Diamonds are categorized by the presence of nitrogen in their crystal structure. Type I diamonds have nitrogen and are the most common diamonds, whereas type II diamonds have no detectable nitrogen in their crystal structure. Type I diamonds are further classified according to the aggregation state of the nitrogen in the crystal structure. Thus, type Ia diamonds are natural diamonds with aggregated nitrogen. Type Ib diamonds, on the other hand, are synthetic diamonds with un-aggregated nitrogen. Type IIa diamonds, on the other hand, have so very little nitrogen, that’s almost undetectable.
Diamond typing is carried out using FTIR. The carbon absorption band for diamond occurs between the wavenumbers 2660 cm-1 to 1330 cm-1 and the nitrogen absorption band occurs between the wavenumbers 1300 cm-1 to 1100 cm-1. The diamond type is determined by the nitrogen absorption band on the FTIR spectrum.
The pdf version of this article shows a comparison of type Ia and type IIa FTIR spectra.
Broadly speaking type IIa diamonds will have a lower IR absorption when compared to type Ia. Therefore, even if the region of research interest falls outside the nitrogen absorption range, it is recommended that one always uses type IIa diamonds when doing IR work.
Please note Almax easyLab (and diamondANVILS.com) supply type IIa synthetic diamond anvils, also known as type IIac. A typical FTIR spectrum for a type IIac diamond anvil is also available on the downloadable pdf.
Click here to download application note – type iia diamond anvil selection.