First-order Raman scattering in natural and ${}_{}{}^{13}{}_{}{}^{}\mathrm{enriched}$ diamond was measured at high pressures up to 15 GPa in a diamond anvil cell. The ratio ${}_{}{}^{12}{}_{}{}^{}\nu$${/}^{13}$ν, where ν is the frequency of the first-order Raman line, decreases with pressure. The deviation of the frequency ratio from the limiting classical value of ${(}^{13}$m${/}^{12}$m${)}^{0.5}$=1.0408, where m is the isotopic mass, becomes stronger at high density. This kind of behavior means that the quantum contribution to the physical properties of diamond increases with density. This is typical of systems with Coulomb interaction, e.g., metals and high-density plasma.

• Received 26 August 1994

DOI:https://doi.org/10.1103/PhysRevB.50.13860