Polarised infrared cathodoluminescence from platelet defects in natural diamonds

Abstract

WE report here that the large platelet defects occasionally found in natural diamonds emit polarised cathodoluminescence in the near infrared. Lattice defects in natural diamonds attract as much scientific enquiry today as at any time since the anomalous ‘spike’ diffuse X-ray reflections from single-crystal diamonds were discovered¹. That platelet-type defects segregated on {100} planes of the diamond matrix could well account for the ‘spike’ X-ray reflection observations^1,2 was shown by Frank³. Transmission microscopy⁴ revealed just such platelets, with diameters generally in the range 10 to 100 nm, profusely present within the most common form of diamond (Type Ia). Mendelssohn⁵ described a specimen containing platelets on {100} that were more than two orders of magnitude greater in diameter than those that had become well known through transmission electron microscopy. The large platelets emitted a green–yellow cathodoluminescence which made them conspicuous when they lay not deeper below the specimen surface than the penetration range of the bombarding electron beam (∼ 10 µm). Work in our laboratory has shown that platelets on {100} span the gamut of sizes from the largest observed by Mendelssohn down to the more familiar sub-micrometre objects seen electron-microscopically. This knowledge has been acquired by combining X-ray topography (using both normal Bragg reflections and the anomalous ‘spike’ reflections), cathodoluminescence topography, ultraviolet absorption topography and transmission electron microscopy at 100 kV and 1 MeV (refs 6,7). Platelets exceding 5–10 µm in maximum dimension are rare compared with those of sub-micrometre size; but our experience affords us a good chance of identifying solely by X-ray topography regions within diamonds where there reside platelets having extreme dimensions exceeding 1–2 µm (and such platelets we dub ‘giant’).