The effect of rare earth (RE3+) ionic radii on transparent lanthanide-tellurite glass-ceramics: correlation between ‘hole-formalism’ and crystallization

Abstract

Rare earth (RE) doped transparent tellurite glass-ceramics (GCs) are widely explored for their application as advanced photonic materials. Specifically, the growth of RE based “anti-glass” crystalline phases in transparent GCs enhances their effective functionality. Enormous studies on the properties of GCs revealed that optimization of glass composition and heat-treatment schedule are the foremost factors that affect the transparency of the GCs. Nevertheless, the direct effect of RE³⁺ ions on the crystallization mechanism of glass has hardly been reported. Therefore, a base glass of the composition La₂O₃–Gd₂O₃–TiO₂–TeO₂ (LGTT) is doped with RE³⁺ ions (Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, and Dy³⁺) with varied ionic radii. Rietveld refinement from XRD of GCs confirms that (La/Gd)₂Te₆O₁₅ phases are precipitated, where dopants Ce, Pr and Nd occupy La sites, while Sm, Eu, Tb, and Dy occupy Gd sites. The configurational heat-capacity (ΔC_p) from DSC is found to be lower for larger ionic radii REs (Ce, Pr, Nd) than smaller ionic radii REs (Sm, Eu, Tb, Dy) thereby exhibiting higher chemical ordering followed by faster crystal growth rate in the former. However, the transparency retention profile of different RE-doped GCs follows the trend Eu:Tb > Sm:Dy > Nd > Pr > Ce, confirming the ionic radii effect. In addition to the ionic radii effect, in this work, we propose a ‘Hole-Formalism’ concept to explain the observed trend. It is further corroborated with an identical neutron diffraction pattern and ΔC_p values for these ion pairs Sm³⁺ : Dy³⁺ (4f⁵ : 4f⁹) and Eu³⁺ : Tb³⁺ (4f⁶ : 4f⁸) results in a similar crystallization mechanism, transmission profile and fine-scale microstructures of these RE-based tellurite GCs.