On the dilation of diamond by nitrogen impurity aggregated in A defects

Abstract

Lattice parameter measurements by modern methods of photography of transmitted divergent–beam X–ray diffraction patterns are reported. A small (less than 10 μm diameter) source of divergent CuKα X–rays is produced by an electron beam impinging on a thin copper film evaporated on to one surface of the diamond, which is placed in a scanning electron microscope. Patterns of high dispersion and high resolution are obtained by use of a 700mm camera length and an Ilford L4 nuclear emulsion as the recording medium. A synthetic diamond volume containing only about one atomic part per 10 6 nitrogen impurity served as a pure–diamond standard. This was compared with a natural diamond of nearly pure IaA spectral type, unusually rich and uniform in content of nitrogen impurity aggregated in A–defect form (N concentration circa 1135 atomic parts per 10 6 ). In both specimens, volumes free from dislocations and bounded by well–polished damage–free surfaces were probed. Advances in pattern–measuring methods included a film–stacking technique to improve signal–to–noise ratio, digitized microdensitometry and the measurement of pattern shifts relative to invariant datum points in the pattern provided by angularly sharp features arising from coherent multiple diffraction effects. Procedures were developed to correct for diffraction–line–profile asymmetry (which depends upon lattice perfection as well as other diffraction parameters). An absolute lattice parameter determination on the pure diamond yielded , a 0 = 0.356710(4)nm, considered to be a good value. After subtracting the calculated dilatation contribution (24%) due to a small population of {001} platelet defects in the nitrogen–rich diamond, the dilatation due to A defects derived is Δa 0 /a 0 = (0.6 ± 0.25) x 10 –6 [μ A(1282cm–1) /cm –1 ], where μ A(1282cm–1) is the infrared absorption at 1282cm –1 due to A defects, in units of cm –1 . 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, Δa 0 /a 0 = 0.036c N(A) , which indicates that a nitrogen atom in an A defect occupies circa 1.11 times the volume of the normal carbon atom it replaces. This dilatation is compared with those expected from computed structures of the A defects.