The VN3 defect in diamond: A quantum mechanical simulation of the vibrational spectra and EPR properties.

The electronic structure and the equilibrium geometry of the VN 3 defect in diamond are computed by using a quantum mechanical approach, the main ingredients being an all electron gaussian type basis set, a global hybrid functional, the supercell scheme (216 atoms) and the CRYSTAL code. For the first time the Fermi contact and the hyperfine coupling tensor B of the open shell structure (S z = 1/2) are evaluated and compared with available experimental EPR (electron paramagnetic resonance) data. The agreement is excellent. The infrared (IR) spectrum is also computed. It results to be very close to the one of the so called B defect (VN 4), and with similarities to the one of the other members of the VN x (x = 0, 1, 2) family, so that the attribution of specific experimental peaks to one of the five mentioned compounds is difficult and questionable. The peaks measured by Sutherland (Nature, 174 , 901, 1954) and attributed to the B center might, at least in part, come from both VN 3 and VN 4. Being the ground state of the latter a closed shell, and then EPR inactive, a combined use of the two spectroscopies (IR and EPR) is mandatory for the identification of the relative weight of the two defects. Image 1 [ABSTRACT FROM AUTHOR]