Preparation of “natural” diamonds by HPHT annealing of synthetic diamonds

In this paper, IIa, Ib, N-doped and N–H co-doped diamonds were studied, and the interaction mechanisms between hydrogen and nitrogen in the diamonds were investigated in detail.

The structure and mechanism of formation of platelets in natural type Ia diamond

Observations have been made on medium- and large-sized platelets in a natural type Ia diamond. Attempts have been made to detect any regularity in the structure of the platelets by means of electron diffraction, but none was found. The displacement associated with the platelets has been determined to be extrinsic, normal to the plane of the platelet and of magnitude 0.4 ‹010›. This value has led to the proposal of a model for the platelet structure in which the platelet consists of an interstitial layer of carbon atoms pentagonally bonded to the surrounding diamond matrix. The way in which the detailed bonding may be varied, while complete tetrahedral bonding is maintained, can lead to irregular structures consistent with the diffraction evidence. Possible mechanisms for producing interstitial carbon atoms are considered and the role of nitrogen in this context and in providing possible nuclei for platelet growth is discussed.

Conversion of platelets into dislocation loops and voidite formation in type IaB diamonds

Type la natural diamonds have been heated in the temperature range of 2400-2700°C under stabilizing pressures. The specimens studied are mainly regular type IaB diamonds. Transmission electron microscopy studies of treated speci­mens show that platelets are converted to interstitial ½ a 0 <011> dislocation loops; voidites are also formed. When all the platelets have been converted, the ex­perimental features associated with them also disappear, i. e. the X-ray extra reflections (spikes), the B' local-mode absorption and the lattice absorption in the one-phonon region termed the D spectrum. It is discovered that when diamonds are heated under graphite-stable rather than diamond-stable conditions, the rate of conversion is considerably enhanced; for instance, at 2650°C there is an increase in the rate of about three orders of magnitude. This enhancement is considered to be due to the instability of the diamond structure itself and a reason for this enhancement is suggested.

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 A p /µm -1 = (9.0 ± 2.1) x 10 -3 I (B´)/cm -2 , where A p is 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 A p 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 a 2 0 ( a 0 is the diamond face-centred cubic unit cell edge), but could form platelets with two nitrogen atoms per unit area a 2 0 ;. Another constraint on platelet structure appears out of the analysis when considering the known expansion of the platelet cell by ca . 0.36 a 0 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.