Deep-ultraviolet transparent monolithic sol–gel derived silica–REPO4 (RE = Y, La–Lu except Pm) glass-ceramics: characterization of the crystal structure and ultraviolet absorption edge, and application to narrow-band UVB phosphors

Abstract

Monolithic silica glasses containing nanocrystals of rare-earth (RE) orthophosphates (REPO₄, where RE = Y, La–Lu except Pm) were prepared by a cosolvent-free sol–gel method. Despite the large refractive index mismatch between the REPO₄ nanocrystals and the host silica glass, these glasses are highly transparent in the deep-ultraviolet (deep-UV, DUV) spectral region (≲300 nm) because Rayleigh scattering is suppressed by the small crystal size (∼5–10 nm) and narrow size distribution. The encapsulation of the nanocrystals in the silica matrix increases the stability range of the monoclinic monazite form. The UV absorption edge originates from the electronic transitions in PO₄ units for RE = Y, La, Gd, and Lu and from the 4f–5d or charge transfer transition for other RE³⁺ ions with partially filled 4f shells. Glasses containing Gd³⁺ ions exhibit a bright narrow UV photoluminescence (PL) band at ∼313 nm, and the PL efficiency is significantly increased by the addition of Pr³⁺ ions acting as photosensitizers for Gd³⁺ ions. The absorbance, and internal and external quantum efficiencies of the PL band of a glass containing Gd_0.9Pr_0.1PO₄ nanocrystals under excitation at 230 nm were ∼0.91, ∼0.70, and ∼0.63, respectively. Because of the high quantum efficiency, this type of glass is promising as a narrow-band UV phosphor used for UVB (280–320 nm) phototherapy.