Structure and intense UV up-conversion emissions in RE3+-doped sol–gel glass-ceramics containing KYF4 nanocrystals

Transparent nano-glass-ceramics containing KYF₄ nanocrystals were successfully obtained by the sol–gel method, doped with Eu³⁺ and co-doped with Yb³⁺ and Tm³⁺ ions. Precipitation of cubic KYF₄ nanocrystals was confirmed by X-ray diffraction and high-resolution transmission electron microscope images. Excitation and emission spectra let us to discern between ions into KYF₄ nanocrystals and those remaining in a glassy environment, supplemented with time-resolved photoluminescence decays, that also clearly reveal differences between local environments. Unusual high-energy up-conversion emissions in the UV range were obtained in Yb³⁺–Tm³⁺ co-doped samples, and involved mechanisms were discussed. The intensity of these high-energy emissions was analyzed as a function of Yb³⁺ concentration, heat treatment temperature of precursor sol–gel glasses and pump power, determining the optimum values for potential optical applications as highly efficient UV up-conversion materials in UV solid-state lasers.