Optical study of rare earth metaborates doped with europium as a local structural probe

The optical analysis of Eu3+ ion in rare earth metaborates, RE(BO2)(3), RE = La, La/Gd and Gd, is given. The evolution of the splittings: of the F-7(1) sublevels with the ionic radii of RE points out the limit of the stability of the metaborate phases along the rare earth series. The electrostatic crystal field (cf) effect on the F-7 multiplet was evaluated on the basis of the phenomenological cf theory using respectively 17, 19 and 18 F-7(JM) Stark components (J=0-4). The actual RE point symmetry is C-2 but the C-2v symmetry was used in the simulation giving a good fit with r.m.s. standard deviations of 4.7, 5.7 and 3.7 cm(-1) between the calculated and experimental F-7(0-4) energy levels. Moreover, when VUV radiation is absorbed by rare earth metaborates the presence of Gd3+ ions enhances considerably the visible luminescence intensity. The shift of the intrinsic absorption band from La to Gd shows the covalency reduction in the metal-ligand bonding.