Intermediate-range ordering and topochemical heterogeneity in binary and ternary silicate glasses

Abstract

The fact that glasses can be synthetically produced and engineered allows plenty of opportunity to control their structure. Even though it may sound simple, the glass formation is controlled by physical-chemical principles and any change in the composition, temperature, cooling rate, processing type, etc. impacts the final material - and consequently its properties. While glasses are considered non-crystalline solids due to the absence of longrange periodicity, they show a regular construction, defined by the short-range and intermediate-range order. Their characteristics are described by the topology, which denotes the basic geometrical arrangement of the structural units and allocation of the atoms. At macroscale, for a glass of the same composition, the structure and properties reveal to be homogeneous, independently of the processing, temperature or precursor material. However, at microscale, the same glass may show a different picture, revealing a topological heterogeneity of a few nanometers. Due to technological limitations, the main difficulty is to directly access this region. There is a consensus that the topological heterogeneity, however, manifests as a significant peak at very low temperatures (about 5 K) or low-frequencies (about 1THz or 33 cm 1) by collective vibrational modes. Since the main model for estimating the phonon contribution to the specific heat in a crystal, the Debye model, does not predict any peak at low temperature and there are no models to describe these manifestations in vitreous materials, usually it is considered an anomaly. This anomalous peak has been called Boson peak. Even though it remains as one of the major debated and unsolved problems of condensed-matter physics, intense investigations in these almost 50 years brought an enormous knowledge about most of its characteristics. In order to access the intermediate-range order and the topochemical heterogeneity of selected binary and ternary glass network formers made by reactive powder sintering process, investigations of the vibrational density of states in the region of the Boson peak has been conducted. Foremost, this study describes that the feature of the Boson peak is governed by topological heterogeneity as well as topochemical heterogeneity. Together with other characterization methods, this has been shown as a powerful descriptive route to understand glass functionality and glass structure in a more extended perspective. Even though it is important for the wide relevance of fundamental knowledge of glasses, this is notably important for high-technological glasses and in which bottom-up strategies are necessary to design new glass compositions with straightforward applications.