Infrared spectroscopy of natural <i>Type Ib</i> diamond: Insights into the formation of <i> <b>Y</b> </i> -centers and the early aggregation of nitrogen

Abstract The growing interest in the spectroscopic properties of Type Ib diamonds has revealed several complexities associated with nitrogen (N)-related defects that have yet to be identified. One defect in particular, the Y-center, produces a characteristic spectrum in the N-region (∼1000–1400 cm−1) that is extremely common in the IR spectra of diamonds with a dominant Type Ib component. In this study, a suite of 178 Type Ib +IaA diamonds with variable N-aggregation states (%IaA) and N contents was examined to evaluate the effect of Y-center absorbance on the results obtained from standard deconvolution of the N-region. To achieve this, a new deconvolution routine was developed that incorporates the Y-center spectrum (obtained by decomposition of Type Ib spectra), and an updated spreadsheet (Caxbd_Inherit_2024-Ib, Online Materials1) for processing the IR spectra of Type Ib diamonds is provided here. It is shown that neglecting Y-center absorption during least-squares fitting of the IR spectra of Type Ib + IaA diamonds results in poor-quality fits of the N-region that may result in erroneous C- and A-center contents. The identity of the Y-center, and the relevant absorption coefficient, have not been constrained. However, several studies have shown that Y-centers are structurally related to single-substitutional N (called NS0 or C-centers); thus, the Y-center contents were calculated using the C-center absorption coefficient. Using the new deconvolution method, it is shown that neglecting Y-centers may result in N-aggregation states that vary by ±10 %IaA and total N contents (Ntot) that may be overestimated by &amp;gt;100 at. ppm. The samples studied here have an average Ntot of ∼100 at. ppm, and errors in %IaA and Ntot may be much larger for diamonds with higher Ntot. Such errors translate to potentially significant discrepancies in the calculated mantle residence times on the order of hundreds of millions of years. Comparisons of the normalized Y-center content and %IaA show that Y-centers are an intermediate defect that is produced from 0 to 40 %IaA at the expense of C-centers and then consumed from 40–100 %IaA to produce A-centers. A strong linear correlation with some IR peaks between 1400 and 1350 cm−1 (e.g., 1358 cm−1) is observed. Evidence supporting the assignment of such peaks to defects containing interstitial carbon and nitrogen (Ci and Ni) is described, suggesting that the formation of Y-centers is driven by interstitial-assisted aggregation. Moreover, the Y-center itself may be an intermediate form of NS0 linked to Ci or Ni or larger interstitial complexes that become unstable with increasing N-aggregation (mantle residence time/temperature). Evidence for alternative hypotheses for the identity of the Y-center involving O- and Ni-related defects and X-centers is also discussed.