The luminescent nature of orange coloration in natural diamonds: optical and EPR study

A natural orange diamond crystal from Siberian kimberlite has been studied using complementary spectroscopic techniques. The IR-spectra of the crystal revealed presence of low concentration of nitrogen A, B and C centers. In addition to these common defects a Y-center and 1240 cm−1 band were clearly detected. Absorption UV–Vis spectra exhibited the 490 nm band and an unknown very broad band with diffuse maximum at 320–380 nm, with low energy edge extending far into the visible region. Photoluminescence spectra showed presence of N3, 440 nm, 455 nm, S1, 575.8 nm vibronic systems as well as zero-phonon lines at 418.2, 525.1, 560.8, 645.7 and 799.7 nm. Luminescence of the 575.8 nm system with broad vibronic band peaked at 690 nm noticeably contributes to the color of the diamond, i.e. its orange color is not only due to absorption but also to emission of light. Electron Paramagnetic Resonance revealed several structural defects: nitrogen-related P1, M5, M6 centers, oxygen-related OK1 and N3 centers, previously unreported nickel M7 center and tentatively phosphorus-related W24 center. The M5 and M6 centers involving two nitrogen atoms separated by ca. 8–10 interatomic distances represent an unusual long-range bonding. It is suggested that M6 and M7 might be related to the IR Y-center and to the broad band with maxima at 320–380 nm in the optical spectra of the diamond.