Nitrogen Structure Determination in Treated Fancy Diamonds via EPR Spectroscopy

At a Glance

Metadata	Details
Publication Date	2022-12-07
Journal	Crystals
Authors	Ira Litvak, Avner Cahana, Yaakov Anker, Sharon Ruthstein, Haim Cohen
Institutions	Bar-Ilan University, Ariel University
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study successfully correlated specific carbon-centered paramagnetic radicals (spin centers) with the resulting fancy colors (green, blue, yellow) in treated natural Type Ia A/B diamonds using Electron Paramagnetic Resonance (EPR) spectroscopy, supported by FTIR and UV-Vis analysis.

Core Achievement: Established a schematic mechanism showing how irradiation and thermal annealing treatments manipulate the concentration and configuration of stable carbon-centered radicals (N1, N4, P2/W21), directly determining the final diamond color.
Nitrogen Structure Correlation: The structure of bulk nitrogen atoms in the diamond was found to be identical to the nitrogen structure adjacent to the carbon-centered radicals, validating EPR as a tool for characterizing overall crystal structure.
Color Center Identification: Blue color is primarily correlated with the appearance of the N4 carbon radical species, while yellow color is attributed to the presence and increase of N1 species.
Treatment Differentiation: Blue color requires irradiation followed by low-temperature annealing (400-600 °C) to eliminate the yellow component. Yellow color requires irradiation followed by high-temperature annealing (500-1000 °C) to eliminate the blue component.
Concentration Metrics: Bulk nitrogen concentration (ppm) is 3-4 orders of magnitude higher than the concentration of stable carbon radicals (spins/mg), indicating that the color centers are highly efficient optical absorbers.
Material Requirements: Successful color enhancement depends heavily on initial bulk nitrogen concentration: Green (>1000 ppm), Blue (300-600 ppm), and Yellow (<200 ppm).

Technical Specifications

Parameter	Value	Unit	Context
Diamond Type Studied	Ia A/B	N/A	Natural, transparent, brilliantly shaped
Electron Irradiation Energy	0.8-2	MeV	LINAC treatment range
Green Protocol Irradiation Time	1.5	h	No subsequent thermal treatment
Blue/Yellow Protocol Irradiation Time	2	h	Followed by thermal treatment
Blue Annealing Temperature	400-600	°C	Low-T annealing (10-20 min)
Yellow Annealing Temperature	500-1000	°C	High-T annealing (10-20 min)
Green Diamond N Concentration	>1000	ppm	High bulk N content
Yellow Diamond N Concentration	<200	ppm	Low bulk N content
GR1 Optical Center Absorption	740.9 and 744.4	nm	Neutral isolated vacancy (V°) defect (Green/Blue)
N3 Optical Center Absorption	415.2	nm	Three substitutional N atoms + vacancy (3N + V) (Yellow)
Green N Concentration (Molar)	2.52 x 10^-1	Molar	Average bulk N concentration
Green Radical Concentration (Molar)	2.64 x 10^-4	Molar	Average carbon-centered radical concentration
Diamond Density Used for Calculation	3.53	g/cm³	Standard diamond density
FTIR Measurement Range	600-1500	cm^-1	One-phonon region (Nitrogen absorption)

Key Methodologies

The study utilized a multi-step process involving high-energy irradiation and controlled thermal annealing, followed by comprehensive spectroscopic analysis.

Material Selection: 15 natural, transparent Type Ia A/B diamonds were selected and grouped based on the intended final color (Green, Blue, Yellow).
Irradiation Treatment: Diamonds were exposed to electron irradiation using a LINAC (Linear Electron Accelerator) at an energy range of 0.8-2 MeV.
- Green Protocol: 1.5 hours of irradiation only.
- Blue/Yellow Protocols: 2 hours of irradiation.
Thermal Treatment (Annealing): Applied post-irradiation to blue and yellow groups to refine color by eliminating undesired centers.
- Blue Protocol: Annealing at 400-600 °C for 10-20 minutes.
- Yellow Protocol: Annealing at 500-1000 °C for 10-20 minutes.
Spectroscopic Characterization:
- FTIR Spectroscopy: Measured nitrogen absorption peaks (A, B, C centers) in the one-phonon region (600-1500 cm^-1) to determine bulk nitrogen concentration.
- UV-Vis Spectroscopy: Measured absorption spectra (350-800 nm) to identify optical color centers (e.g., GR1, H3, N3).
- Fluorescence: Assessed using a 365 nm UV lamp to observe changes in fluorescence patterns (e.g., blue fluorescence in Type Ia due to nitrogen impurities).
- EPR Spectroscopy: Used to identify and quantify stable carbon-centered paramagnetic centers (radicals N1, N4, P2/W21) via g-values and hyperfine interaction (A), correlating radical structure with color.

Commercial Applications

The precise control and characterization of nitrogen-related defects in diamond are critical for several high-value industries, particularly those relying on defect engineering and material authentication.

Gemological Authentication and Grading: Provides definitive methods (EPR fingerprinting) to distinguish between natural, untreated diamonds and those enhanced via irradiation and annealing, crucial for the fancy color diamond trade.
Quantum Technology (NV Centers): The manipulation and understanding of nitrogen-vacancy (N-V) related defects (like N4, which is a precursor or related structure) are fundamental for creating high-quality Nitrogen-Vacancy (NV) centers used in quantum computing, sensing, and magnetometry.
High-Purity Diamond Synthesis: The correlation between bulk nitrogen concentration and resulting defect structures offers feedback for optimizing CVD or HPHT growth processes to control defect incorporation for specific electronic or optical applications.
Advanced Optical Materials: Engineering specific optical centers (GR1, H3, N3) allows for the production of diamonds with tailored absorption and emission properties for use in specialized optics and laser components.
Material Science Research: Provides a robust methodology for studying defect kinetics (formation and bleaching of radicals during irradiation and annealing) in wide bandgap semiconductors.

View Original Abstract

Color induction in nitrogen-contaminated diamonds was carried out via various procedures that involve irradiation, thermal treatments (annealing), and more. These treatments affect vacancy defect production and atom orientation centers in the diamond lattice. Natural diamonds underwent color enhancement treatments in order to produce green, blue, and yellow fancy diamonds. The aim of this study was to follow the changes occurring during the treatment, mainly by EPR spectroscopy, which is the main source for the determination of the effect of paramagnetic centers (carbon-centered radicals) on the color centers produced via the treatments, but also via visual assessment, fluorescence, UV-vis, and FTIR spectroscopy. The results indicate that diamonds containing high levels of nitrogen contamination are associated with high carbon-centered radical concentrations. Four paramagnetic center structures (N1, N4, and P2/W21) were generated by the treatment. It is suggested that the N4 structure correlates with the formation of blue color centers, whereas yellow color centers are attributed to the presence of N1 species. While to produce blue and yellow colors, a thermal treatment is needed after irradiation, for treated green diamonds, no thermal treatment is needed (only irradiation).

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst12121775

References

2013 - Optical Defects in Diamond: A Quick Reference Chart [Crossref]
2018 - Natural-Color Green Diamonds: A Beautiful Conundrum [Crossref]
2007 - Natural Type IA Diamond with Green-Yellow Color Due to Ni-Related Defects [Crossref]
2005 - Characterization and Grading of Natural-Color Yellow Diamonds [Crossref]
2002 - Characterization and Grading of Natural-Color Pink Diamonds [Crossref]
1978 - Investigating Artificially Coloured Diamonds [Crossref]
2002 - HPHT Synthesis of Diamond with High Nitrogen Content from an Fe3N-C System [Crossref]