Quenched Ge–Sb–Te (GST) compounds exhibit strongly disordered metastable structures whose average structure corresponds to a distorted rocksalt type with trigonal symmetry. Depending on the composition and thermal treatment, the metrics remain more or less pseudocubic. The corresponding stable phases show regular sequences of distorted rocksalt-type blocks that formally result from layer-like cation defect ordering. These thermodynamically stable layered phases can gradually be approached by annealing the metastable (pseudo)cubic compounds that are accessible by quenching high-temperature phases. The relaxation of Te atoms in the vicinity of the defect layers leads to van der Waals gaps rather than defect layers in an undistorted matrix. The partially ordered phases obtained show defect layers with an average distance and arrangement depending on the composition and the thermal treatment of the samples. This variation of the nanostructure influences the lattice thermal conductivity (κL) and thus the thermoelectric figure of merit (ZT). This results in ZT values up to 1.3 at 450 °C for bulk samples of Sb2Te3(GeTe)n (n = 12 and 19). The stability ranges of the various phases have been examined by temperature programmed X-ray powder diffraction and can be understood in conjunction with the changes of the nanostructure involved. The real structure of phases Sb2Te3(GeTe)n (n = 3–19) has been investigated by high-resolution electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM)-high-angle annular dark-field (HAADF) with respect to the stoichiometry and synthesis conditions. The correlation of the nanostructure with the thermoelectric properties opens an interesting perspective for tuning thermoelectric properties.
The element distribution in the crystal structure of the stable phase of the well-known phase-change material Ge 2 Sb 2 Te 5 was determined at temperatures up to 471 uC using single crystals synthesized by chemical transport reactions. Because of the similar electron count of Sb and Te, the scattering contrast was enhanced by resonant diffraction using synchrotron radiation (beamline ID11, ESRF). A simultaneous refinement on data measured at the K-absorption edges of Sb and Te as well as at additional wavelengths off the absorption edges yielded reliable occupancy factors of each element on each position (a = 4.2257(2) Å, c = 17.2809(18) Å, P3m1, R 1 (overall) = 0.037). The dispersion correction terms Df 9 were refined and match experimental ones obtained from fluorescence spectra by the Kramers-Kronig transform. The structure contains distorted rocksalt-type blocks of nine alternating cation and anion layers, respectively, which are separated by van der Waals gaps between Te atom layers. Ge atoms prefer the cation positions near the center of the rocksalt-type block (occupancy factors Ge 0.60(4) Sb 0.36(2) ), Sb atoms the one near the van der Waals gap (Ge 0.33(7) Sb 0.66 (4) ). Anti-site disorder is not significant. During heating up to 471 uC and subsequent cooling, a reversible structural distortion was observed. The refinements show that with increasing temperature the first pair of anion and cation layers next to the van der Waals gap becomes slightly detached from the block and increasingly resembles a GeTe-type layer. Thus, the difference between interatomic distances in the 3 + 3 cation coordination sphere of the mixed Ge-Sb position next to the gap becomes more pronounced. The element distribution, in contrast, neither changes during the heating experiment nor upon long-time annealing. Thus, the behavior of 9P-Ge 2 Sb 2 Te 5 single crystals is predominantly under thermodynamic control.
Rewritable data-storage media and promising nonvolatile random-access memory are mainly based on phase-change materials ͑PCMs͒ which allow reversible switching between two metastable ͑amorphous and crystalline͒ modifications accompanied by a change in physical properties. Although the phase-change process has been extensively studied, it has not been elucidated how and why the metastable crystalline state is kinetically stabilized against the formation of thermodynamically stable phases. In contrast to thin-film investigations, the present study on bulk material allows to demonstrate how the cubic high-temperature phase of GeTe-rich germanium antimony tellurides ͑GST materials͒ is partially retained in metastable states obtained by quenching of bulk samples. We focus on compositions such as Ge 0.7 Sb 0.2 Te and Ge 0.8 Sb 0.13 Te, which are important materials for Blu-ray disks. Bulk samples allow a detailed structural characterization. The structure of a multiply twinned crystal isolated from such material has been determined from x-ray diffraction data ͑Ge 0.7 Sb 0.2 Te, R3m, a = 4.237 Å, c = 10.29 Å͒. Although the metrics is close to cubic, the crystal structure is rhombohedral and approximates a layered GeTe-type atom arrangement. High-resolution transmission electron microscopy ͑HRTEM͒ on quenched samples of Ge 0.8 Sb 0.13 Te reveal nanoscale twin domains. Cation defects form planar domain boundaries. The metastability of the samples was proved by in situ temperature-dependent powder diffraction experiments, which upon heating show a slow phase transition to a trigonal layered structure at ca. 325°C. HRTEM of samples annealed at 400°C shows extended defect layers that lead to larger domains of one orientation which can be described as a one-dimensionally disordered long-periodical-layered structure. The stable cubic high-temperature modification is formed at about 475°C. Powder diffraction on samples of Ge 0.8 Sb 0.13 Te with defined particle sizes reveal that the formation of the stable superstructure phase is influenced by stress and strain induced by the twinning and volume change due to the cubic → rhombohedral phase transition upon quenching. The associated peak broadening is larger for small crystallites that allow relaxation more readily. Consequently, the degree of rhombohedral distortion as well as the appearance of superstructure reflections upon annealing is more pronounced for small crystallites. The same is true for samples which were slowly cooled from 500°C. Hence, the lattice distortion accompanying the phase transition toward a stable trigonal superstructure is, to a certain degree, inhibited in larger crystallites. This kinetic stabilization of metastable states by stress effects is probably relevant for GST phase-change materials.
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