Luminescence properties of the Ca-alpha-sialon:Eu solid solution Tomasz Pawlik a , Daniel Michalik a,⇑ , Malgorzata Sopicka-Lizer a , Radoslaw Lisiecki b , Barbara Adamczyk a , Materusz Plawecki c , Lukasz Mieszczak c , Wiktoria Walerczyk b a Silesian University of Technology, Faculty of Materials Engineering and Metallurgy, Krasin ´skiego 8, 40-019 Katowice, Poland b Institute of Low Temperature and Structure Research, ul. Okólna 2, 50-422 Wroclaw, Poland c University of Silesia, Institute of Materials Science, 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland article info Article history: Received 5 November 2015 Received in revised form 16 February 2016 Accepted 1 March 2016 Available online xxxx Keywords: Ca,Eu-a-sialon Solid solution Photoluminescence Eu oxidation state abstract The Ca,Eu-a-sialon powders with the mixed solid solution composition have been manufactured via the solid-state reaction process in flowing nitrogen in a graphite furnace at a relatively low temperature of 1650 °C without an external overpressure. XRD data with Rielveld refinement and XPS measurements were used for characterization of the lattice constants and the surface chemical composition. The mono- phase Ca-Eu-a-sialon was obtained with the nominal composition of Eu 0.048 Ca 0.702 Si 7.75 Al 2.25 O 0.75 N 15.25 . The highest emission intensity in a yellow-orange region at 590 nm and quantum efficiency of 66% was found for this pure Ca,Eu-a-sialon. Estimation of m,n values from the lattice constant and EDS results showed a small deviation from the nominal composition of designed a-sialon. XPS results demonstrated significant changes of the chemical composition in the oxidized surface of phosphor particles. Possible reasons of emission redshift and relationship between the actual solid solution composition and lumines- cence properties are discussed in terms of simultaneous presence of Eu 2+ and Eu 3+ ions in the sialon crys- tal lattice and residual oxynitride glass. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction In the last few years rare-earth-doped (oxo)nitridosilicates with sialons [1–3], oxynitrides (MSi 2 N 2 O 2 ) [4,5], nitrides (M 2 Si 5 N 8 ) [6,7] and others CaAlSiN 3 [8], MgYSi 2 O 5 N, etc. [9] were reported as the best candidates for optical wavelength conversion in while light- emitting diodes (LEDs) because the spectral properties of RE ions (i.e. Eu 2+ and Ce 3+ ) are strongly affected by the surrounding envi- ronment [10]. Thus higher formal charge of nitrogen and higher covalent character of RE-N bonding led to longer excitation and emission wavelength as well as to a smaller Stokes shift responsi- ble for high conversion efficiency. Among these (oxy)nitride phos- phors Eu:Ca-a-sialon with a strong absorption in the UV-range and a broad emission band at over 580 nm took significant interest because of the unique properties of its crystal lattice with the appropriate interstices and a broad range of solid-solution. Solid solutions in the MASiAAlAOAN system have a-Si 3 N 4 derived structure with the general composition of M m/m Si 12(m+n) Al m+n O n N 16n , where M represents the relevant stabilising cation (Li, Ca, Mg, Y or RE with exception of the largest cations) and m is a valence of cation M. The stabilising cation could occupy two available interstitial sites per unit cell of the a-Si 3 N 4 structure. A range of a-sialon solubility depends on the cationic size and a pre- cise control of oxygen to nitrogen ratio. Accordingly, the lattice constants of solid solution will reflect the relevant substitution: the SiAN bond length of 1.74 Å for AlAN (1.87 Å) and AlAO (1.75 Å) as well as the size of M cation because of its insertion into interstices in the structure [11]. Further Shen & Nygren’s studies showed that lattice expansion of the a-sialon phase with increas- ing content of M ions was due to the substitution of AlAN units for SiAN, while the lattice parameters did not depend on the ionic radius of the M ion [12]. To date this seemed a reasonable approach in relating the composition to the lattice parameters in the a-sialon structure. The extensive studies on Eu:Ca-a-sialon phosphors during the last decade showed that the composition (m, n values) of the host lattice significantly affects the luminescence properties of the phosphor [10,13]. It has been generally accepted that increase of Eu concentration shifted dominating emission towards longer wavelength up to x = 0.14 Eu at.% [14–18], however the limit of concentration quenching was the process dependent [17]. Further- more, the red-shift of the emission band was observed for nitrogen-rich Ca-a-sialon:Eu compositions [2,13,16] because of the nitrogen-rich surroundings of Eu 2+ ions and resultant larger ligand field splitting of the 5d levels. The highest dominating http://dx.doi.org/10.1016/j.optmat.2016.03.003 0925-3467/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: daniel.michalik@polsl.pl (D. Michalik). Optical Materials xxx (2016) xxx–xxx Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Please cite this article in press as: T. Pawlik et al., Opt. Mater. (2016), http://dx.doi.org/10.1016/j.optmat.2016.03.003