Folded network and structural transition in molten tin

The fundamental relationships between the structure and properties of liquids are far from being well understood. For instance, the structural origins of many liquid anomalies still remain unclear, but liquid-liquid transitions (LLT) are believed to hold a key. However, experimental demonstrations of LLTs have been rather challenging. Here, we report experimental and theoretical evidence of a second-order-like LLT in molten tin, one which favors a percolating covalent bond network at high temperatures. The observed structural transition originates from the fluctuating metallic/covalent behavior of atomic bonding, and consequently a new paradigm of liquid structure emerges. The liquid structure, described in the form of a folded network, bridges two well-established structural models for disordered systems, i.e., the random packing of hard-spheres and a continuous random network, offering a large structural midground for liquids and glasses. Our findings provide an unparalleled physical picture of the atomic arrangement for a plethora of liquids, shedding light on the thermodynamic and dynamic anomalies of liquids but also entailing far-reaching implications for studying liquid polyamorphism and dynamical transitions in liquids.

Visualization of tetrahedral disordering in amorphous germanium through local atomic motifs

The atomic arrangements in amorphous solids, unlike those in crystalline materials, remain elusive. The details of atom ordering are under debate even in simplistic random network models. This work presents further advancements in the local atomic motif (LAM) method, first through the introduction of an optimized alignment procedure providing a clearer image of the angular ordering of atoms in a model. Secondly, by applying stereographic projections with LAMs, the angular ordering within coordination shells can be quantified and investigated. To showcase the new capabilities, the LAM method is applied to amorphous germanium, the archetype of covalent amorphous systems. The method is shown to dissect structural details of amorphous germanium (a-Ge) from the continuous random network (CRN) model and a reverse Monte Carlo (RMC) refined model fitted to high-resolution X-ray scattering measurements. The LAMs reveal well defined dihedral ordering in the second shell. The degree of dihedral ordering is observed to be coupled to bond length distances in the CRN model. This coupling is clearly not present within the RMC refined model. The LAMs reveal inclusions of third-shell atoms occupying interstitial positions in the second shell in both models.

Structural properties of liquid aluminosilicate with varying Al2O3/SiO2 ratios: Insight from analysis and visualization of molecular dynamics data

Molecular dynamics (MD) simulations and visualizations were explored to investigate the changes in structure of liquid aluminosilicates. The models were constructed for four compositions with varying Al2O3/SiO2 ratio. The local structure and network topology was analyzed through the pair of radial distribution functions, bond angle, bond length and coordination number distributions. The results showed that the structure of aluminosilicates mainly consists of the basic structural units TO[Formula: see text] (T is Al or Si; y = 3, 4, 5). Two adjacent units TO[Formula: see text] are linked to each other through common oxygen atoms and form continuous random network of basic structural units TO[Formula: see text]. The bond statistics (corner-, edge- and face- sharing) between two adjacent TO[Formula: see text] units are investigated in detail. The self-diffusion coefficients for three atomic types are affected by the degree of polymerization (DOP) of network characterized by the proportions of nonbridging oxygen (NBO) and Q[Formula: see text] species in the system. It was found that Q4 and Q3 tetrahedral species (tetrahedron with four and three bridging oxygens, respectively) decreases, while Q0 (with four nonbridging oxygen) increase with increasing Al2O3/SiO2 molar ratio, suggesting that a less polymerized network was formed. The structural and dynamical heterogeneities, micro-phase separation and liquid–liquid phase transition are also discussed in this work.