Judd–Ofelt analysis and radiative properties of LiLa (1x) Eu x (PO 3 ) 4 M. Ferhi a,⇑ , C. Bouzidi a , K. Horchani-Naifer a , H. Elhouichet a,b , M. Ferid a a Laboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications, Centre National des Recherches en Sciences des Matériaux, Technopole de Borj Cedria, BP 73, 8027 Soliman, Tunisia b Département de Physique, Faculté des Sciences de Tunis, Campus ElManar 2092, Tunisia article info Article history: Received 24 March 2014 Received in revised form 1 August 2014 Accepted 5 August 2014 Available online xxxx Keywords: Polyphosphates Eu 3+ Luminescence Judd–Ofelt theory Optical band gap abstract Judd–Ofelt (J–O) theory has been applied to a series of LiLa 1x Eu x (PO 3 ) 4 (x = 5%; 10%; 15%; 20% and 30%) polycrystalline powders based on their emission spectra. J–O intensity parameters X k (k = 2, 4) and var- ious radiative properties such as the radiative transition probability (A), radiative life time (s R ), branching ratio (b R ) and stimulated emission cross-section (r e ) have been determined. The observed trend for J–O parameters (X 2 < X 4 ) reveals the ionic character of La(Eu)–O bond as well as the symmetry environment around the Eu 3+ ion in LiLa(PO 3 ) 4 . The experimental decay curves of the 5 D 0 level in LiLa 1x Eu x (PO 3 ) 4 have a single exponential profile. The registered long life time, low multiphonon relaxation rates (W NR ), high quantum efficiency (g) and the best optical gain parameter (r e x s exp ) have been discussed as function of Eu 3+ concentration, chemical composition and crystal structure of the host matrix and particle-size dis- tribution. A comparative study reveals that LiLa 1x Eu x (PO 3 ) 4 is promising for use in optical devices as well as for developing visible red lasers to emit at 613 nm. The optical band gap energies of LiLa 1x Eu x (PO 3 ) 4 have been calculated from the diffuse reflection spectral measurements. The results were discussed as a function of Eu 3+ concentration and compared to reported theoretical results. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Over the past few decades, rare earth polyphosphates with gen- eral formula MLn(PO 3 ) 4 (M = alkali metal, Ln = rare-earth metal) were the subject of many important researches due to their poten- tial technological and commercial applications [1–5]. These mate- rials present favorable compromise between the requirements of low multiphonon relaxation, relatively high thermal stability, high chemical durability, low production cost and ease of fabrication and apply [6–8]. Their optical properties depend strongly on the crystal structure and chemical composition of the host matrix. Cur- rently, alkali lanthanide phosphates were extensively investigated for their interesting optical applications as active media in ultrafast solid-state lasers [9–17]. LiNd(PO 3 ) 4 is useful as a solid-state laser material with less degree of fluorescence quenching owing the large Nd–Nd distance [9,10]. KLn(PO 3 ) 4 (Ln = Nd, Gd) crystals were used as second harmonic generation due to their asymmetric prop- erties [18,19]. Furthermore, Li + has been introduced into different phosphor host matrix such as oxide GdO 3 :Eu 3+ [20] and mona- zite-type YPO 4 :Eu 3+ [21] acting as a co-activator. The Li + ion has been also used as a charge compensator in Sr 2 SiO 4 :Eu 3+ [22] and KSr 4 (BO 3 ) 3 :Eu 3+ [23]. They show that Li + substitution remarkably affects the crystallinity, morphology and enhances the photolumi- nescence intensity as well as the efficiency of phosphors [20–23]. The aim of this paper is to profit on the Li + presence in the host compound and prove quantitatively its effect on the spectroscopic properties of LiLa 1x Eu x (PO 3 ) 4 using Judd–Ofelt theory. The optical band gap (E opt ) of the title compound has been investigated and compared to theoretical value reported in literature. These proper- ties are discussed as function of Eu 3+ concentration, particle-size distributions, crystal structure of LiLa(PO 3 ) 4 and compared to other materials reported previously. 2. Experimental procedures The synthesis and characterizations of LiLa 1x Eu x (PO 3 ) 4 (x = 5%; 10%; 15%; 20% and 30%) polycrystalline powders as well as the techniques used for excitation and emission spectral measure- ments were detailed in our previous work [4]. Lifetime measure- ments were performed with a Perkin Elmer Model LS-55B spectrofluorimeter by monitoring the emission wavelength at 613 nm under excitation at 393 nm and using xenon lamp. Diffuse reflectance spectra of the sintered polycrystalline LiLa 1x Eu x (PO 3 ) 4 pellet samples were measured in wavelength range of 250–2500 nm, on a double-beam Lambda Series UV/Vis/NIR http://dx.doi.org/10.1016/j.optmat.2014.08.003 0925-3467/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +216 79 325 280; fax: +216 79 325 314. E-mail address: ferhi.mounir@gmail.com (M. Ferhi). 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: M. Ferhi et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.08.003