You know oxygen in air is colorless and you may have heard that liquid oxygen is pale blue, but did you know oxygen also occurs in pink, orange, red, black, and even a metallic form? At atmospheric pressure, liquid oxygen cools into solid oxygen, which forms blue crystals. However, as the pressure is increased and/or the temperature is lowered, phase transitions occur that change the structure of oxygen and also its color. Six phases of solid oxygen are known:
α-phase: light blue — forms at 1 atm below 23.8 K, monoclinic crystal structure
- β-phase: faint blue to pink — forms at 1 atm below 43.8 K, rhombohedral crystal structure (at room temperature and high pressure, transforms to tetraoxygen, O4)
- γ-phase: faint blue — forms at 1 atm below 54.36 K, cubic crystal structure
- δ-phase: orange — forms at room temperature at pressure of 9 GPa
- ε-phase: dark-red to black — forms at room temperature at pressures greater than 10 GPa
- ζ-phase: metallic — forms at pressures greater than 96 GPa
Tetraoxygen (O4) is a transient state, although octaoxygen (O8) is stable. The metallic phase of oxygen exhibits superconductivity.