New Disorder Induced Phase Transitions of Classical Rare Gases in Porous Vycor Glass

Abstract

We report the results of x-ray diffraction studies of Ar and Kr confined in Vycor glass. The freezing and melting temperatures of both Ar and Kr were suppressed well below their bulk freezing points. On solidification, both samples crystallized in a disordered hexagonal close packed structure similar to that observed in molecular dynamics simulations of confined solids. A new solid-solid phase transition is observed at a reduced temperature of T ͞T f ഠ 0.5 when confined to Vycor. Below this temperature the disordered hexagonal closed packed structure coexists with the fcc structure.[S0031-9007 (98)06675-7] PACS numbers: 61.43.Gt, 61.10.NzThe effects of finite size and confinement on gases, liquids, and solids adsorbed in mesoporous materials, such as porous glasses and zeolites, is a topic of current interest for classical and quantum systems. For example, in classical systems the liquid-gas transition is enhanced with respect to the bulk [1,2] while the liquid-solid transition is suppressed [2,3]. Hysteresis, which appears to be a stable thermodynamic behavior, is also introduced into these transitions. Quantum transitions show equally dramatic behavior. For example, the superfluid transition of 4 He in Vycor is substantially suppressed while the critical exponents remain unchanged. However, confinement in aerogel results in only a small suppression but substantially different critical exponents [4,5]. 3 He, on the other hand, has superfluid phases in high porosity aerogels [6,7], but not in lower porosity confining geometries.Microscopic structural studies of materials confined to mesoporous media have only recently been undertaken, and a variety of different behaviors have been reported. In some cases, such as Hg in Vycor [8], the structure of the confined solid is identical to that of the bulk while in other cases, such as H 2 O and D 2 in Vycor [9,10], confinement stabilizes new structures not present in the bulk. Confinement can also suppress crystalline order, such as O 2 in xerogel [11], resulting in a glassy solid phase. Confinement has not, however, been observed to introduce new phase transitions in any systems that have been studied to date.In this Letter, we report x-ray diffraction studies of Ar and Kr adsorbed in Vycor. As in many other systems, a suppression of the liquid-solid transition hysteresis between freezing and melting is observed. On freezing, both Ar and Kr are observed to form a disordered hexagonal close packed (dhcp) structure, characterized by random plane stacking, in contrast to the bulk fcc structure. A new phase transition, which is absent from the bulk phase diagram, occurs at roughly half of the freezing temperature below which both Kr and Ar exhibit coexistence of the dhcp and fcc phases. This solid-solid structural phase transition is repeatable on cycling and no appreciable hysteresis is observed. Thus, the coexistence phase appears to be a true disorder-induced thermodynamic phase, rather than a metastable phase.The confining media used in these studies was Vycor glass. Vy...

The physics of premelted ice and its geophysical consequences

“…The shift, now known as the Gibbs-Thomson effect, has been studied with a variety of substances dispersed on inert substrates or confined in porous media. A sampling is given in the references in four groups: weakly interacting molecular species such as rare gas atoms in porous media ͑Brewer et Tell and Maris, 1983;Liezhao et al, 1986;Shimoda et al, 1986;Kondo et al, 1987;Brown et al, 1988;Bruschi et al, 1988;Hiroi et al, 1989;Jackson and McKenna, 1990;Rall et al, 1991;Duffy et al, 1995;Beaudoin et al, 1996͒, ice in porous media ͑Blachere andYoung, 1972;Gay et al, 1992;Maruyama et al, 1992;Ishizaki et al, 1996;Mori et al, 1996͒, dispersed metal particles ͑Takagi, 1954Gladkich et al, 1966;Wronski, 1967;Coombes, 1972;Peppiatt and Sambles, 1975;Buffat andBorel, 1976͒, andice in soils ͑Hoekstra andDelaney, 1974;Konrad and Morgenstern, 1981;Smith and Tice, 1988͒. The typical behavior of the first group is that melting begins appreciably below the bulk transition when the pores are filled. Although the first one or two adsorbed layers prefreeze, due to substrate attractive forces stronger than the interactions within the material.…”

Observation of a Superfluid Component within Solid Helium

“…The confinement in aerogel suggests the unusual coexistence of small bcc and hcp crystallites that lets us expect further changes in the quantum behavior of this system [13].…”

Molecular dynamics study on freezing of Lennard-Jones argon in an open-ended cylindrical pore

“…Freezing and melting have been studied by many experimental [1][2][3][4][5][6][7] and theoretical techniques [8][9][10][11][12]. In experimental studies, the characteristic features of nanopores, such as network structures, pore size distributions and thermal gradients make the situation complicated, and accordingly it becomes difficult to elucidate the relationships of cause and effect because too many factors are involved.…”