Temperature variations of the Raman frequencies in diamond

Abstract

With a high dispersion spectrograph, a diamond of the ultraviolet transparent type and the mercury λ 2536.5 excitation, the principal Raman frequency has been measured and is found to decrease from 1333.2 cm^-1 at 85° T. to 1316.0 cm^-1 at 975° T. Identical results are obtained with a diamond of the ultraviolet opaque type and the λ 4358 excitation. The temperature dependence of the more prominent second order Raman frequencies has also been investigated. The relative change of the principal frequency with relative change of volume decreases from a high value at 85° T. to a constant small value above 500° T. and differs greatly from the so-called Gruneisen constant. The different lattice and superlattice frequencies behave differently in respect of their temperature variation and this has been explained as due to differences in the rate of change with temperature of the different force constants which determine the vibration frequencies of the diamond lattice. The 1332 line has a finite width which increases with temperature. This may be explained as due to the changes of lattice frequency arising from the thermal fluctuations of density in the volume elements of the crystal, if the effective linear dimensions of a volume element are assumed to be about 8 times as large as the edge of the unit cubic cell.