Electron paramagnetic resonance of terbium in a zirconium fluoride glass

Electron paramagnetic resonance of trivalent terbium ions is reported in a glass based on zirconium fluoride at liquid helium temperatures and at frequencies in the range 9 to 30 GHz. The results are interpreted as arising from a nonKramers doublet ground state with g/sub ///=17.3+or-0.5, A/sub ///=0.209+or-0.001 cm-1 and with a zero-field-splitting parameter delta taking a continuous range of values from zero to at least 1.0 cm-1. A simple theoretical model is developed that satisfactorily reconstructs the essential features of the observed resonance spectra. It is concluded that terbium ions act as network formers. They occupy a relatively well defined type of site, similar to those found in crystalline hosts, with a dominant crystal-field term of axial symmetry but with a continuous range of small site-to-site variations.