The effect of isotopic changes on the {001} platelet infrared absorption in diamond

Abstract

Normal synthetic diamonds consist of 99% ¹²C with 1% ¹³C in the host lattice, with dispersed ¹⁴N as the main impurity. Two variants were produced by changing natural isotopic abundances, in one case of the atoms of the host lattice and in the other of the nitrogen impurity. These, together with normal specimens, were treated under a stabilizing pressure at a temperature sufficiently high to produce {001} platelets and the associated B' localized-mode infrared absorption peak. No isotopic effects were observed when B' peaks in ¹²C diamonds with a substantial proportion of the nitrogen (up to about 67%) present as the ¹⁵N isotope (the balance being ¹⁴N) were compared with the B' peaks in ¹²C diamonds containing only ¹⁴N. This implies either that nitrogen atoms, if present, are a minor constituent of the platelets or that such atoms, if a major species, remain stationary in the vibrational mode responsible for the B' absorption peak. Specimens consisting almost entirely of ¹³C, with the nitrogen impurity present as ¹⁴N, showed a shift in the wavenumber of the B' peak. Using simple harmonie theory, and taking into consideration the experiment described above which shows no influence of nitrogen atoms on the B' peak, one would anticipate this shift to be in the ratio of about (12/13)^½, caused by the change in the carbon mass only. The observed shift was, however, smaller than expected. It is suggested that this anomalous shift is caused by anharmonic effects, enhanced by the proximity of the B' local mode energy to the Raman energy. It would be valuable if a theoretical study could be made to test the validity of this suggestion.