Structural Evidence for the Spin Collapse in High Pressure Solid Oxygen

Abstract

Single crystal x-ray diffraction measurements have been carried out on epsilon oxygen up to 30 GPa to examine the behavior of the constituent (O2)4 units. An isostructural phase transition is evidenced by lattice parameter and intracluster (O8) distance discontinuities and clear changes in the equation of state at 18.1±0.5 GPa on compression. This transition corresponds well to the predicted collapse of the molecular magnetic moment from spin-liquid, ϵ1, to a spinless, ϵ0, states of the O8 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.