Date Published: March 01, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Lothar Wondraczek, Zhiwen Pan, Theresia Palenta, Andreas Erlebach, Scott T. Misture, Marek Sierka, Matthieu Micoulaut, Uwe Hoppe, Joachim Deubener, G. Neville Greaves.
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.
As one of the most dramatically visible phase transitions, melting processes are of fundamental importance across all fields of condensed matter science. Regarding the microscopic mechanism which leads to the transformation of a solid into a liquid, our understanding rests largely on the early works of Lindemann1 and Gilvarry.2 These led to a criterion of melting, whereby structural instability occurs when the amplitude of atomic thermal vibrations exceeds ≈10% of the interatomic distance. Based on Einstein’s model of harmonic atomic oscillation and heat capacity,3 the theory successfully predicts melting temperatures of close‐packed solids, but has serious shortcomings for less‐dense materials in only considering the average atom reduced to a simple cubic lattice, and in underestimating the vibrational dynamics.4 The vast range of network structures (with free volume and complex dynamic parameters like cooperative motion, bond rupture, and interactions between solid and liquid phases) do not follow the Lindemann criterion.
Using a combination of experimental and theoretical techniques, we have elucidated the origins of low‐temperature melting by studying zeolite frameworks with common topology, where melting is decelerated at the glass transition to tractable timescales. Employing a simple kinetic model the unusual compressed exponential kinetics (β > 1) that characterize decelerated melting are accurately reproduced by a two‐stage reaction sequence: (1) an order–order superheated transition from the nanoporous (expanded) crystal to a low density intermediate phase (β = 2, τ1), followed by (2) an order–disorder transition where the aperiodic solid LDA phase melts heterogeneously to a final HDL phase (β = 1, τ2). The very different temperature dependencies of the two reaction times τ1 and τ2 (extrapolated across the supercooled regime) define boundaries for the two reactions, which subtend the viscosities of the classic glass formers carnegieite and albite and the zone of surface melting. At the glass transition, they define the distinct domains of zeolite, LDA and HDL in time–temperature space. Appealing to a range of NMR, IXS and INS experiments, the two reactions involved in decelerated melting have been independently identified. Finally, modeling predictions and experimental observations of the kinetics of decelerated melting have been tested against extensive MD simulations, where confirmation of the two‐stage process has been obtained from the developments in total energy and internal pressure. Moreover, by visualizing atomic trajectories associated with the classical subunits that define faujasite topology, we have been able to uniquely identify their retention in the homogeneous superheated zeolite‐LDA reaction, and their destruction in the heterogeneous LDA‐HDL melting reaction that follows.
LSX Zeolite, Ion Exchange, and Melt‐Quenched Glass Synthesis: Synthesis of the LSX followed the recommended procedure of the International Zeolite Commission (IZA40). For this, reagent grade chemicals sodium aluminate, potassium hydroxide, sodium hydroxide, and sodium silicate (water glass), and double‐distilled water were employed, resulting in a white crystalline powder which was filtered washed, and finally dried at 100 °C (Table1). Between experiments, all powders were stored in dry desiccators.
The authors declare no conflict of interest.