Novel superstructure of the rocksalt type and element distribution in germanium tin antimony tellurides

•A novel superstructure of the rocksalt-type in the system Ge–Sn–Sb–Te is elucidated.•It combines the cubic stacking of the HT phase with building blocks of the RT phase.•It indicates the ordering mechanism during the phase transition of GST materials.•A hierarchical structure motif is promising with respect to the reduction of κL.•Resonant diffraction reveals the element distribution in 39R-Ge3SnSb2Te7.

A superstructure of the rocksalt-type observed in quenched CVT-grown single crystals of Ge3.25(7)Sn1.10(3)Sb1.10(3)Te6 was elucidated by X-ray diffraction using fourfold twinned crystals (space group P3¯m1, a=4.280(1) Å, c=20.966(3) Å). The structure is built up of distorted rocksalt-type building blocks typical for long-range ordered GST materials and substitution variants thereof. In contrast to those phases, an exclusive ABC-type cubic stacking sequence of the Te-atom layers is present. High-resolution electron microscopy reveals spheroidal domains with this structure (average diameter 25 nm) whose stacking direction is perpendicular to the 〈1 1 1〉 directions of the basic rocksalt-type structure. Additional slab-like domains with a lateral extension up to 1 µm occasionally result in a hierarchical structure motif. Due to the similar electron counts of the elements involved, resonant diffraction was used in order to elucidate the element distribution in rocksalt-type building blocks of the stable layered compound 39R-Ge3SnSb2Te7 (R3¯m, a=4.24990(4) Å, c=73.4677(9) Å). Sb tends to occupy the atom site close to the van der Waals gaps while Ge concentrates in the center of the building blocks.

Graphical abstractHigh-resolution transmission electron micrograph, SAED pattern and reciprocal lattice section of X-ray single crystal data of Ge3.25Sn1.1Sb1.1Te6 with an 11P-type superstructure of the rocksalt type.Figure optionsDownload full-size imageDownload as PowerPoint slide