Electronic Structure of Rare Earth Ions in the Sr2MgSi2O7 Persistent Luminescence Host Material

Laamanen, Taneli^1,2; Hölsä, Jorma^1,3; Lastusaari, Mika^1,3; Novák, Pavel⁴

¹ University of Turku, Department of Chemistry, FI-20014 Turku, Finland, E-mail: taanlautu [dot] fi
² Graduate School of Materials Research (GSMR), Turku, Finland
³ Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland
⁴ Academy of Sciences of the Czech Republic, Institute of Physics, Prague

The alkaline earth magnesium disilicates (M₂MgSi₂O₇, M: Ca, Sr and Ba; e.g. [1]) doped with Eu²⁺ and co-doped with rare earth (R) ions as Dy³⁺ and Nd³⁺ are the most efficient persistent luminescence materials presently existing. They emit for up to 24 h at room temperature after ceasing the irradiation. The role of the R²⁺/R³⁺ ions acting as traps for the charge carriers in the persistent luminescence materials has been suggested but is lacking the definite proof. As a first step to ascertain this hypothesis, one needs to know the exact position of their energy levels in the host’s band structure. These positions are not known since they are notoriously difficult to determine experimentally though this information would be very important because the thermally controlled persistent luminescence mechanism of Sr₂MgSi₂O₇:Eu²⁺,R³⁺ is sensitive even to slight changes in the trap level structure.

In this work, the electronic structure of the Sr₂MgSi₂O₇:R materials was studied with density functional theory (DFT) calculations using the WIEN2k package [2]. The GGA and GGA+U methods were used to describe the strongly correlated R 4f electrons in the best manner available.

A good agreement was found between the experimental and calculated band gap energies in Sr₂MgSi₂O₇. The energy positions of both the 4fⁿ ground state and the lowest 4f^n-15d¹ excited state of all R²⁺ and R³⁺ ions in the band structure of the host were calculated for the first time using DFT. The 4fⁿ ground state positions of R³⁺ are either close to or within the valence band (VB). The 4fⁿ ground state positions of R²⁺ are reproduced well, though they are systematically at lower energy relative to the conduction band (CB) than suggested by the empirical model [3]. The lowest excited 4f^n-15d¹ states of R³⁺ are systematically (ca. 0.5 eV) lower in energy than those of the corresponding R²⁺ states. The present DFT results provide an important step towards the independent verification of the empirical model [3] predicting the energy level locations of all the R²⁺/R³⁺ ions. Eventually, the introduction of the R³⁺ ions created density of states close to CB possibly due to significant interaction between the R³⁺ ion and the CB states of the host. These states can act as shallow electron traps and may partly help to explain the drastic enhancement of persistent luminescence with the R³⁺ co-doping.

[1] T. Aitasalo, J. Hölsä, M. Kirm, T. Laamanen, M. Lastusaari, J. Niittykoski, J. Raud, R. Valtonen, Radiat. Meas. 42 (2007) 644-647.
[2] P. Blaha, K. Schwarz, G. K. H. Madsen, D. Kvasnicka, J. Luitz, In: K. Schwarz (Ed.), WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties, Vienna University of Technology, Austria, 2001.
[3] P. Dorenbos, J. Phys.: Condens. Matter 15 (2003) 8417-8434.