Single crystal x-ray diffraction measurements have been carried out on epsilon oxygen up to 30 GPa to examine the behavior of the constituent $({\mathrm{O}}_2{)}_4$ units. An isostructural phase transition is evidenced by lattice parameter and intracluster (${\mathrm{O}}_8$) distance discontinuities and clear changes in the equation of state at $18.1\pm 0.5\mathrm{GPa}$ on compression. This transition corresponds well to the predicted collapse of the molecular magnetic moment from spin-liquid, ${\epsilon }_1$, to a spinless, ${\epsilon }_0$, states of the ${\mathrm{O}}_8$ structure. The collapse of the molecular magnetic moment has to date only been supported by spectroscopic data and, until now, has not been substantiated structurally.

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• Molecular magnetism
• Pressure effects
• Structural properties
• Pressure techniques
• X-ray diffraction