Studies of the influence of alkaline earth fluorides (MgF2, CaF2, SrF2, BaF2) and Nb2O5 on the structural and optical properties of fluorophosphoniobate glasses and glass-ceramics

Author(s)

Leandro Olivetti Estevam da Silva

• Document type: Master's Dissertation
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
• Defense date: 2024-07-12

Examining board members

Danilo Manzani; Fábio José Caixeta; Euclydes Marega Junior

• Advisor: Danilo Manzani

Abstract

Fluorophosphoniobate glasses have high technological and scientific potential as a host matrix for optically active species (such as rare earth ions) due to their wide transparency between the ultraviolet and near infrared, high solubility of rare earth ions, chemical stability and low phonon energy. The implications for the structural, thermal and optical properties of glasses caused by altering the composition of the glass matrix through the use of different concentrations of niobium have been widely studied in the literature, but there is a lack of information on the modifying role that different alkaline earth metals can play on these matrices from the point of view of their properties, as well as exploring the precipitation of crystalline phases with these variables. This research has therefore set out to explore in depth the implications of these compositional and experimental variations in the synthesis of fluorophosphoniobate glasses and glass-ceramics with different alkaline earth metals. New glass matrices, with a molar compositional rule of (80-y)NaPO3-(y)Nb2O5-20XF2 (X = Mg2+, Ca2+, Sr2+ and Ba2+), were successfully synthesized, as were their respective glass-ceramics after heat treatment above Tg. The matrices not doped with rare earth ions allowed their optical, structural and thermal properties to be explored, while the matrices doped with Eu3+ and codoped with Yb3+/Tm3+ allowed the luminescent properties of these different matrices to be explored. Analysis by DSC, refractive index, UV-Vis absorption and optical bandgap calculation revealed that the covalent character of the samples increases linearly with increasing Nb2O5 concentration, which increases their glass transition temperatures, refractive index and shifts the UV absorption band to longer wavelengths due to the decrease in optical bandgap energy. In addition, varying the alkaline earth metal between Mg2+, Ca2+, Sr2+ and Ba2+ revealed how the metals with a smaller ionic radius act by increasing the covalent character of the samples, possibly due to the lower availability of fluoride to act electrostatically in the glass matrix due to the greater affinities of fluoride with the metals with a smaller ionic radius, as shown by 19F nuclear magnetic resonance. Different alkaline earth metals have shown a significant impact on the thermal properties of glasses and consequently on the crystalline phases formed after controlled heat treatment. Glass-ceramics containing Ca2+ and Sr2+ showed the formation of crystalline phases with lower phonon energy, such as the Ca5(PO4)3F and Sr5(PO4)3F phases (increasing the upconversion emission intensity of the Yb3+/Tm3+ pair by up to 16 times), while none of the samples containing Ba2+ crystallized after heat treatment. The glass sample containing CaF2 and 20 mol% Nb2O5, after heat treatment, showed an increase in upconversion emission intensity of 16 times compared to glass (when codoped with Yb3+/Tm3+) and a drop of up to 70% in multiphonon relaxation rates (when doped with Eu3+), indicating that the rare earth ions occupy the same environment after heat treatment. The study provided an in-depth understanding of the modifying role of different alkaline earth metals and the impact of Nb2O5 in a fluorophosphate matrix, not only in terms of glass properties, but also in terms of the precipitation of crystalline phases from controlled heat treatments. (AU)

• FAPESP's process: 22/01762-0 - Influence of earth-alkali fluorides (MgF2, CaF2, SrF2, BaF2) and Nb2O5 on the structural and optical properties of fluorophosphoniobate glasses
• Grantee: Leandro Olivetti Estevam da Silva
• Support Opportunities: Scholarships in Brazil - Master