Liquid–solid and solid–solid phase transitions of oxygen in a single cylindrical pore

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To study the liquid–solid and solid–solid phase transitions of a confined O2, we performed x-ray diffraction measurements of O2 confined inside the cylindrical pores of six kinds of regular mesoporous adsorbents (MCM-41 and SBA-15) with different pore radii (R=2.1, 2.9, 3.9, 4.5, 5.3, and 7.0 nm) as a function of temperature. Hysteresis effects between the γ-to-β and β-to-γ solid–solid phase transitions are considerably larger than those between freezing and melting and the depression of the β-to-γ transition point is slightly smaller than that of the melting point. This suggests that the β-to-γ rather than the γ-to-β transition point represents the thermodynamic phase transition temperature between the γ and β solids. The nature of the phase transitions between the γ and β solids is almost unchanged by confinement. The depression of the melting point is analyzed based on a simple capillary theory by invoking an empirical relation between liquid–solid interfacial free energy and enthalpy of fusion proposed by Turnbull. The O2 confined to the cylindrical mesopores of R=2.1 nm gives rise to an amorphous diffraction pattern even at the lowest temperature studied (27 K). Unusual behavior of the peak width against temperature strongly suggests that the amorphous pattern originates from the formation of the γ-phase clusters rather than the vitrification of a confined liquid.