| Strain-induced symmetry changes in diamond were examined by measuring the first order Raman spectrum of single crystals shocked along the (110) and (100) directions. Uniaxial strain along these directions is predicted to completely or partially lift the triple degeneracy of the ambient Raman line.;Peak longitudinal stresses ranging from 12 to 45 GPa (1% to 3.5% density compression) were achieved in plate impact, uniaxial strain loading experiments. Time-resolved Raman spectra with 10ns resolution were obtained during shock compression to determine the shifted frequencies. The degenerate Raman line was observed to split due to the presence of large nonhydrostatic stresses. This is the first reported observation of such splittings during shock experiments. Data collected for the (110) orientation indicate that the observed spectra are accurately described by three singlets, suggesting a complete lifting of the degeneracy. Raman spectra collected for the (100) orientation were consistent with singlet and doublet lines, suggesting a partial removal of the degeneracy. All frequency-shifts were observed to increase with density compression.;A three parameter, perturbation model was used to analyze the results. The data for the (110) orientation provided a unique determination of the three parameters. The data for the (100) orientation was in good agreement with the predictions based on these parameters, providing and independent check. Unlike the previous parameter values determined from uniaxial stress measurements, the present values demonstrate a good fit to the observed frequency shifts over the entire stress range considered. The centroids of the shifted frequencies for each orientation were compared with static high pressure measurements of the degenerate Raman line. Good agreement was demonstrated for compression to 3.5%, suggesting that the centroid location is not influenced by the nonhydrostaticity. The static high-pressure measurements are also in good agreement with the predictions based on the present work.;The present results show that the three perturbation parameters determined in this work may be used to predict the Raman frequency shifts and splittings for other loading conditions. |
