Structural and spectroscopic studies of Eu 3+ doped Lu 2 O 3 –Gd 2 O 3 solid solutions Radenka M. Krsmanovic ´ Whiffen a,⇑,1 ,Z ˇ eljka Antic ´ a , Adolfo Speghini b , Mikhail G. Brik c , Barbora Bártová d , Marco Bettinelli b , Miroslav D. Dramic ´ anin a a University of Belgrade – Vinc ˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia b Dipartimento di Biotecnologie, Università di Verona and INSTM, UdR Verona, Strada Le Grazie 15, I-37134 Verona, Italy c Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia d LSME&CIME, École Polytechnique Fédérale de Lausanne, Station 12, CH-1015 Lausanne, Switzerland article info Article history: Received 2 December 2013 Received in revised form 23 January 2014 Accepted 27 January 2014 Available online xxxx Keywords: Nanophosphors Sesquioxides Rare earths Optical properties Gd 2 O 3 Lu 2 O 3 abstract A series of europium doped (Lu x Gd 1x ) 2 O 3 (x = 1, 0.75, 0.5, 0.25 and 0) nanocrystalline powders were pre- pared using a polymer complex solution method based on a polyethylene glycol (PEG) as fuel. The sam- ples were systematically characterized by powder X-ray diffraction, scanning and transmission electron microscopies and luminescence spectroscopy. The powders consisted of well-crystalline, cubic phase nanoparticles of 20–50 nm in size, which unit cell parameter increased with Gd content complying with Vegard’s law. Upon blue light excitation all samples exhibited strong red luminescence typical of trivalent europium ion. The maximum splitting of the 7 F 1 manifold changed linearly with the composition change and decreased with lowering of the crystal field strength. Relatively long lifetime values were obtained for 5 D 0 (1.4 ms) and 5 D 1 (120 ls) levels. For all samples we estimated theoretical densities, refractive index coefficients, optical filling factors and Z eff , in order to estimate the Judd–Ofelt intensity parameters and branching ratios. The calculated lifetime of 5 D 0 level was in line with experimentally obtained lumi- nescence lifetime values. Relative integrated emissions were measured on all samples and Gd 2 O 3 sample proved to have a maximum amount of the characteristic Eu 3+ luminescence. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction In the last two decades an extensive research has been per- formed for the development of inorganic nanophosphors with im- proved physical and emissive properties. These materials gained a special place in the material science research due to numerous applications ranging from solid-state lighting and novel flat dis- plays to high-resolution X-ray detectors and sensors in biomedi- cine. In particular cubic sesquioxides M 2 O 3 (M = Y, Gd, Lu and Sc) are well-recognized hosts for their good chemical stability, ade- quate thermal conductivity, high light output and simple structure ðspace group Ia  3Þ that offers low-symmetry sites for rare earth or transition metal activators substitution. For example, Eu 3+ doped Y 2 O 3 is a well-known red phosphor [1,2] while Eu 3+ :Y 2 O 3 –Gd 2 O 3 solid solution is commercially used as scintillating precursor for high-density transparent ceramics utilized in X-ray computed tomography medical imaging and mammography [3]. In our previous works [4,5] we investigated structural and luminescencent properties of europium doped (Y x Gd 1x ) 2 O 3 and (Y x Lu 1x ) 2 O 3 nanocrystalline phosphors (x = 0, 0.25, 0.5, 0.75 and 1). Both Y 2 O 3 –Gd 2 O 3 and Y 2 O 3 –Lu 2 O 3 solid solutions show strong red emission and long decay time, and these good properties re- mained preserved in the polycrystalline ceramic samples produced under high pressure-high temperature treatments, as demon- strated in our recent papers [6,7]. In this work we expanded our investigation to (Lu x Gd 1x ) 2 O 3 nanopowders (x = 0, 0.25, 0.5, 0.75 and 1) doped with trivalent europium ions (denoted hereafter as LuGdO). Both gadolinium and lutetium oxides, having high densi- ties (q(Gd 2 O 3 ) = 7.407 g cm 3 and q(Lu 2 O 3 ) = 9.42 g cm 3 ) and high Z-numbers (64 for Gd and 71 for Lu), are able to provide high stopping power for ionizing radiation [8]. On the other hand, the same bixbyite cubic structure of Lu 2 O 3 and Gd 2 O 3 and the similar ionic radii of Lu 3+ and Gd 3+ ions enable their excellent mixing into solid solutions, while their large band gaps (5.4–5.5 eV) [9] can http://dx.doi.org/10.1016/j.optmat.2014.01.039 0925-3467/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Address: Mike Petrovic ´a Alasa 12-14, Vinc ˇa, 11001 Belgrade, Serbia. Tel.: +381 113408195; fax: +381 113408607. E-mail address: radenka@vinca.rs (R.M. Krsmanovic ´ Whiffen). 1 Current address: Chimie Paris Tech, Chimie de la Matière Condensée de Paris, UMR-CNRS7574, 11 Rue P. & M. Curie, 75231 Paris Cedex 05, France. Tel.: +33 153737945. Optical Materials xxx (2014) xxx–xxx Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Please cite this article in press as: R.M. Krsmanovic ´ Whiffen et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.01.039