Rare earth (RE) doped oxyfluoride glass ceramics possess interesting optical properties with applications in telecommunications and optoelectronics, such as solid state lasers, optical amplifiers, etc. These materials combine the transparency and mechanical and chemical resistance of aluminosilicate glasses with the low phonon energy and facile incorporation of RE ions in the fluoride crystals. The incorporation of RE ions in the crystalline phases enhances the laser emission intensity, a major property of these materials. Transparency is achieved when crystal size is in the nanometric scale, usually below 40 nm, which avoids light scattering. A strict control of the nucleation and crystal growth processes is therefore necessary which requires a deep knowledge of the crystallisation mechanisms. The great activity and publications in this field in the last decades merit a review providing a comparative study of the different nanoglass ceramic systems, their structural and optical characterisation and their main properties and applications. This is the objective of this review paper which includes 232 references. A general discussion on glass nucleation and crystallisation theories and more relevant crystallisation parameters and characterisation techniques are put forward in the first section of the review, focused on nanocrystallisation processes in oxyfluoride systems. In the second section, the principal RE doped glass ceramics are presented. After a general introduction about the luminescence processes, including up-and down-conversion, the behaviour of RE elements in glasses and crystals are discussed. Glass ceramic compositions have been divided as follows: glass ceramics with a glass composition following Wang and Ohwaki's oxyfluoride glass ceramic, 1 ; and glass ceramics with different matrix compositions, arranged by crystalline phases. Relevant properties, mainly optical and laser, are described in each system along with the most relevant applications of these materials.

Nanocrystallization of LaF 3 in a glass of composition 55SiO 2 -20Al 2 O 3 -15Na 2 O-10LaF 3 (mol%) has been achieved by heat treatment above the glass transition temperature. A maximum crystal size of 14 nm has been attained, with the crystalline fraction and crystal size dependent on the time and temperature of thermal treatment. The effect of lanthanum fluoride crystallization is noticeable from the microstructural and compositional changes in the glass matrix, which have been studied using several techniques, including viscosity, dilatometry, X-ray diffraction, and quantitative Rietveld refinement, transmission electron microscopy, and differential scanning calorimetry. The crystallization mechanism is shown to occur via regions of La-and Si-phase separation in the glass, from which the fluoride crystals develop during heat treatment. The interface between the glass matrix and the crystals in the demixed ranges is enriched in network formers, mainly SiO 2 , creating a viscous barrier, which inhibits further crystal growth and limits the crystal size to the nanometric range.

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The influence of the addition of 1 mol% Tm2O3 on the nanocrystallization of LaF3 in a glass of composition 55SiO2–20Al2O3–15Na2O–10LaF3 (mol%) has been studied. Tm2O3 affects the phase separation in the glass and delays the onset of crystallization with respect to the undoped glass. Additionally, the maximum LaF3 crystal size is slightly greater than that in the undoped glass–ceramics. The microstructural and compositional changes in the glass matrix have been studied using several techniques, including viscosity, dilatometry, X‐ray and neutron diffraction (XRD, ND), quantitative Rietveld refinement, transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and Raman spectroscopy. Photoluminescence measurements indicate that the Tm3+ ions are distributed between the glassy matrix and LaF3 crystals. Eu2O3 has been used as structure probe and part of the Eu3+ ions are reduced to Eu2+ when incorporated in the LaF3 nano‐crystals. Up‐conversion spectra under IR‐excitation show a higher intensity of the blue emission in the Tm‐doped glass–ceramic compared with that in the glass.