Bimodal phase percolation model for the structure of Ge-Se glasses and the existence of the intermediate phase

Abstract

International audienceA detailed nuclear magnetic resonance and Raman study of GexSe1−x glasses indicate that the glass structure is composed of intertwined microdomains of GeSe2 and Sen. Static nuclear magnetic resonance spectra of glasses ranging from 0≤x≤1/3 reveal the absence of Ge-Se-Se fragments in the structure. High temperature nuclear magnetic resonance showing considerable line narrowing confirms this observation. More importantly, the fraction of Se-Se-Se obtained by integration of nuclear magnetic resonance lines matches closely the percentage predicted for a bimodal phase model and is not consistent with the existence of Ge-Se-Se fragments. Raman spectra collected on the same glass also confirm the existence of GeSe2 domains up to high selenium concentrations. The mobility of the Sen phase observed at high temperature while the GeSe2 phase remains rigid is consistent with their respective underconstrained and overconstrained structural nature. The proposed bimodal phase percolation model is consistent with the original Phillips and Thorpe theory however it is clearly at odds with the intermediate phase model which predicts large amounts of Ge-Se-Se fragments in the structure. A calorimetric study performed over a wide range of cooling/heating rates shows a narrow composition dependence centered at ⟨r⟩=2.4 in contrast with the wide reversibility window observed by Modulated Differential Scanning Calorimetry. This suggests that the observation of the reversibility window associated with the intermediate phase in Ge-Se glasses could be an experimental artifact resulting from the use of a single modulation frequency

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

“…The majority of intertetrahedral connections are, however, formed from Se n bridges where n is an integer 2. These findings are consistent with previous FPMD simulations on GeSe 4 glass at ambient pressure [59,60] and indicate the absence of a network that is phase separated on the nanoscale into GeSe 2 fragments and Se n chains, a scenario that would be accompanied by a small proportion of AB configurations and by large proportions of both AA and BB configurations [16,21]. The FPMD models find a broad distribution of chain lengths as indicated by Fig.…”

“…13(d). The presence of large proportions of AA, BB, and AB configurations is confirmed by recent investigations using 77 Se nuclear magnetic resonance (NMR) spectroscopy [22,56,61], and the presence of edge-sharing Ge(Se 1/2 ) 4 tetrahedra with AA configurations is supported by Raman spectroscopy experiments [12,15,21,22].…”

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

“…Data points are also given for the "dry" samples prepared by Bhosle et al (2012a). The pairs of vertical dashed (black) or chained (red) lines, and associated horizonal arrows, mark compositions for which ∆Hnr ≃ 0 as found in the present work or in the work of Boolchand et al (2001a), respectively. experiments that employed different cooling rates (Lucas et al, 2009), also shows a minimum around x = 0.20 (Figure 5). …”

High-temperature elasticity and viscosity of GexSe1−xglasses in the transition range

“…The structure of these glasses might be slightly more complicated though, as will be further discussed, with Se occurring in at least three different sites 11 and possible "clustering" -at the scale of the structural units 12,13 . 77 Se experimental solid state Nuclear Magnetic Resonance and Raman scattering investigations have suggested a bimodal phase network, where the glass structure is composed of intertwined GeSe 2 and Se n microdomains 14 . This assumption was based on the apparent absence of Se-Se-Ge fragments in the structure.…”