Effect of concentration in Ge-Te liquids: A combined density functional and neutron scattering study

Abstract

International audienceThe structural properties of three compositions of Ge-Te liquids (Ge10Te90,Ge15Te85,Ge20Te80) are studied from a combination of density functional based molecular dynamics simulations and neutron scattering experiments. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, neighbor distributions and compare our results with experimental findings. Most noticeable is the good agreement found in the reproduction of the structure in real and reciprocal space, resulting from the incorporation of dispersion forces in the simulation. This leads to Ge and Te coordination numbers which are lower than in previous studies and which can now be followed with temperature, while also strongly depending on the chosen cutoff distance. Results show a gradual conversion of higher coordinated species (TeIV, GeV) into lower coordinated ones at lower temperature, while leaving anticipated coordinations from the octet rule (TeII and GeIV) nearly unchanged. Structural correlations are characterized as a function of temperature and composition. The vibrational density of states is also measured from inelastic neutron scattering for different compositions and temperatures, and compared to the simulated counterpart which exhibits a reasonable agreement at low frequency

Quantitative measure of tetrahedral-sp3geometries in amorphous phase-change alloys

“…One of the main results of taking into account such an empirical term is that an increased agreement on structure is obtained, which solves the well-known bond distance problem encountered in standard DFT-based simulations of tellurides [12]. This leads indeed to a reduction of the coordination numbers and a better reproduction of the experimental pair distribution function g(r) as also demonstrated recently in the liquid phase for the present investigated Ge-Te compositions [29], as discussed next.…”

“…This picture has been challenged in the literature from FPMD simulations disregarding dispersion forces but such simulations (as the green curve in Fig. 6) usually lead to a systematic overestimation of the Ge-Te bond lengths and Ge coordination numbers with respect to experiments [30] as also revealed in the liquid phase [29]. They also lead to a significantly higher fraction of octahedra as we have calculated η T = 23.4% for Ge 20 Te 80 without Eq.…”

“…The tetrahedral Ge fraction in Ge-Te is very difficult to estimate given that only indirect evidences can be provided by Raman or x-ray photoemission spectroscopy. DFT simulations which usually disregard dispersion forces, lead to structural models that exhibit a "bond-distance" problem [12,29], i.e., Ge-Te bond distance lengths longer than the experimental measured ones. Longer bond lengths may erroneously lead to an increased fraction of octahedral Ge species.…”

Concepts and applications of rigidity in non-crystalline solids: a review on new developments and directions

“…Ge 20 Te 80 only three angles are found to have small angular excursions. These involve the first three neighbours of Ge at distances of about 2.69 Å [118,123], and with angles found at (≃ 98°). This defines a pyramid with a triangular basis having the Te-Te bonds as edges, and a Ge at the remaining vertex, similar to the pyramidal geometry found in As 2 Se 3 for which three rigid angles are also obtained [59].…”

“…The neat estimate of constraints from MD simulations [117,118] also permits to address an important issue in the field of phase change materials, i.e. the detection of Ge tetrahedral units and the measurement of their population with e.g.…”