The effects of alumina content and heat treatment on upconversion properties of codoped (ErF3-YbF3) oxyfluoride glass ceramics were investigated. Results showed that alumina content had an effect on phase separation and viscosity of the glass. Due to the high viscosity of low alumina content glass, the phase separated areas were smaller in these specimens. Increasing the heat treatment temperature led to the incorporation of Er3+ ions into CaF2 crystals and also increased the Yb3+ concentration in them. This increase improved the energy transfer and back transfer process between Er3+ and Yb3+ ions and as result upconversion intensity was increased. 1. Introduction At present, there is great interest in luminescent materials for efficient frequency conversion from infrared to visible radiation, mainly because a visible source pumped by a near infrared laser is useful for high-capacity data storage optical devices [1]. This process can be obtained by upconversion mechanisms, where several infrared photons can be absorbed by the material doped with rare earth ions (RE) in order to populate more energetic levels. Therefore, both the fluorescence lifetime and the stimulated emission cross-section of the RE excited level should be maximized, whereas the nonradiative decay mechanisms should be minimized [2]. Oxyfluoride glass ceramics are ambivalent materials. Despite the fact that they are mainly oxide glasses, they can exhibit optical properties of fluoride single crystals when they are doped with rare earth ions. They are often called nanocomposite materials. Their weird character is obtained by a classical melting and quenching preparation in air followed by an adapted thermal treatment during which fluoride phases are crystallized. The size, size distribution, and volume concentration of fluoride crystallites are crucial for photonic applications. For example, to be a promising optical functional material, the size of the crystallites should be smaller than at least half of the wavelength of the light used while the size distribution should be narrow and the crystallites should possess a homogeneous spatial distribution. In this way, according to the scattering theory developed by Rayleigh [3], complete transparency of a light transmitting material can be attained [4]. A refractive index difference between the amorphous and crystalline phases of less than 0.1 is also required. However, according to Beall and Pinckney [5], based on Hopper’s model, crystal sizes of 30？nm and differences in refractive index of 0.3 may be acceptable, provided that the crystal
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