Optical spectroscopy of Nd 3+ in LiLa 9 (SiO 4 ) 6 O 2 crystals Enrico Cavalli a, * , Gianluca Calestani a , Alessandro Belletti a , Marco Bettinelli b , Adolfo Speghini b a Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica e Chimica Fisica, Università di Parma, viale G. P. Usberti 17/a, 43100 Parma, Italy b Dipartimento Scientifico e Tecnologico, University of Verona and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, 37134 Verona, Italy article info Article history: Available online 17 November 2008 PACS: 78.55.Hx Keywords: Luminescence Neodymium Apatite Judd–Ofelt theory abstract LiLa 9 (SiO 4 ) 6 O 2 (LLS) crystals activated with Nd 3+ have been grown from a lithium molybdate flux. Their absorption and emission spectra and fluorescence decay profiles have been measured at different tem- peratures. The spectra are affected by a significant inhomogeneous broadening as a consequence of the partial disorder of the crystal structure. The room temperature absorption spectra have been analyzed using the Judd–Ofelt approach. The X 2 , X 4 and X 6 intensity parameters have been evaluated and used to calculate the spontaneous emission probabilities, the branching ratios and the radiative lifetime for the 4 F 3/2 emitting level. These values have been compared with the experimental decay time in order to estimate the effect of the non-radiative processes in this material. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Lanthanide doped silicate crystals with the apatite structure have excellent thermal, optical and mechanical properties and have been demonstrated to be promising laser hosts for the Nd 3+ , Ho 3+ and Yb 3+ ions [1,2]. LiLa 9 (SiO 4 ) 6 O 2 (LLS) is an interesting apa- tite host lattice affected by an intrinsic structural disorder that can be conveniently exploited to realize broadband and/or tunable emitting materials. In this paper we present the results of a preli- minary study concerning Nd 3+ -doped LLS, carried out in order to develop a new laser crystal operating in the NIR region. 2. Experimental 2.1. Crystal growth and structural data Well shaped, transparent and free of cracks LLS single crystals with size up to 0.5  1  2 mm 3 were grown from a Li 2 MoO 4 flux in the 1360–940 °C temperature range [3]. The charge to solvent molar ratio was 0.4% and the cooling rate was 5 °C/h. They were doped with Nd 3+ by replacing 1% or 5% La 2 O 3 with Nd 2 O 3 in the growth mixture. LLS belongs to the hexagonal system, with space group P6 3 /m and cell parameters a = b = 9.691 Å and c = 7.162 Å, Z =1 [4]. In this crystal two cationic sites are present. The first one is occupied exclusively by La 3+ coordinating seven oxygen ions to form a distorted pentagonal bipyramid. In the second site, the coordination polyhedron is a ninefold distorted threefold capped trigonal prism. It is occupied in part (75%) by La 3+ and in part by Li + (25%) ions with a random distribution. This implies some struc- tural disorder and then perturbations of the crystal field around the Nd 3+ ions replacing La 3+ in both sites. As a consequence, the spectral features are expected to be significantly broadened even at low temperature. 2.2. Spectroscopic measurements The absorption and luminescence spectra and the emission de- cay profiles of the grown crystals have been measured at 10 and 298 K using the experimental setup already described in a previous paper [5]. 3. Results and discussion The 10 and 298 K polarized absorption spectra of LLS:Nd (5%) are shown in Fig. 1. The observed transitions have been assigned on the basis of the energy level scheme of the Nd 3+ ion having the 4f 3 electronic configuration [6]. As expected, the Stark compo- nents of the low temperature manifolds are relatively broad, their full width at half maximum being of the order of 70–100 cm 1 . Moreover their number confirms the multisite occupancy of the Nd 3+ ion in this material. The transition to the non-degenerate 2 P 1/2 state in particular, is composed of at least two lines at 429.5 and 431.5 nm. The full width at half maximum (FWHM) of the strongest one at 431.5 nm in the r spectrum is about 60 cm 1 , an intermediate value between that typical of ordered crystals 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.009 * Corresponding author. Tel.: +39 0521 905436; fax: +39 0521 905556. E-mail address: enrico.cavalli@unipr.it (E. Cavalli). Optical Materials 31 (2009) 1340–1342 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat