Kinetics of Decelerated Melting

Abstract

Melting presents one of the most prominent phenomena in condensed matter science. Its microscopic understanding, however, is still fragmented, ranging from simplistic theory to the observation of melting point depressions. Here, a multimethod experimental approach is combined with computational simulation to study the microscopic mechanism of melting between these two extremes. Crystalline structures are exploited in which melting occurs into a metastable liquid close to its glass transition temperature. The associated sluggish dynamics concur with real‐time observation of homogeneous melting. In‐depth information on the structural signature is obtained from various independent spectroscopic and scattering methods, revealing a step‐wise nature of the transition before reaching the liquid state. A kinetic model is derived in which the first reaction step is promoted by local instability events, and the second is driven by diffusive mobility. Computational simulation provides further confirmation for the sequential reaction steps and for the details of the associated structural dynamics. The successful quantitative modeling of the low‐temperature decelerated melting of zeolite crystals, reconciling homogeneous with heterogeneous processes, should serve as a platform for understanding the inherent instability of other zeolitic structures, as well as the prolific and more complex nanoporous metal–organic frameworks.

Decoding entangled transitions: Polyamorphism and stressed rigidity

“…that may well be investigated experimentally both from the viewpoint of rigidity and polyamorphism. Finally, it is also well known that molecular systems can undergo LDA-HDA transformations as exemplified by metal-organic glasses 40 or amorphous water, 3 this transformation being now also observed in mesoporous materials 31 which associate the pressure-driven void collapse with a non-trivial decelerated melting kinetics. A neat definition of network connectedness might be particularly difficult to establish for certain of these systems but it is important to emphasize that the MD based constraint count has a general basis and can here be applied as well.…”

“…The obvious difference between the reduction of void at low pressure and the short-range evolution at larger pressure that also results in the constraint variation is actually compatible with results obtained for densified borosilicate glasses 29 which show that there is a decoupling between void collapse at low pressure and the short-range collapse at elevated pressures, a situation that is also met in pressurized zeolithes. 30,31 A detailed analysis of the constraints [Figs. 7(b) and 7(c)] shows that the increase in rigidity is strongly driven by Ge atoms, and n BS c increases indeed to about 3 which can be associated with a nearly octahedral environment (r Ge = 2n BS c = 6).…”

Local Deformation of Glasses is Mediated by Rigidity Fluctuation on Nanometer Scale

“…d) The presence of structural heterogeneity and rigidity fluctuation in vitreous silica is rationalized on a MD trajectory of amorphous SiO 2 (adapted from ref. ) highlighting regions of high and low network density which alternate on the scale of a few nanometers (yellow balls: Si; red balls: O). The marked network regions have a diameter of ≈1 nm.…”

Boson peak, heterogeneity and intermediate-range order in binary SiO2-Al2O3 glasses

“…The underlying excess of vibrational states is often taken as a universal signature of structural heterogeneity at intermediate length scale 2 , resulting from dynamic heterogeneity in the liquid state which leads to spatial gradients in fictive temperature 3,4 . Heterogeneity then manifests not only in the excess of heat capacity, but also in the terahertz frequency range (0.1–3 THz) which is detectable by low-frequency Raman spectroscopy 5 , or in the inelastic scattering of X-rays (IXS) and neutrons (INS) 6,7 . While the exact consequences of such intermediate-range structural features remain unclear, various links have been identified with macroscopic properties, e .…”