Topological versus chemical ordering in network glasses at intermediate and extended length scales

Abstract

Atomic ordering in network glasses on length scales longer than nearest-neighbour length scales has long been a source of controversy. Detailed experimental information is therefore necessary to understand both the network properties and the fundamentals of glass formation. Here we address the problem by investigating topological and chemical ordering in structurally disordered AX2 systems by applying the method of isotopic substitution in neutron diffraction to glassy ZnCl2. This system may be regarded as a prototypical ionic network forming glass, provided that ion polarization effects are taken into account, and has thus been the focus of much attention. By experiment, we show that both the topological and chemical ordering are described by two length scales at distances greater than nearest-neighbour length scales. One of these is associated with the intermediate range, as manifested by the appearance in the measured diffraction patterns of a first sharp diffraction peak at 1.09(3) A(-1); the other is associated with an extended range, which shows ordering in the glass out to 62(4) A. We also find that these general features are characteristic of glassy GeSe2, a prototypical covalently bonded network material. The results therefore offer structural insight into those length scales that determine many important aspects of supercooled liquid and glass phenomenology.

Supercooled liquids with enhanced orientational order

“…Further isothermal measurements were performed on glycerol films of different thicknesses (9,15,18,26,30,55, 89 nm): the temperature of the IDE was varied in the same range as above, but in steps of 5 K. The sample was held at each annealing temperature for 2 h. The conversion rate into the OL phase, v, was estimated by fitting isothermal scans with linear decays v ¼ Dl/(t À t 0 ), where Dl is the fraction of transformed film, and t 0 indicates the onset of measurement. The value of Dl is obtained from the time evolution of the dielectric strength under the assumption that De is the sum of the contribution of the two liquid phases averaged over their volume fractions according to We ruled out the possibility that the larger values of De and of t observed in the medium-range crystalline order phase are actually induced by the presence of water or other contamination in the films.…”

“…Evidence of such structures has been found in metallic glass formers [14][15][16] , where the presence of quasi-crystalline clusters with a few topological defects has been detected by diffraction techniques, such as atomic-scale high-resolution transmission electron microscopy and by X-ray, neutron and electron diffraction. An enhanced degree of topological order may be achieved via structural rearrangements, either by annealing below the glass transition temperature (physical aging) 17 or pressurizing 18 , which permits the nucleation of MRO regions in proximity to the quasi-crystalline clusters and the consecutive propagation of a similarly ordered structure.…”

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

“…According to Fourier transform theory, a sharp peak of width ∆q i at a position q i in S(q) ≃ S NN (q) is associated with real-space ordering of periodicity 2π/q i and correlation length 2π/∆q i (Salmon, 1994). Indeed, the real-space periodicity associated with these features is directly observable for several network-forming glasses, including Ge 0.333 Se 0.667 (Salmon, 1994(Salmon, , 2006Salmon et al, 2005Salmon et al, , 2006. The composition dependence of the periodicity and correlation length associated with each of the first three peaks in the measured S(q) functions is shown in Figures 7 and 8, respectively.…”

“…For example, the mean coordination numbern is obtained directly from g NN (r). In addition, the peak positions and widths in S NN (q) describe the atomic ordering in a glass network on different length scales (Salmon, 1994;Salmon et al, 2005;Zeidler and Salmon, 2016). One of these length scales is associated with an intermediate range, and manifests itself by the appearance of a first sharp diffraction peak (FSDP) in S NN (q) at q FSDP , where q FSDP r nn ≃ 2.2 − 2.8 for glassy Ge x Se 1−x and r nn is the nearest-neighbor bond distance.…”

Relaxation and physical aging in network glasses: a review

“…Fragility -structure relationships Glass fragility is also found to display a relationship with atomic ordering on intermediate and extended ranges, a relationship that also connects to the notion of dynamic heterogeneities (see below). Specifically, the structure can be characterized in terms of topological and chemical ordering from neutron diffraction experiments in real (pair correlation function g(r)) and reciprocal space (static structure factor S(k)) [63]. It turns out that the ordering for GeO 2 , SiO 2 and ZnCl 2 at distances greater than the nearest neighbor lengthscale can be rationalized in terms of an interplay between the relative importance of two length scales [64].…”