Quantum effects in diamond isotopes at high pressures

Abstract

An influence of quantum effects on the equation of states and phase transitions in compressed matter is of primary interest in the physics of giant planets and in astrophysics. To gain some insight into the problem we carried out precision Raman studies of diamond isotopes $^{12}\text{C}, ^{13}\text{C}$, and their mixture $^{12.5}\text{C}$ in the pressure range up to 73 GPa using helium as a hydrostatic pressure-transmitting medium. The ratio of Raman frequencies of $^{12}\text{C}$ and $^{13}\text{C}$ diamonds, which differs from the classical value (1.0408), was found to slightly, but nonmonotonically, change up to 60 GPa. One can propose that the quantum effects in diamonds initially are enhanced on compression to \ensuremath{\sim}30 GPa and then decrease up to the maximum experimentally measured pressure. This behavior probably unveils hidden features of covalent interaction in crystals. Examination of the isotopically mixed $^{12.5}\text{C}$ diamond shows that the effective mass determining the Raman frequency varies under compression from 12.38 a.u. at ambient pressure to 12.33 a.u. at pressure of 73 GPa.