On the measurement of population density and size of platelets in type I a diamond and its implications for platelet structure models

This paper is concerned with the way that nitrogen impurity in type Ia diamond is distributed between the defects responsible for the principal impurity-dependent infrared absorption maxima in the 7-11 µm wavelength range. The defects involved are the platelets on diamond {100} planes, which produce the B´ absorption peak at 7.3 µm, and the A and B defects identified by their absorption maxima at 7.8 µm and 8.5 µm, respectively. The relation between the strength of the B´ absorption and the platelet area per unit volume measured electron-microscopically is found to be Ap/µm-1 = (9.0 ± 2.1) x 10-3I(B´)/cm-2, where Apis the platelet area per unit volume, expressed in square micrometres per cubic micrometre, and I(B´) is the integrated absorption, i. e. the area under the B´ peak, expressed in reciprocal centimetres squared. Features of the experimental methods employed include application of weak-beam microscopy for accurate measurement of platelet size and shape, counting a large sample platelet population in specimen thicknesses accurately determined in terms of extinction distance and measurement of infrared absorption through the same thin slice in which at a closely adjacent point platelet counting was performed. The constraints that the relation between Ap and I(B´) imposes on platelet structure models, when taken in conjunction with the correlations between A, B and B´ absorptions established by G. S. Woods (Proc. R. Soc. Lond. A 407, 219-238 (1986)), are analysed. Currently accepted models of nitrogen-containing A and B defects are assumed, and the analysis presupposes that platelets are formed by the conversion of A defects into platelets and B defects. It is found that conversion of A defects cannot supply sufficient nitrogen to form platelets containing four nitrogen atoms per area unit a20(a0 is the diamond face-centred cubic unit cell edge), but could form platelets with two nitrogen atoms per unit area a20;. Another constraint on platelet structure appears out of the analysis when considering the known expansion of the platelet cell by ca. 0.36 a0 normal to the platelet plane. It is that the atomic packing density in the platelet structure cannot be more than about three-quarters that of the perfect diamond structure, and this restriction applies whether the platelet is nitrogen-free or contains all the nitrogen released from A defects.