New Results
- Strain in diamond crystals evaluated by Raman
- Example of Raman mapping of strain distribution in a diamond crystal in a composite
Strain in diamond crystals evaluated by Raman
A study of a single crystal with the upper facet in the form of a truncated triangle was performed. A series of spectra recorded on the surface of the crystal showed that positions and relative intensities of the peaks varied widely across the sample. Most of Raman lines were asymmetric and broad suggesting non-hydrostatic residual stress. During analysis we observed splitting of the diamond peak into two components - a doublet and a singlet (partially removing of degeneracy). The biaxial stress value was determined from the splitting of the zone center optical phonon at ν0=1332 cm-1 frequency according following equations:
- for singlet: σ = -1.08 GPa/cm-1 (νs - ν0)
- for doublet: σ = -0.384 GPa/cm-1 (νd - ν0)
For an isolated, stress-free crystal:
A perfect diamond crystal Raman line appears at 1332 cm-1 and is due to triply degenerate optical phonons at the Brillouin zone centre.
In the presence of strain:
- the cubic symmetry of the crystal is lowered the triply degeneracy is lifted, leading to peak splitting.
- Depending on the direction of the stress relative to crystallographic directions the degeneracy is partially (doublet + singlet) or completely lifted (3 singlets).
- The lifting of degeneracy occurs for uniaxial and biaxial crystallographic strains, but no due to hydrostatic strains.
Example of Raman mapping of strain distribution in a diamond crystal in a composite
Examples of different band shapes observed within the same grain and a distribution of stress within the grain. Red corresponds to 3 GPa, yellow – 2.25 GPa, green – 1.875 GPa, turquoise - 1.25 GPa, and dark blue – 0.375 GPa.