The [V−C═C−V] Divacancy and the Interstitial Defect in Diamond: Vibrational Properties

Abstract

We describe a particularly stable isolated divacancy [VCCV] in diamond, in which the two vacant sites are separated by two bonded carbons. Its structure, vibrational properties, and stability are described by using Density Functional Theory (DFT). We validated the calculated CC stretching frequencies for eleven molecules. The isolated divacancy [VCCV] is found to be very stable and is separated by a high barrier of 5 eV from the divacancy of two missing adjacent carbons, V2. The predicted characteristic CC stretching frequency of the isolated divacancy [VCCV] is 1607 cm−1, close to Raman bands at 1620−1630 cm−1 observed in various nanodiamonds, offering an alternative interpretation for the assignment of this vibrational band. We conclude that the alternative interpretation based on the CC stretching mode of an interstitial defect can be ruled out. Our accurate and validated vibrational calculations offer further evidence that the vibrational structure of the 3H optical center in diamonds should be assigned to the interstitial defect because it provides excellent agreement with the observed vibrational frequencies of three isotopologues.

Comparison between cluster and supercell approaches: the case of defects in diamond

“…2 of Refs. [7,6]). The vacancy defect has been created through the substitution of the central atom with a ghost atom (see next paragraph).…”

“…The cluster approach is widely used to study local defects [1][2][3][4][5][6][7][8][9][10], chemisorption at crystalline surfaces [11][12][13][14][15][16], molecular crystals [17][18][19] and, in extreme cases, the properties of bulk solids [20,21]. A particular advantage of this method, in which the infinite system is approximated by a finite cluster containing a relatively small number of atoms, is that sophisticated many-body techniques of quantum chemistry are available that can be used to estimate electron correlation effects [22,23] and for the characterization of excited states.…”

“…Furthermore, the cost of such calculations increases quickly, so in most of the cases the largest clusters considered contain rather a small number of atoms (say less than 100-200). [6,7] Comparisons with periodic or embedded clusters performed in the past [6,37,38] can be considered only qualitative, since differences in functionals, basis set, computational parameters and adopted computer code can introduce severe differences in the results. The cluster approach is particularly favorable for a study of defects in semiconductors, since long range electrostatic and relaxation effects are expected to be negligible.…”

“…For this reason the cluster model has been extensively used also in recent years for the study of local defects in diamond, and actually the most complete investigations have been performed with this technique. [6][7][8][9] However, a clear evidence of the convergence with the cluster size of the various properties has not been provided. In some cases, when both supercell and cluster schemes have been used in the same investigation, different functionals have been used for the two cases, making meaningless the quantitative comparison of the results.…”

“…In some cases, when both supercell and cluster schemes have been used in the same investigation, different functionals have been used for the two cases, making meaningless the quantitative comparison of the results. [6] Finally, when properties that can hardly be defined as local, as the IR or Raman vibrational spectrum, have been computed, border effects were not discussed. More explicitly: how border effects perturb the very simple Raman spectrum of pristine diamond, containing a single peak at 1332 cm −1 ?…”