Author's personal copy Design of oxy-fluoride glass-ceramics containing NaLaF 4 nano-crystals A. de Pablos-Martín a , G.C. Mather a , F. Muñoz a , S. Bhattacharyya b,c , Th. Höche c , J.R. Jinschek d , T. Heil e , A. Durán a , M.J. Pascual a, ⁎ a Instituto de Cerámica y Vidrio (CSIC), C/Kelsen 5, Campus de Cantoblanco, 28049 Madrid, Spain b Department of Condensed Matter Physics and Materials Science, Tata, Institute of Fundamental Research, Colaba, Mumbai-400005, India c Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstraße 15, D-04318 Leipzig, Germany d Europe Nanoport, FEI Company, Achtseweg Noord 5, 5651 GG Eindhoven, The Netherlands e Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany abstract article info Article history: Received 23 November 2009 Received in revised form 30 March 2010 Keywords: Oxy-fluoride glass-ceramics; NaLaF 4 nano-crystals; photonic applications Fluoride glass-ceramics doped with lanthanide ions are very promising materials for optical devices. In this study, transparent fluoride glass-ceramics have been obtained from oxy-fluoride glasses in the Na 2 O–Al 2 O 3 – SiO 2 –LaF 3 system by heat treatment slightly above T g , upon which NaLaF 4 nano-crystals with a size of up to 20 nm precipitate. The crystallisation mechanism is suggested to involve the formation of a SiO 2 -enriched shell around the nano-crystals which hampers their further growth, limits Ostwald ripening, and promotes uniformity in size. This desired enrichment of lanthanide ions inside nano-crystals with an adjustable and uniform size distribution is essential for future photonic applications. To validate the crystallisation mechanism of this glass system, the structure and composition of the crystalline phase, glass matrix and the developing interface have been studied by viscosity, dilatometry, X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. © 2010 Elsevier B.V. All rights reserved. 1. Introduction New, rare-earth (RE) doped, oxy-fluoride glass-ceramics have recently been developed, with exciting optical properties for potential applications in telecommunications, including solid-state lasers and optical-amplifier fibres. The advantage of this class of materials is the combination of the mechanical and chemical resistance of alumino- silicate glasses with the low phonon energy of fluoride crystals. The preferred location of RE dopant ions is in the crystalline phase [1–5], instead of in the glass matrix. This enhances the characteristic laser- emission intensity that is highly desirable for optical applications [6]. One of the most commonly adopted fluoride phases for lanthanide doping is LaF 3 . However, the use of double-fluoride crystals [7–9] (e.g. NaYF 4 [10], NaLaF 4 :Ln 3+ [11], NaGdF 4 :Tm 3+ , NaGdF 4 :Nd 3+ , NaLaF 4 : Tm 3+ and NaLaF 4 :Nd 3+ [12]) is also gaining more attention. The requirement of transparency in these materials is achieved by limiting the crystal size to the nanometric scale with the effect that light scattering is significantly suppressed [13]. A thorough knowl- edge of the associated crystallisation mechanisms is therefore necessary to strictly control the nucleation and growth processes of the nano-crystals in a glassy matrix. This work is focussed on the design and synthesis of oxy-fluoride glass-ceramics containing NaLaF 4 nano-crystals achieved through a rigorous control of crystallisation. The glass-ceramics have been characterized by XRD and TEM to elucidate the associated crystal- lisation mechanisms. In non-isochemical systems, the precipitated crystalline phase has a different composition compared to the vitreous matrix. Hence, the chemical composition of the glassy matrix changes concomitantly with the evolving nucleation and crystallisation of the nano-crystals. Consequently, important parameters, such as viscosity, growth rate and the composition of the glass–crystal interface also change during this process. If the viscosity of the glass–crystal interface decreases due to an enrichment of glass formers in the nano-crystals, an enhancement in the crystal-growth velocity takes place. In contrast, if modifier components are enriched in the crystals, the viscosity of the glass– crystal interface increases and, consequently, a diffusion barrier forms. Thus, the crystal growth velocity decreases, and the crystal size does not proceed beyond the nano scale [14]. In the latter case, the variation in size and number of the RE-doped nano-crystals can be controlled. Evidence of this nanometre-scaled diffusion barrier around the nano-crystal, and thereby the preferred crystallisation mechanism, will be shown in this work. 2. Experimental Glass with a nominal composition of 70 SiO 2 . 7 SiO 2 . 8 Na 2 O. 8K 2 O. 7 LaF 3 (mol%), hereafter denoted 70Si–7La, was prepared by melting reagent grade SiO 2 sand (Saint Gobain, 99.6%), Al 2 O 3 Journal of Non-Crystalline Solids 356 (2010) 3071–3079 ⁎ Corresponding author. Tel.: +34 917355840; fax: +34 917355843. E-mail address: mpascual@icv.csic.es (M.J. Pascual). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.04.057 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol