Effect of ligand field symmetry on upconversion luminescence in heat-treated LaBGeO5:Yb3+, Er3+ glass

In this paper, we report upconversion (UC) luminescence enhancement in LaBGeO5:Yb3+, Er3+ glass-ceramics (GCs), surface crystallized glass-ceramics (SCGCs) and ceramics compared with the as-melt glass fabricated by the conventional melt-quenching technique. Based on structural investigations, we find that the nucleation and crystallization of trigonal stillwellite LaBGeO5:Yb3+, Er3+ nanocrystals occur first at the glass surface before the following volume crystallization. The local site symmetry around rare earth (RE) ions which was evaluated using the Eu3+ ions as a probe together with Judd-Ofelt theory calculations exhibits a clear increase with the devitrification of the glass. Consequently, complete crystallization of the glass leads to largest enhancement in the UC emissions of the LaBGeO5:Yb3+, Er3+ ceramics. We ascribe the enhancement of UC luminescence in the LaBGeO5:Yb3+, Er3+ GCs, SCGCs, and ceramics to the structural ordering and the improvement of site symmetry surrounding RE ions that minimizes the rate of nonradiative relaxation process.