Crystal field analysis of Er 3+-doped glasses: germanate, silicate and ZBLAN

Abstract

A simple model of average crystal field Hamiltonian based on the principle of descending symmetry and using the group chain scheme is proposed to describe the average of local symmetries occupied by rare earth ions in a disordered medium. Only three crystal field parameters, one for each rank, have to be fitted in this Hamiltonian. Based on the absorption and emission spectra measured in a conventional way at 13 K, the Stark levels in manifolds from 4I 15/2 to 2G(1) 9/2 of Er 3+ in two oxide glasses: germanate and silicate and one fluoride glass: ZBLAN, are investigated. The scalar crystal field strength is analysed, and former models about optical properties of rare earth ions in glasses are discussed. The crystal field strength is decreasing from silicate to germanate and finally to fluoride glass. In the oxide glasses, the rare earth ions can occupy positions with symmetries distorted from D 4, D 4 h and their isomorphic groups. In the fluoride glass, the rare earth ions can occupy positions more freely with symmetries distorted from D 4, D 4 h and also from D 3, D 3 d and their isomorphic groups. In all studied glasses, the cubic crystal field terms give the major contribution to the crystal field strength around the Er 3+ ion when the weak axial distortion removes completely the degeneracy.