ON THE DILATATION OF DIAMOND BY NITROGEN IMPURITY AGGREGATED IN A-DEFECTS

Lattice parameter measurements by modern methods of photography of tra nsmitted divergent-beam X-ray diffraction patterns are reported. A sma ll (less than 10 mu m diameter) source of divergent CuK alpha X-rays i s produced by an electron beam impinging on a thin copper film evapora ted on to one surface of the diamond, which is placed in a scanning el ectron microscope. Patterns of high dispersion and high resolution are obtained by use of a 700 mm camera length and an Ilford L4 nuclear em ulsion as the recording medium. A synthetic diamond volume containing only about one atomic part per 10(6) nitrogen impurity served as a pur e-diamond standard. This was compared with a natural diamond of nearly pure IaA spectral type, unusually rich and uniform in content of nitr ogen impurity aggregated in A-defect form (N concentration ca. 1135 at omic parts per 10(6)). In both specimens, volumes free from dislocatio ns and bounded by well-polished damage-free surfaces were probed. adva nces in pattern-measuring methods included a film-stacking technique t o improve signal-to-noise ratio; digitized microdensitometry and the m easurement of pattern shifts relative to invariant datum points in the pattern provided by angularly sharp features arising from coherent mu ltiple diffraction effects. Procedures were developed to correct for d iffraction-line-profile asymmetry (which depends upon lattice perfecti on as well as other diffraction parameters). An absolute lattice param eter determination on the pure diamond yielded a(0) = 0.356710(4) nm, considered to be a good value. After subtracting the calculated dilata tion contribution (24%) due to a small population of {001} platelet de fects in the nitrogen-rich diamond, the dilatation due to A defects de rived is Delta a(0)/a(0) = (0.6 +/- 0.25) x 10(-6) [mu(A(1282 cm-1))/c m(-1)], where mu(A(1282 cm-1)) is the infrared absorption at 1282 cm-l due to A defects, in units of cm-l In terms of C-N(A): the fractional atomic concentration in the diamond of nitrogen impurity present in A defects, and subject to more uncertainty, Delta a(0)/a(0) = 0.036c(N( A)), which indicates that a nitrogen atom in an A defect occupies ca. 1.11 times the volume of the normal carbon atom it replaces. This dila tation is compared with those expected fi-om computed structures of th e A defects.