Infrared absorption studies of the annealing of irradiated diamonds

Abstract

Abstract Natural (types Ia and IIa) and synthetic (type Ib) diamonds have been irradiated with energetic electrons and neutrons and then heated at temperatures up to 1400°C. Attendant changes in the infrared absorption spectra, especially above the Raman frequency (1332 cm−1), have been monitored. The most prominent absorption to develop in the infrared region proper, on annealing both type Ia and type Ib specimens, whether electron- or neutron-irradiated, is the H1a line at 1450 cm−1. Measurements taken of neutron-irradiated type Ia specimens show that the strength of this line is specimen-dependent, and that it is a linear function of radiation dose. Isochronal annealing studies show that the onset of the line occurs during heating at 250°C for type Ia specimens and at 650°C for type Ib specimens. The absorption begins to weaken during heating at 1100°C, but it is very persistent, surviving an anneal of 4 hours at 1400°C, albeit with diminished intensity. Three other weaker lines at 1438, 1358 and 1355 cm−1 develop with the 1450 cm−1 line, but differ from it and from each other in subsequent annealing behaviour. Type Ib synthetic diamonds also show the development, at 650°C, of a second system of four apparently related lines at 1502,1430,1408 and 1384 cm−1, of which the one at 1502 cm−1 is the strongest. This system disappears during heating at 850°C. It is shown that the centre responsible for the 1502 cm−1 line involves one nitrogen atom only, and that the line is interpretable as a C-N bond vibration. Several other weaker peaks, notably at 1531, 1570 and 1573 cm−1, have been observed in the infrared spectra as a result of irradiation and annealing. Their dependences on diamond type, on the nature of the irradiation and on annealing conditions are described. Finally, a new line, termed H1c, has been found in the near-infrared region at ∼1936nm, accompanying the already-reported H1b line at ∼2026nm, which develops on annealing irradiated type Ia specimens. The onset of these lines occurs on heating at 650–700°C, and they anneal out completely during 4 hours at 1400°C. The strengths, widths and precise positions of the lines, which are thought to be of electronic rather than vibrational origin, are specimen-dependent. Their relationships to intrinsic nitrogenous defects are discussed: H1b depends on the presence of A aggregates whilst H1c almost certainly depends on B aggregates for its formation.