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.
Performance of nearly fixed offset asymmetric channel-cut crystals for X-ray monochromators
