Positron annihilation and optical studies of natural brown type I diamonds

• Positron annihilation measurements of natural brown type I diamonds were performed.
• Natural brown type I diamonds were found to contain large vacancy clusters.
• Vacancy cluster defects were absent in clear diamonds.
• The concentration of vacancy clusters increased with brown colouration intensity.
• These results indicate that large vacancy clusters are the brown centres.

Positron annihilation measurements of natural brown type I diamonds revealed that they contain monovacancy-type defects and large vacancy clusters. The latter defect type was absent in clear diamonds. Both the intensity of the positron vacancy cluster lifetime and the S-parameter were found to increase with the intensity of brown colouration determined by optical absorption measurements. No correlation was found between the nitrogen content and the positron annihilation parameters.