Electron paramagnetic resonance and equilibrium atomic configurations studies of binuclear niobium molecules in Li-Nb phosphate glass dielectrics

COMP 151

Sabrina A. Arrington-Peet, s.a.arrington@nsu.edu, Rakhim R. Rakhimov, rrakhim@nsu.edu, and Vladimir I. Gavrilenko, vgavrilenko@nsu.edu. Center for Materials Research, Norfolk State University, Norfolk, VA 23504
The purpose of this research was to investigate paramagnetic properties of binuclear niobium complexes Nb─O─Nb with two nonequivalent Nb4+ ions in lithium niobium phosphate glass (LNPG) using electron paramagnetic resonance (EPR). Equilibrium atomic configurations of the system were studied by total energy ab initio method. Experimental EPR spectrum analysis revealed nonequivalent distribution of the charge and electron spin density between two Nb atoms. The niobium atoms could exist in either of the two forms, Nb4+─Nb4+ or Nb3+─Nb5+. The theoretical analysis of the complex revealed that the Nb atoms could in fact exist in either of these forms depending upon the state of the Nb complex. The Nb atoms in freestanding molecule are equivalent with oxygen atom having only one local equilibrium position in the center of the molecule, indicating centro-symmetric structure. However, when the Nb complex was embedded in the glass the potential energy and atomic geometry changed due the distortions of the molecules. The effect of external potential in LNPG resulted in an appearance of two nonequilibrium geometries for the central oxygen atom shifted from the center by DR ≈ 0.3 to 0.4. The model correctly predicts the appearance of the two nonequivalent geometries of the complex embedded into LNPG, which was confirmed experimentally. Theoretical value of the distortion exceeds experimental value by approximately a factor of 2. This indicated that further modification of the model is required.