Sequences of high Bragg-angle (0
B
= 74°) 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 x 3.2 mm
2
in area, 0.7 mm thick, possessed an unusually well developed (110) 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, a
0
, of the (111) growth sector (nitrogen content 88 + 7 parts per 10
6
atomic) relative to that of the (110) sector: Aa
0
/ a
0
= 1.18 + 0.07 x 10
-5
. In terms of measured infrared absorption coefficient at 1130 cm
-1
, this gives Aa
0/a
0
= (2.95 + 0.27) x 10
-6
[p(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
c
N
, the fractional atomic concentration of substitutional nitrogen, as A
a
0
/
a
0
= (0.14 + 0.02)
c
N
, 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.
Research of an algorithm for crystal lattice parameter identification based on the gradient steepest descent method