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

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/Tf≈0.5 when confined to Vycor. Below this temperature the disordered hexagonal closed packed structure coexists with the fcc structure.