Rare-earth orthovanadates: Covalency, chemical bonding, and optical spectra

Investigations of electronic structure and optical spectra were made for yttrium orthovanadate, and for rare earth orthovanadates RVO₄, where R = Ce, Nd, Eu, Tb, Dy, Gd, and Yb. The Hartree-Fock-Slater model was used in conjunction with a numerical discrete variational method to calculate energy levels and wavefunctions for molecular clusters (VO₄)^3- and (RO₈)^13- found in the orthovanadate crystal lattice. Analysis of the MO charge and spin densities reveals a significant involvement of rare earth 4f orbitals in chemical bonding, through hybridization of of R-5p and mixing with O-2p atomic orbitals. The MO energy level diagrams provide a satisfactory semiquantitative interpretation of the experimental excitation, reflection, and luminescence spectra. Energy transfer from the vanadate ion to the rare-earth ion is understood in terms of covalent mixing between metal and shared O-2p orbitals for neighboring (VO₄)^3- and (RO₈)^13- clusters. The relative luminescent efficiency of some rare-earth elements is explained on the basis of the calculated energy level diagrams.