Insulator-metal transition and valence instability in EuO near 130 kbar

Optical reflection spectroscopy reveals an insulator-metal transition in EuO at 140 kbar ($T=300$ K). The pressure-volume relation starts to exhibit "softening" near 130 kbar. Both observations indicate a continuous valence change commencing at a pressure considerably lower than predicted. The pressure dependence of the 4f-5d absorption edge suggests that the valence instability is driven by the onset of ferromagnetic ordering and the concomitant exchange splitting of conduction-band states.