Role of the Eu3+ ions in the formation of transparent oxyfluoride glass ceramics

Abstract

Increasing interest in finding host materials for active optical devices, such as lasers and optical amplifiers that utilize intra-4 f transitions of rare earth ions, has developed much work in different crystals and glasses.1–3 Oxyfluoride matrices have received special attention from the report by Wang and Ohwaki4 of efficient Er31 upconversion in transparent SiO2–Al2O3–PbF2–CdF2–YbF3 glass ceramics, increased by a factor 100 with respect to the glasses. After this pioneer work, different papers5–14 have analyzed oxyfluoride glass ceramics doped with rare earth ions, mainly Er31 or Pr31, and the highest upconversion efficiencies have been obtained with SiO2 based matrices. It has been found that after thermal treatment in oxyfluoride glasses, close to the crystallization temperature, it is possible to obtain a glass ceramic in which fluoride nanocrystals are embedded in a primarily oxide glass matrix. If the rare earth ions are preferentially incorporated into these crystalline fluoride environments during the ceramming process, an improvement of their optical properties could be achieved. The glass ceramic remains highly transparent in spite of the high ~20%–30%! volume fraction of the nanocrystal phase.7,8 Thus, it represents a unique class of material that combines the particular optical properties of rare earth ions in a fluoride host with the elaboration and manipulation advantages of oxide glasses. The increase in the upconversion efficiency has been explained considering the lower phonon energy in the fluoride nanocrystals, which reduce the nonradiative decay rates, and the shortening of distances between the rare earth ions, which favors the energy transfer processes. In reference to the structure of the nanocrystals, most works propose it as PbxCd12xF2 , as determined from x-ray diffraction patterns. Moreover, it has been assumed that the rare earth ions reside as dopants in these nanocrystals. However, it is difficult to conclude whether the rare earth ions are incorporated into the crystalline phase in glass ceramics or they stay in the interface between the nanocrystals and the glassy phase. On the other hand, the Eu31 ion has been widely used as a probe to investigate the local structure around rare earth ions in condensed matter. This is because the electronic transition between the 7F0 and 5D0 nondegenerate levels is matchless to apply the fluorescence line narrowing ~FLN! technique to analyze changes from site to site in the energy level diagram, lifetime, linewidth, energy transfer processes between optical ions, and migration of energy between ions in sites close in energy but spectrally different. The optical properties of the rare earth ions in glass ceramic and therefore their interest for optical devices depend on the final environment of these ions. In this work the optical properties of the Eu31 ions in a SiO2 based oxyfluoride glass ceramic and in the precursor glass are analyzed in order to determine where the ions reside after the ceramming process.