Cr⁴⁺ doped oxide materials attract a substantial attention of researchers as the active media of tunable and femtosecond solid-state lasers operating in the spectral range of 1,1-1,6 mm. Such lasers can be used in fiber optics communications, ophthalmology, laser ranging, etc. However, the wide application of these lasers is suppressed by absence of the efficient hosts for this ion. Cr⁴⁺:Y₃Al₅O₁₂ and Cr⁴⁺:Mg₂SiO₄ single crystals, used in practice, have rather low fluorescence quantum yield and short excited state lifetime of Cr⁴⁺. Another problem is low available concentrations of Cr⁴⁺ and presence of the parasitic oxidation states of chromium in above crystals.

The search for new, more efficient laser hosts for Cr⁴⁺ ion is the actual problem. The attempts to find such a host among single crystals and glasses, continued during the last 25 years, had no great success: there were found some crystals, very promising from the point of view of their spectroscopic characteristics, and having good admittance for Cr⁴⁺. However, these crystals cannot be grown from melt as the samples of laser quality, because of sub-solidus polymorph transitions, and/or because of strongly incongruent melting, and/or because of very high selective evaporation of the melt components.

That is why, at the last decade researchers pay much attention to another kind of optical materials, ultra-transparent nano-glass-ceramics as to the alternative hosts for Cr⁴⁺. These materials reproduce the valuable spectroscopic properties of the corresponding single crystals in substantial extent. On the other hand, nano-glass-ceramics can be rather easily obtained as high-quality bulk samples of any shape (including single mode optical fiber) with high concentrations and uniform distribution of a dopant over the sample.

Erlier, we studied Cr⁴⁺:LiGaSiO₄ single crystals for the first time. Cr⁴⁺ possess the attractive spectroscopic properties in this crystal. Its structure is favorable for Cr⁴⁺ formation, and unfavorable for the formation of the parasitic Cr²⁺ and Cr³⁺ ions. However, the incongruent melting of the compound prevents the possibility to grow the high-quality Cr:LiGaSiO₄ single crystals from melt. Meanwhile, an ultra-transparent Cr:LiGaSiO₄ nano-glass-ceramics looks very promising, and that was the subject of our studies in Refs. [1, 2, 3]. In this talk we present the studies of different crystalline phases formation in the parent Cr-Li-Ga-Si-O glass depending on the temperature and duration of crystallization, as well as on the particular composition of the parent glass.

The samples of a parent glass (vitreous precursors) were sintered by melting the charge in air or in pure argon, and keeping the melt above the liquidus temperature (~ 1350 ^oC), with stirring during 0,5 to 50 hours, until the melt become completely transparent. For different charge compositions the temperature and duration of melting, necessary for the synthesis, varied in considerable ranges. In particular, heavily chromium doped compositions required extremely long melting exposure (several tens of hours), whereas chromium-free and silica-enriched charges became transparent just after several minutes of melting. The charges containing TiO₂, ZrO₂ or WO₃ required considerably increased temperatures for a complete dissolution of all the solid particles, flowing in the melt, whereas the addition of Li₂O, BaO or B₂O₃, on the contrary, reduced the melting temperature.

After the melt achieved complete transparency, and after the complete dissolution of all the solid inclusions happened, they were quenched by removing from a hot zone. High quality transparent glass samples were obtained. The colour of the samples was yellowish-green, except chromium-free samples, which were colourless. Our attempts to quench the opaque Cr-Li-Ga-Si-O melts (even without visible solid inclusions) resulted in obtaining of opaque, partially crystallized samples.

The controlled crystallization of the glass samples was performed by their heat treatment using different thermal-temporal regimes. The crystalline phases formed in the samples were monitored by XRD analysis.

Several different crystalline phases were observed in the studied glass-ceramic samples. Not taking into the consideration well-known phases observed in the samples with very specific initial compositions (like spodumene-related phases in the heavily SiO₂-enriched samples), we will discuss four different phases, observed in the most part of the studied samples. They are:

1.      Well-known stable eucryptite-like α-LiGaSiO₄ polymorph modification. This is the most attractive laser host for Cr⁴⁺, hence its formation is the most desirable;

2.      The metastable γ-LiGaSiO₄ modification. The particular structure of this crystalline polymorph has not been studied, because of difficulties in obtaining the macroscopic single crystals of γ-LiGaSiO₄, and to study it by single-crystalline X-ray diffraction methods, although some similarities between the structures of α- and γ- LiGaSiO₄ do, obviously, exist, because the most part of the α- LiGaSiO₄ and γ-LiGaSiO₄ XRD-peaks are very close to each other. γ-LiGaSiO₄ is not as attractive laser host for Cr⁴⁺, as α- LiGaSiO₄. However, Cr⁴⁺ doped nano-glass-ceramic containing γ-LiGaSiO₄ crystallites also has rather strong fluorescence in the range 1,2-1,5 µm;

The rest two crystalline phases are very specific ones, they were not observed earlier anywhere, to our knowledge, and are absent in XRD-catalogues. Both these phases have no Cr⁴⁺ fluorescence, but can cause the scattering in the samples. Therefore, their presence in laser Cr:LiGaSiO₄ nano-glass-ceramics is strongly non-desirable. Both these phases are, obviously partially ordered, each of them have just two very strong XRD-peaks.

3.      First of them is the phase, which we have titled as X-phase. It has XRD peaks at 2Q ~ 22^o, and less intensive peak at 2Q ~ 45^o. The latter peak corresponds to the inter-plane spacing of 2,0Å that fits very well the typical height of [SiO₄] tetrahedron, whereas the former peak corresponds to the inter-plane space of 4,0Å that fits very well to the double typical height of [SiO₄] tetrahedron, or to the typical lengths of [Si₂O₇] doubled tetrahedron.

4.      The phase, which we have titled as Y-phase. It has two XRD peaks of comparable intensities at 2Q ~ 27^o, and at 2Q ~ 56^o. The former peak corresponds to the inter-plane spacing of 3,3Å that fits very well the interatomic Li-Li distance in the cubic Li₂O crystalline phase.

The content of these crystalline phases depend on the temperatures and duration of crystallization, as well as on the parent glass compositions. These dependencies, as well as some our versions about the structures and compositions of the X- and Y-phases are presented and discussed in the talk.

References

[1] K.A.Subbotin, V.A.Smirnov, E.V.Zharikov, L.D.Iskhakova, V.G.Senin V.V.Voronov, I.A.Shcherbakov. - Optical Materials, v.32 iss. 9, (2010) p. 896–902.

[2] K.A.Subbotin, S.M.Arakelian, V.V.Voronov, V.G.Senin, M.N.Gerke, V.A.Smirnov, D.A.Nikolaev, E.V.Zharikov, I.A.Shcherbakov.– J. Crystal Growth, v.328 iss. 1, (2011) p. 95–101.

[3] K.A.Subbotin, A.A.Veber, D.A.Nikolaev, V.G.Senin, V.A.Smirnov, Yu.N.Osipova, E.V.Zharikov, I.A.Shcherbakov.– Optics and Spectroscopy v. 114, iss. 5, pp. 71– 75

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