JOURNAL OF RARE EARTHS, Vol. 29, No. 12, Dec. 2011, P. 1130 Foundation item: Project supported by Turku University Foundation, Jenny and Antti Wihuri Foundation (Finland) and the Academy of Finland (contract #117057/2006, #134459/2010) and research mobility agreements (112816/2006/JH, 116142/2006/JH, 123976/2007/TL) between the Academy of Finland and the Academy of Sciences of the Czech Republic, and the Czech research project (AVOZ10100521 (PN)) Corresponding author: Taneli Laamanen (E-mail: taanla@utu.fi; Tel.: +358-2-3336731) DOI: 10.1016/S1002-0721(10)60611-4 Defect aggregates in the Sr 2 MgSi 2 O 7 persistent luminescence material Jorma Hölsä 1,2 , Taneli Laamanen 1,3 , Mika Lastusaari 1,2 , Pavel Novák 4 (1. Department of Chemistry, University of Turku, FI-20014 Turku, Finland; 2. Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland; 3. Gradu- ate School of Materials Research (GSMR), Turku, Finland; 4. Institute of Physics, Academy of Sciences of the Czech Republic, CZ-16253 Prague 6, Czech Republic) Received 30 August 2011; revised 13 September 2011 Abstract: The crystal and electronic structure of the Eu 2+ doped and defect containing Sr 2 MgSi 2 O 7 persistent luminescence material were studied using the density functional theory (DFT). The defects may act as energy storage or even luminescence quenching centres in these materials, however their role is very difficult to confirm experimentally. The probability of vacancy formation was studied using the total en- ergy of the defect containing host. Significant structural modifications in the environment of the isolated defects, especially the strontium va- cancy, as well as defect aggregates were found. The experimental band gap energy of Sr 2 MgSi 2 O 7 was well reproduced by the calculations. The defect induced electron traps close to the host’s conduction band were found to act as energy storage sites contributing to its efficient per- sistent luminescence. The interactions between the defects were found to modify both the Eu 2+ 4f 7 ground state energy as well as the trap structure. The effect of charge compensation induced by the rare earth co-doping on the defect structure and energy storage properties of the persistent luminescence materials was discussed. Keywords: strontium magnesium disilicate; vacancy; defect aggregate; persistent luminescence; density functional theory calculations; rare earths Persistent luminescence materials store energy from solar radiation or artificial lighting and release it as light gradually at room temperature. The Eu 2+ doped alkaline earth magne- sium disilicates (M 2 MgSi 2 O 7 :Eu 2+ ,R 3+ ; M: Ca, Sr, Ba; R: Nd, Dy, Tm) are among the most efficient ones [1–5] . The devel- opment of even more efficient materials is hindered by the lack of knowledge on the effect of charge compensation de- fects and structural distortions resulting from the charge/size mismatch between the trivalent rare earth (R 3+ ) and M 2+ ions. The defects may play either a desired or unwanted role as en- ergy storage or luminescence quenching centers, respectively. The total number of intrinsic and extrinsic (e.g. dopant ions) lattice defects may be large in a polycrystalline mate- rial. The intrinsic defects include cation vacancies, ' ' M V (in the Kröger-Vink notation where '' denotes a defect with dou- ble negative net charge [6] ), and oxygen vacancies, x x O V (double positive), as well as interstitial ions (Fig. 1). Cation vacancies exist e.g. due to the charge compensation when R 3+ replaces M 2+ ( x M R ), whereas oxygen vacancies may be created due to the reducing preparation conditions. Defect aggregates may also exist since the aggregation probability should increase with the increasing defect concentration. The energies of isolated defects (mainly oxygen vacancy) in the electronic band structure of simple oxide materials (e.g. MgO [7–9] , ZrO 2 [10–12] , and SrTiO 3 [13–15] ) have been studied with ab initio density functional theory (DFT) calculations. In addition, the energies of some R 3+ (usually Er 3+ ) contain- ing defect aggregates in selected hosts (AlN [16] , SiC [17] , GaN [18,19] , and GaAs [20] ) have been explored. The band gap energy of a small number of persistent luminescence hosts (SrAl 2 O 4 [21–24] , Ca 2 MgSi 2 O 7 [25] , Sr 2 MgSi 2 O 7 [26] , CdSiO 3 [27] ) has recently been studied using DFT. However, the defect energies in these hosts’ band structure remain unclear. The defect aggregation effects in such hosts are also unknown. The goal of the present work was to study the role of the vacancies and defect aggregates as possible electron or hole trapping sites contributing to the energy storage efficiency. The modifications in the crystal structure of the Sr 2 MgSi 2 O 7 material due to these defects were studied using the DFT calculations. Eventually, the electronic structure of the non- or Eu 2+ doped and isolated vacancy ( ' ' Sr V and x x O V ) as well as defect aggregate ( x Sr Eu + ' ' Sr V , x Sr Eu + x x O V and ' ' Sr V + x x O V ) con- taining Sr 2 MgSi 2 O 7 was explored. Fig. 1 Pure (left) and lattice defect containing SrO ( ' ' Sr V : double negative, x x O V : double positive, x Sr R : single positive, x Sr Eu : zero net charge, right)