Structure and properties of densified silica glass: characterizing the order within disorder

The broken symmetry in the atomic-scale ordering of glassy versus crystalline solids leads to a daunting challenge to provide suitable metrics for describing the order within disorder, especially on length scales beyond the nearest neighbor that are characterized by rich structural complexity. Here, we address this challenge for silica, a canonical network-forming glass, by using hot versus cold compression to (i) systematically increase the structural ordering after densification and (ii) prepare two glasses with the same high-density but contrasting structures. The structure was measured by high-energy X-ray and neutron diffraction, and atomistic models were generated that reproduce the experimental results. The vibrational and thermodynamic properties of the glasses were probed by using inelastic neutron scattering and calorimetry, respectively. Traditional measures of amorphous structures show relatively subtle changes upon compacting the glass. The method of persistent homology identifies, however, distinct features in the network topology that change as the initially open structure of the glass is collapsed. The results for the same high-density glasses show that the nature of structural disorder does impact the heat capacity and boson peak in the low-frequency dynamical spectra. Densification is discussed in terms of the loss of locally favored tetrahedral structures comprising oxygen-decorated SiSi4 tetrahedra. A method for characterizing order in disordered materials such as glass has been developed by an international team of scientists. The atoms in most solid materials are arranged in a regular pattern. Other materials are amorphous, with the positions of each atom more random. However, even these amorphous materials can exhibit some level of ordering over short distances. Shinji Kohara from the National Institute for Materials Science in Ibaraki, Japan, Philip Salmon from the University of Bath, UK, and their colleagues have devised a way of characterizing order within disordered silica glass. The team increased the temperature of silica while keeping it under constant pressure, thereby inducing a transition from a low- to high-density state. Their description of a structural collapse with increasing density could provide a clear structural signature for defining amorphous materials. There is a fundamental divide in symmetry between crystalline and glassy materials, where the structural disorder in glass leads to unique material properties and a myriad of applications. The provision of metrics for describing the order within disorder remains, however, an open challenge, especially on length scales beyond the nearest neighbor that are characterized by rich structural complexity. Here, we address this challenge by applying the method of persistent homology to characterize the structure of silica glass. The structural disorder is systematically engineered by preparing the glass under different high-pressure and temperature conditions, which impacts on the low-frequency vibrational and thermodynamic properties.