Luminescence and energy transfer in europium Tellurite anti-glass phases

The luminescence and energy transfer properties of two lower symmetrical europium tellurite anti-glass phases are reported, viz. Eu_1.79TeO_x, which has a pseudo-tetragonal structure, and Eu_1.08TeO_x, which has a monoclinic ordered crystal structure.

In the pseudo-tetragonal phase the Eu³⁺ ions occupy two major types of sites. In the monoclinic phase there are at least seven different crystallographic sites available to the Eu³⁺ ions. At low temperatures transfer of excitation energy to the sites with the lowest ⁵D₀ energy levels occurs, and only these sites luminesce. Above 10 K backtransfer to higher ⁵D₀ levels takes place.

The energy migration process, which becomes important at temperatures above 20 K for the pseudo-tetragonal phase and above 10 K for the monoclinic phase, is two-phonon assisted. For both compounds the assistance mechanism up to about 50 K is two-site non-resonant, and at higher temperatures it is one-site resonant.