Structure, topology, rings, and vibrational and electronic properties of GexSe1−xglasses across the rigidity transition: A numerical study

Abstract

The structural, electronic, and vibrational properties of glassy Ge x Se 1−x are studied using density-functionalbased molecular dynamics. The focus is on four compositions (x = 10%,20%,25%,33%) spanning the rigidity transitions and representing typical compositions of flexible, intermediate, and stressed rigid systems. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions and compare our results with experimental findings, when available. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal space which allows tracking the effect of Ge composition on the structure. Ring statistics and ring correlations are examined and followed across the rigidity transition, and the details of typical small rings show a much more complex picture than established previously. A comparison is made with simple bond models and their validity is discussed. Topological constraint analysis is performed and shows that the onset of rigidity changes substantially the angular motion inside the Ge tetrahedra, which displays increased soft bending motions in the stressed rigid phase. We then investigate the vibrational properties via the vibrational density of states and the dielectric function (infrared absorption), and discuss them with respect to experimental findings. Finally, the electronic properties are computed and show an excellent agreement with respect to previous first-principles simulations and to experiments.

Pressure-induced structural changes in the network-forming isostatic glassGeSe4: An investigation by neutron diffraction and first-principles molecular dynamics

“…10. Our model reproduces the EDOS profile found in previous calculations at ambient pressure [26] and shows a decrease of the band gap energy with increasing pressure [ Fig. 11(a)].…”

“…The experimental work is complemented by a set of Car-Parrinello [45] FPMD simulations that give an accurate account of the atomic-scale structure in the Ge x Se 1−x family of network-forming materials, as indicated by detailed comparisons with experiment for compositions that include GeSe [46,47], Ge 2 Se 3 [17,[48][49][50], GeSe 2 [26,34,[51][52][53][54][55][56][57][58], GeSe 4 [26,56,[59][60][61][62], and GeSe 9 [26,63]. The combined techniques yield a self-consistent picture for GeSe 4 glass over the entire pressure range in which there is little change to the topological and chemical ordering at pressures up to ∼8 GPa.…”

Is cement a glassy material?

“…We note that the PDFs of those three systems does not show significant differences that would clearly allow distinguishing a glassy from a crystalline phase. They actually present a similar shape than the PDFs of silicate [21] and chalcogenide glasses [22]. The first peak in the 1Å region is associated to H-O bonds.…”

Topological Ordering and Viscosity in the Glass-Forming Ge–Se System: The Search for a Structural or Dynamical Signature of the Intermediate Phase

“…Modeling methods can, however, access information on higher-body correlation functions, and Micoulaut et al (2013) used first-principles molecular dynamics to investigate the structure of several Ge x Se 1−x glasses with compositions spanning the intermediate phase. Although a compelling structural signature of the intermediate phase was not found, constraint-counting algorithms show that broken bond-bending constraints are associated with the stressed-rigid phases at x = 1/3 and x = 0.40.…”