Electron-density distribution in CuFeS2 as determined by 63,65Cu NMR in an internal magnetic field

NMR spectra of Cu 63 and Cu 65 in an internal magnetic field were studied experimentally. The electric field gradient (EFG) at Cu nuclei in chalcopyrite CuFeS2was evaluated ab initio by using a cluster approach.Calculations were carried out in the framework of the self-consistent field restricted open-shell Hartree-Fock method (SCF-LCAO-ROHF). The largest cluster for which calculations were carried out had the formula Cu9Fe10S28n (R~6 Å, 47 atoms), where n is the cluster charge. The best agreement of the quadrupole parameters (quadrupole frequency νQ and EFG tensor asymmetry parameter η) that were determined experimentally (νQ = 1.29 MHz, η = 0.34) and were calculated (νQ = 1.40 MHz, η = 0.50) was obtained for the cluster Cu9Fe10S28-4. Maps of electron-density distribution in the neighborhood of the Cu quadrupolar nucleus were built for the cluster Cu9Fe10S28-4. It was suggested based on an analysis of the obtained electron-density distribution that the bond in chalcopyrite is not covalent. The energy-level diagram that was calculated in the ROHF high-spin approximation defined rather well chalcopyrite as a semiconductor with a very narrow LUMO-HOMO gap and was consistent with the notion of chalcopyrite as a gapless semiconductor. (author)