OPTICAL-TRANSITIONS IN DIAMOND AT ULTRAHIGH PRESSURES

MANY technological and scientific applications of diamond arise from its unique properties, which include optical transparency in the ultraviolet to infrared, electrical insulation, thermodynamic stability, and unsurpassed strength and hardness 1. Here we describe optical studies on diamond at ultrahigh pressures (to above 300 GPa) which show that these properties are affected by such stresses. Our spectroscopic measurements show that the optical absorption edge shifts from ultraviolet to red with increasing pressures, and Raman scattering measurements show evidence for new structural transitions, associated with large macroscopic deformation, beginning at a pressure of approximately 150 GPa. The changes are reversible and are associated with intense luminescence peaks at 2.0-2.2 eV under 458-514-nm radiation. Our results may be related to the onset of band-gap closure in the approach to a new high-pressure phase. These spectral features must also be taken into account when diamond is used as optical windows for ultrahigh-pressure investigations 2-5.