Urban, Philipp; Schneider, Matthias N.; Erra, Loredana; Welzmiller, Simon; Fahrnbauer, Felix; Oeckler, Oliver
Temperature dependent resonant X-ray diffraction of single-crystalline Ge2Sb2Te5.
In: ChrystEngComm, Vol. 15, No. 24: pp. 4823-4829
The element distribution in the crystal structure of the stable phase of the well-known phase-change material Ge2Sb2Te5 was determined at temperatures up to 471 degrees C 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) angstrom, c = 17.2809(18) angstrom, P (3) over bar m1, R-1 (overall) = 0.037). The dispersion correction terms Delta f' 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 Ge0.60(4)Sb0.36(2)), Sb atoms the one near the van der Waals gap (Ge0.33(7)Sb0.66(4)). Anti-site disorder is not significant. During heating up to 471 degrees C 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-Ge2Sb2Te5 single crystals is predominantly under thermodynamic control.