Impact of Lanthanide (Nd3+, Gd3+, and Yb3+) Ionic Field Strength on the Structure and Thermal Expansion of Phosphate Glasses

Abstract

Phosphate glasses containing Nd³⁺, Gd³⁺, and Yb³⁺ as lanthanide ions are attractive for applications in laser materials, phototherapy lamps, and solar spectral converters. The composition-structure-property relation in this type of glass system is thus of interest from fundamental and applied perspectives. In this work, the impact of the differing ionic radius of Nd³⁺, Gd³⁺, and Yb³⁺ and consequent field strength on the physical properties of phosphate glasses is investigated, focusing ultimately on thermal expansion effects. The glasses were made by melting with a fixed concentration of the lanthanide ions having 50P₂O₅-46BaO-4Ln₂O₃ nominal compositions (mol %) with Ln = Nd, Gd, and Yb. The investigation encompassed measurements by X-ray diffraction (XRD), optical spectroscopy, density, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and dilatometry. XRD supported the amorphous nature of the glasses, whereas absorption and photoluminescence spectra showed the optical features of the Nd³⁺, Gd³⁺, and Yb³⁺ ions in the glasses. Oxygen speciation by XPS indicated an increase in nonbridging oxygens for the larger radii Nd³⁺ and Gd³⁺ ions relative to the host, contrasting with Yb³⁺. Phosphorus XPS analysis further supported the hypothesis that the P 2p binding energies of the glasses increased with the cation field strength of the lanthanides. The Raman spectra were interpreted based on glass depolymerization effects and the impact of Ln³⁺ ions with high field strength. Particularly, the band position of the symmetric out-of-chain nonbridging oxygen stretch, ν_s(PO₂^-), shifted to higher frequencies correlating with the Ln³⁺ field strength. Dilatometry ultimately revealed a steady decrease in the coefficient of thermal expansion for the glasses, which correlated linearly with Ln³⁺ field strengths and thus indicated to sustain increased glass rigidities. The various analyses performed thus illuminated the structural foundation of the thermomechanical behavior of the glasses connected with changes in the Ln³⁺ field strengths.