Depending on the composition of the glass and the annealing procedure supplied, thermal annealing of silicate glasses in the system SiO 2 /Al 2 O 3 /AlF 3 /Na 2 O/ Gd 2 O 3 /SmF 3 led to the precipitation of cubic or hexagonal NaGdF 4 nanocrystals. The glasses were already phase separated after casting and formed a droplet phase supposedly enriched in fluorides and rare earths. The droplets had sizes mainly in the range from 80 to 120 nm. During annealing at temperatures ≥600 °C, multicore particles of cubic or hexagonal NaGdF 4 were precipitated inside the amorphous droplet phase. Completely transparent hexagonal NaGdF 4 containing nano-glass-ceramics could be derived from this system. Fluorescence spectra and fluorescence decay curves of Sm 3+ show crystal phase dependent effects. Sm 3+ doped hexagonal NaGdF 4 exhibits notably different fluorescence emission spectra and longer fluorescence lifetimes than cubic NaGdF 4 .

A glass with the composition 2BaO·TiO2·2.75SiO2 was annealed at 810 °C for 20 h. This led to surface crystallization. Immediately at the surface of the sample, a layer of Ba2TiSi2O8-type fresnoite crystals (layer I), with a thickness of approximately 7 μm, oriented with the crystallographic [101]-direction perpendicular to the surface, was formed. The pole of the (001)-plane rotates randomly around the [101]-direction. It is assumed that nucleation kinetics is decisive to the direction of growth. In the next layer (layer II), the crystals are oriented with the crystallographic [001]-direction (c-axis) perpendicular to the surface of the sample. This layer occurs at a distance of 7−60 μm from the surface. Here, crystals that are not oriented in that way hinder each other during crystal growth. At a distance > 60 μm, the orientation of the fresnoite crystals is random and is the result of volume crystallization. The main characterization method is electron backscatter diffraction/orientation imaging microscopy.

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A glass of the composition 2SrO?TiO 2 ?2.75SiO 2 was melted. Cooled samples were polished and thermally annealed at 970 uC for 10 min to 20 h to achieve surface crystallisation of Sr 2 TiSi 2 O 8 fresnoite. At the surface, the crystals were immediately oriented with their crystallographic c-axes perpendicular to the surface. Crystal growth occurred in the form of similarly oriented structures. Very homogeneous structures were also observed but they fray into areas of higher orientation diversity at some distance from the surface. The crystallised areas were highly permeated by nanoscale inclusions of residual glass. Kinetic selection occurs and leads to a preferred orientation with the crystallographic c-axis tilted by about 43 ¡ 5u from the surface normal after about 300 mm. Crystal growth is not governed by the fastest growing crystallographic direction or the formation of a diffusion barrier, but rather by the ability to circumvent the residual glass.

Growth of nanocrystals precipitated in glasses with specific compositions can be effectively limited by diffusion barriers forming around crystallites. For the first time, we do experimentally prove this concept of self-limited growth on the nanoscale for a SiO(2)/Al(2)O(3)/Na(2)O/K(2)O/BaF(2) glass in which BaF(2) nanocrystals are formed. As shown by advanced analytical transmission electron microscopy techniques, the growth of these BaF(2) crystals, having great potential for photonic applications, is inherently limited by the formation of a ca. 1 nm wide SiO(2) shell.

Transparent bulk glass-ceramics containing ZnF2, K2SiF6, and KZnF3 nanocrystals are successfully obtained from xKF-xZnF2-(100 - 2x)SiO2 oxyfluoride glasses for the first time to the best of our knowledge. The glass transition temperatures of heat-treated samples increase with time and approach values that resemble the temperatures chosen for thermal treatment. During nucleation and crystal growth, the residual glass around the crystals is depleted in fluoride which as glass component usually leads to a decrease in viscosity. The crystallization behavior notably depends on the glass composition and changes within a small range from x = 20 to 22.5 mol %. The occurrence of liquid/liquid phase separation in dependence of the composition is responsible for the physicochemical changes. Two different microstructures of droplet and interpenetrating phase separation and their compositional evolution are observed by replica transmission electron microscopy technique in the multicomponent glassy system. This study suggests that the size and crystal phase of precipitated crystallites can be controlled by the initial phase separation.

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