Electron paramagnetic resonance and optical spectroscopy of Yb3+ ions inSrF2 andBaF2;an analysis of distortions of the crystal lattice near Yb3+

Abstract

SrF2 and BaF2 crystals, dopedwith the Yb3+ions, have been investigated by electron paramagnetic resonanceand optical spectroscopy. As-grown crystals of SrF2 andBaF2show the two paramagnetic centres for the cubic (Tc) and trigonal (T4) symmetries ofthe Yb3+ions. Empirical diagrams of the energy levels were established and the potentialsof the crystal field were determined. Information was obtained on the SrF2 andBaF2phonon spectra from the electron-vibrational structure of the opticalspectra. The crystal field parameters were used to analyse the crystallattice distortions in the vicinity of the impurity ion and the F−ion compensating for the excess positive charge in T4.Within the frames of a superposition model, it is shown that three F− ionsfrom the nearest surrounding cube, located symmetrically with respect to the C3axis from the side of the ion-compensator, approach the impurityion and cling to the axis of the centre when forming T4. TheF−ion located on the axis of the centre between the Yb3+ion and ion-compensator, also approaches close to the impurity ion.