Direct ab initio simulation of silver ion dynamics in chalcogenide glasses

Mobile ions in amorphous materials have been a serious object of investigation [1], and their dynamics in disordered hosts constitute one of the major unsolved problems in the field of solid state ionics. Certain compositions of silver doped GeSe glasses (close to compositions reported here) have practical potential for novel computer memory devices [2]. The structure of the Ge– Se– Ag glasses has been investigated using standard techniques, including X-ray diffraction [4, 5], neutron diffraction with isotopic substitution [6], EXAFS [7], differential anomalous X-ray scattering (DAS) [8, 9], Modulated Differential Scanning Calorimetry (MDSC), and Raman spectroscopy [10]. Despite this database, the structure of the ternary Ge– Se– Ag glasses has not yet been completely determined. There continues to be a debate on basic aspects of the glass structure (i.e. homogeneity and Ag coordination) especially for Se rich glasses with more than 67% Se. In this work, we focused on Ag-doped glasses containing Ge 25 at% and Se 75 at% that we label later as GeSe 3. Such compositions are of special interest because of proximity to the intermediate phase, which is reported to not age and thus might contribute to formation of memory or other type of devices with special reliability [10]. For the simulations reported in this paper, we use FIREBALL2000 developed by Lewis and co-workers [11]. Total energies and forces were computed within an ab ini-tio local orbital formalism [12]. The exchange–correlation energy was treated within the LDA, for which we used the results of Ceperley and Alder [13], as interpolated by Per-dew and Zunger [14]. The pseudopotential and pseudoa-tomic wave functions were generated in the Troullier – Martins form [15] employing the scheme of Fuchs and Scheffler [16]. The models described here were generated using the melt-quenching method. We placed atoms randomly in a cubic supercell according to the correct stoichiometry [for (GeSe 3) 0.90 Ag 0.10 54 germanium atoms, 162 selenium atoms and 24 silver atoms; for (GeS 3) 0.85 Ag 0.15 51 germanium atoms , 153 selenium atoms and 36 silver atoms] with the minimum distance between atoms 2 Å. The size of the cubic cells was chosen to make the density of these glasses match experimental data. The box size of the 240 atom su-percell of (GeSe 3) 0.90 Ag 0.10 and (GeSe 3) 0.85 Ag 0.15 are respectively 18.601 Å and 18.656 Å with corresponding density 4.98 g/cm 3 and 5.03 g/cm 3 …