Trace nitrogen in diamond
Carbonado diamonds are a variety of polycrystalline diamond found in alluvial deposits in the Central African Republic and Brazil. They are typically black and have high porosity, finding use in industrial applications that require super-hard materials.
Within the diamond structure, the most common impurity element is nitrogen, as it is easily accommodated owing to its atomic radius being almost equal to that of carbon, and can be present in concentrations up to 0.25 wt%. To identify defects in diamonds, cathodoluminescence (CL) is commonly used because there is a strong signal from nitrogen incorporation and because the cathodoluminescence is sensitive to a number of different nitrogen defect types, The characterization of nitrogen within diamonds is important, as the aggregation of nitrogen is mainly controlled by the residence time of the diamond in the mantle and the temperature of the mantle.
Cathodoluminescence spectrum collected from a carbonado diamond.
A hyperspectral CL map was collected using an electron probe microanalyser (EPMA) across a Brazilian carbonado diamond, sample L-5 (Magee, 2001). Peak fitting allows the spectra by fitting Gaussian or other functions allows it to be deconvoluted into lines from the various defects. In this carbonado diamond lines from N-V0, N2V, and N3V defects were identified allowing maps of the distribution of these defect types to be produced.
Defect intensity maps from a carbonado diamond, with the N-V0 defect shown on the left, and the N2V defect on the right.
The N2V defect require high temperature annealing, and so its occurrence has implications regarding the thermal history of the diamond.
Reference
Wilson, N. C., MacRae, C. M., Torpy, A., Davidson, C. J., & Vicenzi, E. P. (2012). Hyperspectral cathodoluminescence examination of defects in a carbonado diamond. Microscopy and Microanalysis, 18(6), 1303-1312. https://doi.org/10.1017/S1431927612013578