Spectroscopic characteristics of langasite (La 3 Ga 5 SiO 14 ) and langatate (La 3 Ga 5.5 Ta 0.5 O 14 ) crystals doped with Eu 3+ S. Georgescu * , O. Toma, A.M. Chinie, L. Gheorghe, A. Achim, A.S. Stefan National Institute for Lasers, Plasma, and Radiation Physics, P.O. Box MG-36, 76900 Magurele, Romania Received 13 February 2007; received in revised form 11 May 2007; accepted 19 May 2007 Available online 10 July 2007 Abstract The spectroscopic properties of two partially disordered crystals langasite (LGS) and langatate (LGT) doped with Eu 3+ were inves- tigated. The fluorescence lifetimes of 5 D 0 and 5 D 1 excited levels are measured. The spectroscopic figures of merit R 0 , R 2 (the areas of the electric-dipole transitions 5 D 0 ! 7 F 0 and, respectively, 5 D 0 ! 7 F 2 divided by the area of the magnetic-dipole one – 5 D 0 ! 7 F 1 ) and the maximum splitting of the 7 F 1 level (DE) are calculated from the luminescence spectra. The spectral overlapping between the luminescence from 5 D 0 and 5 D 1 levels was eliminated using pulsed excitation at 532 nm and suitable delays. We obtained R 0 (LGS) > R 0 (LGT), R 2 (LGS) < R 2 (LGT) and DE (LGS) > DE (LGS). Possible reasons (covalency, J-mixing effects) for these inequalities are discussed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.43.j; 78.55 Keywords: Partially disordered crystals; Langasite; Langatate; Fluorescence; Eu 3+ ; Fluorescence probe 1. Introduction The crystals from the langasite family were initially intended as hosts for laser active media [1], but in present their main application is based on their very good piezo- electric characteristics [2,3]. Nowadays, langasite (La 3 Ga 5- SiO 14 ) tends to replace quartz in high temperature applications [4]. Also, langasite is used for electro-optic Q-switch [5], for gas sensor [6], and so on. Recently, self- tuning of Nd 3+ in langasite crystals was obtained [7]. Two other members of the langasite family, langatate (La 3 Ga 5.5 Ta 0.5 O 14 ) and langanite (La 3 Ga 5.5 Nb 0.5 O 14 ) proved to have superior characteristics in some applica- tions [8]. Langasite, langatate and langanite are partially disordered crystals, i.e., one of the crystallographic posi- tions is shared by two different ions. This results in inho- mogeneously broadened absorption and fluorescence lines. This was first considered an advantage for laser emis- sion: wider absorption lines result in an efficient pumping while wider emission lines means tunability. Besides the inhomogeneous line broadening, the partial disorder could lead, in some instances, to the splitting of the optical lines. Such an effect was observed in other partially disordered crystals [9], namely calcium niobium gallium garnet and calcium lithium niobium gallium garnet doped with Nd 3+ . A multisite structure was observed in the low tem- perature spectra, and was assigned to various configura- tions in the first cationic coordination sphere. Langasite (LGS), langatate (LGT) and langanite (LGN) crystallize in the P321 space group, symmetry class 32 and are isostructural with the calciumgallogermanate (Ca 3 Ga 2 - Ge 4 O 14 ) [10]. The general formula is A 3 BC 3 D 2 O 14 where A represents the dodecahedral positions (distorted Thompson cubes), B – octahedral positions and C, D – tetrahedral positions. La 3+ occupies the position A. The local symme- try at this site is C 2 [11]. In contrast with LGS where Ga 3+ and Si 4+ share with equal probability the tetrahedral posi- tions D, in LGT the octahedral positions B are occupied by two different ions, Ga 3+ and Ta 5+ (also, with equal 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.05.035 * Corresponding author. E-mail address: joe@pluto.infim.ro (S. Georgescu). www.elsevier.com/locate/optmat Available online at www.sciencedirect.com Optical Materials 30 (2008) 1007–1012