Spatial Confinement Effect on the Atomic Structure of Solid Argon

Kengo Nishio, Wataru Shinoda, Tetsuya Morishita, and Masuhiro Mikami

J. Chem. Phys. 122 124715 (2005)

Molecules confined in nanopores show unusual behavior not seen in bulk systems. This Letter reports on molecular dynamics simulations of unusual freezing behavior in confined Ar. Similarly to bulk Ar, liquid Ar confined in pores with a diameter D>15σ(=5.1 nm:σ is the diameter of the Ar atom) crystallizes when the quenching rate is lower than a critical value (Qc). In the pore of 10σ (3.4 nm) in diameter, on the other hand, the behavior is dramatically changed. Crystalline Ar does not appear inside the pore even when the system is cooled at a rate lower than the Qc in the bulk system by over two orders of magnitude. Instead, amorphous Ar characterized by local icosahedral configurations is formed in the pore. We further find that, even when crystalline Ar is formed outside the pore, it does not grow deeply into the pore. This supports that the amorphous Ar is actually the most stable phase in the pore. It is well known that preparing amorphous rare-gas solids is very difficult. Our finding that the amorphous phase of even such a poor glass former is the most stable in the pore suggests that it is possible to prepare amorphous structure selectively by using nano-molds.

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