Electronic structure of defects in Sr 2 MgSi 2 O 7 :Eu 2+ ,La 3+ persistent luminescence material Jukka Hassinen a , Jorma Hölsä a,b , Taneli Laamanen a,c, ⁎, Mika Lastusaari a,b , Pavel Novák d a University of Turku, Department of Chemistry, FI-20014 Turku, Finland b Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland c Graduate School of Materials Research (GSMR), Turku, Finland d Academy of Sciences of the Czech Republic, Institute of Physics, CZ-16253 Prague 6, Czech Republic abstract article info Available online 20 August 2010 Keywords: Disilicate; Persistent luminescence; Electronic structure; Vacancy; Density functional theory calculation The modifications in the crystal and electronic structure due to the introduction of strontium and oxygen vacancies in the Sr 2 MgSi 2 O 7 persistent luminescence host material were calculated using the density functional theory (DFT). The defect energy level structures of the Sr 2 MgSi 2 O 7 :Eu 2+ (,La 3+ ) materials were studied with the thermoluminescence (TL) spectroscopy, as well. Both shallow and deep electron traps due to isolated oxygen and strontium vacancies and hole traps due to an isolated strontium vacancy were located in the energy gap (E g ) of the host. The shallow electron traps can contribute to persistent luminescence since they are readily bleached by the thermal energy at room temperature. The DFT calculations suggest that the strontium vacancies created by the substitution of Sr 2+ with R 3+ have a significant role in the creation of electron traps highly useful in promoting persistent luminescence as shown experimentally. The present results indicate that the differences between the calculated and experimental trap level structures may be due to the defect aggregation. © 2010 Elsevier B.V. All rights reserved. 1. Introduction New energy storage materials based on thermally stimulated (i.e. persistent) luminescence are constantly searched for. However, the tedious experimental trial and error methods used are hindering the rapid success of these efforts due to limited knowledge about the mechanisms in the solid state oxide materials. Recently, the Eu 2+ doped alkaline earth magnesium disilicates (M 2 MgSi 2 O 7 :Eu 2+ ,R 3+ ; M: Ca, Sr, Ba; R: Nd, Dy, Tm) have been found to show persistent luminescence for up to 24 h at room temperature [1–5]. Despite the rapid commercialization of these excellent materials, the role of intrinsic lattice defects (vacancies, interstitials etc.) in the persistent luminescence mechanism(s) is currently not fully understood. Both cation (the Kröger-Vink notation: V M '' ) and oxygen vacancies (V O •• ) are expected to exist in the M 2 MgSi 2 O 7 :Eu 2+ ,R 3+ persistent lumines- cence materials. Defects such as V M '' and V O •• can be created due to the evaporation of MO during the high-temperature solid state reaction frequently necessary for the good performance of the material. Cation vacancies also exist due to charge compensation when the R 3+ dopant replaces the Sr 2+ host cation. More oxygen vacancies are created in the lattice due to the reducing preparation conditions as well. Both isolated defect species [6–19] and defect aggregates [20–22] have been studied in simple oxide materials (e.g. CaO, MgO and TiO 2 ) with density functional theory (DFT) calculations. The host band structure and the defect as well as Eu 2+ energy levels have been studied in selected persistent luminescence host materials [23–29] using DFT as well. However, the connection between the electronic band structure of the host as well as the energy levels of the vacancies and the rare earth (co-)dopants must still be solved. Possible modifications in the electronic structure due to the inclusion of defects should be studied in detail since even small changes in the electronic structure may have significant effect on the persistent luminescence efficiency. A sophisticated approach combining both theoretical and experimental methods is urgently needed to enable the systematic development of new, more efficient materials. In this work, distrontium magnesium disilicates doped with Eu 2+ and co-doped with La 3+ (Sr 2 MgSi 2 O 7 :Eu 2+ ,La 3+ ) were prepared with a solid state reaction. The trap energy level structure of the materials was studied using the thermoluminescence (TL) spectroscopy. The electronic structure of the non-doped Sr 2 MgSi 2 O 7 material including a strontium or oxygen vacancy and the energy positions of the vacancy states were studied with DFT calculations. The changes in the crystal and electronic structure due to the optimization of these materials were studied, too. The relationship between the trap energy level structure derived from the TL measurements and the calculated electronic structure was discussed. Eventually, a persistent lumines- cence mechanism was constructed based on these results. Journal of Non-Crystalline Solids 356 (2010) 2015–2019 ⁎ Corresponding author. University of Turku, Department of Chemistry, FI-20014 Turku, Finland. Tel.: +358 2 3336731; Fax: +358 2 3336700. E-mail address: taanla@utu.fi (T. Laamanen). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.06.035 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol