On the Dilatation of Synthetic Type Ib Diamond by Substitutional Nitrogen Impurity
Abstract
Sequences of high-Bragg-angle (θ B = 74 degrees) double-crystal X-ray topographs taken at the SRS (Daresbury, U.K.) have yielded precise measurements of lattice parameter differences between growth sectors of different crystallographic forms in a large undoped synthetic diamond whose type Ib infrared absorption spectrum (principal peak at 1130 cm-1) indicated atomically dispersed nitrogen, singly substituting for carbon, as the only detectable impurity. The plate-shaped specimen, polished parallel to (110), 5.0 × 3.2 mm2 in area, 0.7 mm thick, possessed an unusually well-developed (1{1}0) growth sector containing nitrogen impurity concentration of only ca. 10-6, which served as an internal standard of pure-diamond lattice parameter with which lattice parameters of nitrogen-containing growth sectors were compared. The specimen's suitability for precision diffractometry was checked by comprehensive tests using optical microscope techniques, cathodoluminescence and single-crystal X-ray topography. The double-crystal combination was silicon reference crystal, asymmetric 175 reflection, with diamond specimen symmetrical 440 reflection. The principal measurement was the increase of the lattice parameter, a0, of the (1{1}1) growth sector (nitrogen content 88 ± 7 parts per 106 atomic) relative to that of the (1{1}0) sector: Δ a0/{a}0 = 1.18 ± 0.07 × 10-5. In terms of measured infrared absorption coefficient at 1130 cm-1, this gives Δ a0/{a}0 = (2.95 ± 0.27) × 10-6[μ (1130 cm-1)/cm-1], which is believed to hold for growth sectors of all crystallographic forms. Combination with the nitrogen assay findings of Woods, van Wyk & Collins (Phil. Mag. B 62, 589-595 (1990)) provides a direct relation to cN, the fractional atomic concentration of substitutional nitrogen, as Δ a0/{a}0 = (0.14 ± 0.02)cN, which indicates that the effective volume of a single substitutional nitrogen atom in diamond is 1.41 ± 0.06 times that of the carbon atom it replaces. This substantial dilatation conflicts with several models for the substitutional nitrogen structure.
- Publication:
-
Philosophical Transactions of the Royal Society of London Series A
- Pub Date:
- December 1991
- DOI:
- 10.1098/rsta.1991.0135
- Bibcode:
- 1991RSPTA.337..497L