Amorphization Induced by Pressure: Results for Zeolites and General Implications

Abstract

We report an ab initio study of pressure-induced amorphization (PIA) in zeolites, which are model systems for this phenomenon. We confirm the occurrence of recently reported low-density amorphous phases that preserve the crystalline topology, and explain the role of the zeolite composition regarding PIA. Our results support the correctness of existing models for the basic PIA mechanism, but suggest that energetic, rather than kinetic, factors determine the irreversibility of the transition. In such a transition, domain nucleation would overwhelm growth and destroy the long-range order [5]. Cohen, Íñiguez, and Neaton [5] (CIN) further propose the PIA transition will be reversible (i.e., the long-range order will be recovered upon decompression) if the crystalline topology (i.e., the atomic coordination and bonding) is preserved in the amorphous phase.Recent work [6] on the nanoporous aluminosilicates known as zeolites has renewed the interest in PIA. It was shown that a zeolite may present two distinct PIA phases: a low-pressure ( 2 GPa) low-density amorphous phase (LDA), which might constitute a ''perfect'' glass with negligible configuration entropy, and a high-pressure ( 6 GPa) high-density amorphous phase (HDA). Further, the crystalline topology is found to be preserved in the LDA phase and lost in the HDA phase, which, according to the CIN picture, implies amorphization will be reversible in the former case and irreversible in the latter. These results, together with other studies [7,8] that, for example, show a striking dependence of the PIA reversibility on the zeolite composition, clearly point at these systems as ideal to test general PIA theories.Here we report an ab initio study that reveals the mechanisms controlling PIA in zeolites and provides important insights pertaining PIA phenomena at large.We used the generalized gradient approximation (GGA) to density functional theory [9] as implemented in the code SIESTA [10]. We wanted to study amorphization occurring in spite of negligible thermal activation and thus focused on low temperature simulations. We proceeded as follows: we started from the experimentally known zero-pressure phase and increased (or decreased) the pressure p by a p of 0.25 GPa. At each new pressure p p, we started from the structure obtained for the previous pressure p, performed a short (100 fs) molecular dynamics simulation at 100 K, with random initial velocities, and relaxed the resulting structure. In this way, we were able to compute the pressure dependence of a phase up to its (meta)stability limit, where it transforms into a new phase that is directly obtained from the calculation. Discontinuities in the calculated volume=enthalpy are indicative of first-order transitions; when the discontinuities appear only in the slope of the volume/enthalpy versus pressure curve, the transitions are of second-order.Zeolites have the general formula A n x=n Al x Si 1ÿx O 2 , where A is a charge-compensating cation. The Si=Al atoms are at the center of corner-sharing O 4 tetrahedra. A...

Poisson's ratio and modern materials

“…93; Fig. 8c), with the associated LDA glasses being topologically ordered 38,79,99 . The m versus α plot 105,106 (Fig.…”

Amorphization of Metal–Organic Framework MOF-5 by Electrical Discharge

“…It is worth noting that DBD is carried out under ambient pressure and has a low gas temperature around 120°C, measured by infrared imaging (Icron 100PHT) [26,27]. Therefore, it provides us a novel method for amorphization of solid materials under mild condition, for pressure-induced amorphization always needs a very high pressure [28,29]. DBD can generate a large amount of free radicals and high-energy electrons, as well as ozone if carried out in air.…”

Pressure-induced penetration of guest molecules in high-silica zeolites: the case of mordenite

“…The compression of zeolites using non-penetrating media has also been exploited to induce the so-called ''pressureinduced amorphization'' (PIA). [26][27][28][29][30][31][32][33][34][35][36][37][38] PIA is observed for a wide range of silicate structures and is accompanied by significant volume reductions, resulting in a new denser material. For some zeolites, PIA is irreversible in the presence of small extraframework cations (e.g.…”