Abstract
Quenching cubic high-temperature polymorphs of (GeTe)nSb2Te3 (n ≥ 3) yields metastable phases whose average structures can be approximated by the rocksalt type with 1/(n + 3) cation vacancies per anion. Corresponding diffraction patterns are a superposition of intensities from individual twin domains with trigonal average structure but pseudo-cubic metrics. Their four orientations are mirrored in structured diffuse streaks that interconnect Bragg reflections along the [001] directions of individual disordered trigonal domains. These streaks exhibit a “comet-like” shape with a maximum located at the low-angle side of Bragg positions (“comet head”) accompanied by a diffuse “comet tail”. 2D extended cation defect ordering leads to parallel but not equidistantly spaced planar faults. Based on a stacking fault approach, the diffuse scattering was simulated with parameters that describe the overall metrics, the concentration and distribution of cation defect layers, atom displacements in their vicinity and the stacking sequence of Te atom layers around the planar defects. These parameters were varied in order to derive simple rules for the interpretation of the diffuse scattering. The distance between Bragg positions and “comet heads” increases with the frequency of planar faults. A sharp distance distribution of the planar faults leads to an intensity modulation along the “comet tail” which for low values of n approximates superstructure reflections. The displacement of atom layers towards the planar defects yields “comets” on the low-angle side of Bragg positions. A rocksalt-type average structure is only present if the planar defects correspond to missing cation layers in the “cubic” ABC stacking sequence of the Te atom layers. An increasing amount of hexagonal ABA transitions around the defect layers leads to increasing broadening and splitting of the Bragg reflections which then overlap with the diffuse scattering. Based on these rules, the diffuse scattering of (GeTe)nSb2Te3 (n = 2, 4, 5, 12) crystals was analyzed by comparing simulated and experimental reciprocal space sections as well as selected streaks extracted from synchrotron data. With decreasing n, both the average distance between faults and thus the slab thickness decrease, whereas the probability of hexagonal ABA transitions increases. The quenched metastable phases can be understood as intermediates between the stable high-temperature phases, which exhibit a rocksalt-type structure with randomly disordered cations and vacancies on the cation position, and the trigonal layered structures, which are stable at room temperature and consist of distorted rocksalt-type slabs separated by equidistant defect layers.
Acknowledgments
The authors thank T. Miller for laboratory single-crystal data collection as well as C. Minke for SEM operation and EDX analyses. We further thank the operating staff of beamline ID11 (ESRF, Grenoble), especially Dr. G. Vaughan and Dr. L. Erra for their help. Martin Ehrensperger is acknowledged for help with the synthesis. Furthermore, we are indebted to Prof. Dr. W. Schnick for his generous support of this work. This investigation was funded by the Deutsche Forschungsgemeinschaft (grant OE530/1-2).
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The online version of this article (DOI: 10.1515/zkri-2014-1829) offers supplementary material, available to authorized users.
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