Chalcogenide phase-change materials, such as GeSb2Te4, Ge2Sb2Te5, show strikingly contrasting optical and electrical properties, leading to promising extensive implementation in both memory devices and thermoelectric applications. For 2(GeTe)·(Sb2Te3), namely Ge2Sb2Te5, two different possible atoms sequences have been reported: -Te-Sb-Te-Ge-Te-Ge-Te-Sb-Te- (S1) [1] and -Te-Ge-Te-Sb-Te-Sb-Te-Ge-Te- (S2) [2]. In this paper, we performed a series of first principles calculations using density functional theory (DFT) to determine electronic and thermoelectric properties of Ge2Sb2Te5 with these 2 different atoms sequences. The related compounds GeTe and Sb2Te3 were also investigated for comparison. Different exchange-correlation functionals (LDA, PBE, WC and HSE potentials) were tested, w/o spin-orbit coupling, which has been found to have important effects. The calculated electronic bands indicate that S1 is a direct band gap semiconductor, whereas S2 shows metallic characters. We also calculated elastic moduli, dielectric constants, Born effective charges, and phonon dispersion within the quasi-harmonic approximation. Based on the above-mentioned calculations results, thermal conductivity has been obtained by solving the Boltzmann transport equation. The most interesting compound for thermoelectric applications was found to be Ge2Sb2Te5 with the S1 sequence. Furthermore, S1 shows robust bandgap under slight biaxial strains (-1.6% to 2%) and a sensitive semiconductor-to-metal transition (-1.6% to -1.8% and 2.1% to 3%). Additionally, the QTAIM theory was employed to explain the differences in the properties of the 2 stackings. 

[1] Kooi, B. J., and J. Th M. De Hosson. "Electron diffraction and high-resolution transmission electron microscopy of the high temperature crystal structures of GexSb2Te3+x(x=1, 2, 3) phase change material." Journal of applied physics 92.7 (2002): 3584-3590.
[2] Petrov, I. I., R. M. Imamov, and Z. G. Pinsker. "Electron-diffraction determination of the structures of Ge2Sb2Te5 and GeSb4Te7." Sov Phys Crystallogr 13.3 (1968): 339-342.