A computational and spectroscopic investigation of hole states in Cu-doped MgO

The influence of a copper substitution in an MgO lattice and the nature of hole states surrounding the dopant are investigated using hybrid quantum mechanical/molecular mechanical calculations at the density functional theory level alongside new Electron Paramagnetic Resonance (EPR) spectroscopic data on a synthesized sample of the material. In calculations, the neutral Cu(II)–MgO system displays Jahn–Teller distortion around the copper center in both tetragonally elongated and compressed configurations, in line with previous computational and experimental findings. Subsequently, the behavior of an electron hole is investigated in Cu(II)–MgO, which has been previously unexamined prior to this work. For this charged system, it is clearly shown that in the ground state a hole will delocalize across all coordinated oxygens. Localization of a hole on an oxygen species is possible but is found to be higher in energy, and a hybrid functional with an increased fraction of Hartree–Fock exchange is necessary. Notably, the calculations show that oxidation of the copper species to Cu(III) is not possible. The spin Hamiltonian parameters for the monoatomic copper centers in bulk MgO for both the neutral and ionized systems are also calculated and compared to experimental values obtained by EPR spectroscopy on a polycrystalline Cu(II)–MgO sample. The computed spin Hamiltonian parameters are shown to be largely consistent with the experimental parameters, and the axial distortion in the directional parameters in the material confirms that the copper dopant causes tetragonal elongation in its bonding with neighboring oxygen atoms.