic metrics or symmetry were detected in tentative refinements using a α‐GeTe‐type structure model.…”

“…These defect layers apparently reduce the phononic part κ p of the thermal conductivity . The fact that the average structure of such compounds corresponds to the NaCl type implies a “cubic” ABC stacking sequence of the Te‐atom layers around the defect layers, in contrast to a “hexagonal” ABA sequence around the van der Waals gaps of stable layered trigonal phases. Compounds with n = 12 or 19 reach ZT values of up to 1.5 or 1.3 at 450 °C, respectively .…”

Diffuse single-crystal scattering corrected for molecular form factor effects

“…The 3D-ÁPDF thus contains information on the deviation of the local structure from the average crystal structure. The method allows for extensive, fast and fairly general modelling of diffuse scattering (Simonov, 2014;Simonov et al, 2014a,b;Urban et al, 2015). The technique depends on the measurement of a complete three-dimensional scattering data set.…”

On single-crystal total scattering data reduction and correction protocols for analysis in direct space

“…In a progression to a more quantitative analysis, short-range ordering parameters were successfully refined from the 3D-ÁPDFs (Simonov et al, 2014b;Urban et al, 2015). Later studies have revealed short-range ionic correlations in the intercalation compounds 0 -Na 0.45 V 2 O 5 (Krogstad et al, 2020) and local structure effects in the thermoelectric -Cu 2Àx Se (Roth & Iversen, 2019).…”