Yellow persistent luminescence of Sr 2 SiO 4 :Eu 2 þ ,Dy 3 þ Danuta Dutczak a,b , Alexander Milbrat a , Arturas Katelnikovas a,c , Andries Meijerink b , Cees Ronda b,d , Thomas J ¨ ustel a,n a Department of Chemical Engineering, M¨ unster University of Applied Sciences, Stegerwaldstr. 39, D-48565 Steinfurt, Germany b CMI, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands c Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania d Philips Research Europe – Eindhoven, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands article info Article history: Received 19 December 2011 Received in revised form 2 March 2012 Accepted 26 March 2012 Available online 3 April 2012 Keywords: Afterglow Persistent luminescence Strontium ortho-silicate Photoluminescence spectroscopy abstract This paper reports the photoluminescence and afterglow of Sr 2 SiO 4 doped with Eu 2 þ and Dy 3 þ . Factors governing the formation of the monoclinic or orthorhombic phase of this ortho-silicate are described and the impact of the crystallographic modification on the luminescence and afterglow under UV and VUV excitation are discussed and insight in factors limiting the efficiency of this yellow afterglow material is given. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Persistent luminescence phosphors find application in many areas, such as security signs, toys, watch dials, interior decoration and optical data storage [1–3]. Especially since the discovery of SrAl 2 O 4 :Eu 2 þ ,Dy 3 þ in the middle of the 1990s [4] extensive research on different afterglow materials has been conducted aimed at tuning their emission color, prolong their persistent luminescence and provide fundamental understanding of the afterglow mechanism. Different synthesis methods, various co- dopants, and fluxes were investigated [5–7]. Those studies have brought new interesting afterglow phosphors emitting mostly in the blue-green spectral range. Until now, the most efficient afterglow phosphors are blue-green emitting Eu 2 þ doped alumi- nates and silicates, viz. Sr 2 MgSi 2 O 7 :Eu,Dy [8], Sr 4 Al 14 O 25 :Eu,Dy [9], SrAl 2 O 4 :Eu,Dy [10]. There are rather few yellow and red emitting afterglow phosphors, e.g. Sr 2 SiO 4 :Eu,Dy [11],Y 2 O 2 S:Eu,- Ti,Mn [12], Ca 2 Si 5 N 8 :Eu,Tm [13], and Mg 2 SiO 4 :Dy,Mn [14] but most of them show short and weak persistent luminescence. Thus, the development of efficient yellow and red emitting after- glow phosphors is still an ongoing challenge [15]. One of the yellow emitting afterglow phosphors is strontium ortho-silicate (Sr 2 SiO 4 ) doped with Eu 2 þ and Dy 3 þ . The Eu 2 þ activated ortho-silicates were first reported by Barry [16] and Blasse et al. in 1968 [17] and were intensively studied as phos- phors for color-tunable white light emitting diodes (LEDs). The luminescence of divalent europium doped ortho-silicates is attrib- uted to the [Xe]4f 6 5d 1 –[Xe]4f 7 transition and is strongly influenced by the local coordination in the host lattice [18]. Strontium ortho- silicate exists in two crystallographic modifications, viz. a 0 -Sr 2 SiO 4 (orthorhombic) and b-Sr 2 SiO 4 (monoclinic) [19]. The transition between the b-phase and the high temperature a 0 -phase occurs at 358 K involves the rearrangement of SiO 4 tetrahedra without disconnection of bonds [20–22]. The a 0 -Sr 2 SiO 4 phase can be stabilized at room temperature by partial replacement of stron- tium ions by barium [23,24]. This paper reports the luminescence and persistent lumines- cence of yellow emitting Sr 2 SiO 4 :Eu 2 þ ,Dy 3 þ and is focused on the correlation between the phase transition and persistent lumines- cence of this phosphor. The Eu 2 þ luminescence in strontium ortho-silicate will be discussed on the basis of crystal structure, covalent interaction and crystal-field strength. 2. Experimental 2.1. Synthesis Ortho-silicates with different concentrations of co-dopants were prepared by conventional high-temperature solid state preparation techniques. Stoichiometric amounts of the starting materials were ground with acetone in an agate mortar to obtain Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.03.055 n Corresponding author. Tel.: þ49 2551 962205; fax: þ49 2551 962502. E-mail addresses: tj@fh-muenster.de, dutczak@fh-muenster.de (T. J ¨ ustel). Journal of Luminescence 132 (2012) 2398–2403