An alternative description for the interaction between the Eu 3+ ion and its nearest neighbours Y.A.R. Oliveira ⇑ , H. Lima, A.S. Souza, M.A. Couto dos Santos Department of Physics, Federal University of Sergipe, 49100-000 São Cristóvão, SE, Brazil article info Article history: Received 29 August 2013 Received in revised form 1 November 2013 Accepted 4 November 2013 Available online 23 November 2013 Keywords: Europium compounds The simple overlap model Crystal field parameters Local symmetry Charge factor abstract The LiYF 4 :Eu 3+ and the Eu (btfa) 3 (4,4-bipy)(EtOH) compounds are being revisited by the method of equiv- alent nearest neighbours (MENNs) and the simple overlap model, this time to suggest a comparison between: the europium local symmetry in complexes containing b-diketones and the S 4 symmetry in the LiYF 4 :Eu 3+ ; and the ionic bonding in lanthanide containing compounds as pure electrostatic attrac- tion. The 7 F 1 level splitting was satisfactorily predicted in both cases by very similar sets of charge factors. This similarity indicates that the lanthanide ion treats the chemical species (N, O and F) in its first neigh- bourhood merely as negative charges. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The lanthanide ion-nearest neighbours (Ln-NN) interaction has been a subject of experimental and theoretical discussions, mainly due to the need of development of new materials to be used in optical devices. From the crystal field point of view, the number of lines of each 5 D 0 ! 7 F J transition in the spectra of systems doped with trivalent europium ions (Eu 3+ ) is almost an unambiguous way of analysing its local symmetry group [1,2]. The electrostatic neu- trality, which is a requirement to ensure the stability of a solid, and the local symmetry have been used by the method of equivalent nearest neighbours (MENNs) to predict the Eu–NN charge of inter- action and the 7 F 1 energy level splitting (DE) [3,4]. The MENN is based on the simple overlap model (SOM, [5]) and the level split- ting is predicted by the Auzel–Malta expression, which describes the maximum splitting of any multiplet of any trivalent lanthanide ions as a function of the crystal field strength parameter, N V [6]. Even though good predictions have been achieved with the MENN, some specific point concerning the magnitude of the charge factors and the local symmetry still remain obscures. For instance, the spectra of Eu-complexes involving b-diketones present the same profile of the 7 F 1 energy sublevels [7–12], and similar to that observed in the LiYF 4 :Eu 3+ crystal [13]. As the number of lines of the 7 F 1 level is the starting point to determine the local symmetry of the Eu 3+ ion, one can suggest that the luminescent ion in these compounds have approximately the same local symmetry. However, the NN in the LiYF 4 :Eu 3+ crystal ([4]) and the Eu (btfa) 3 (4,4-bipy)(EtOH) complex (EuBTFA) [14] are not of the same chemical species. In the former, one has only F  ions and in the latter O 2 and N 3 ions. In this work these systems are being revis- ited in order to present an alternative description for the interac- tion between the Eu 3+ ion and its nearest neighbours. 2. Theory In this study there are no changes in any model related to the crystal field theory. For details about the SOM and the Auzel–Malta model, it is suggested to the interested reader the following Refs. [5,6,15]. The MENN follows three constraints: (i) equivalent NN should be identified through the local symmetry; (ii) using the Auzel–Malta expression for the 7 F 1 level splitting (in which only crystal field parameters (CFP) with k ¼ 2 are operative), the DE should be pre- dicted by a set of charge factors, g j (j running over the NN) ([4,6]); and (iii) the P j g j ¼ 3, the valence of the central ion, in order to guar- antee the local electrical neutrality. In constraint (i), when a symme- try operation displaces one NN to the position of another NN, these neighbours are equivalent and should have the same g j . In this way, there is a reduction of degrees of freedom. The g j are actually the unknowns (not parameters) of the MENN. Further, the reader should realise that the constraints (ii) and (iii) give rise to two equa- tions, which consider the symmetry and electrostatic neutrality of the local site as the most important aspect for the present predic- tions. Therefore, the maximum value of c, the equivalence number of the MENN, must be 2. This is the reason why only sites with high 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.11.005 ⇑ Corresponding author. Tel.: +55 7999009633. E-mail address: yuri.fisica.ufs@gmail.com (Y.A.R. Oliveira). Optical Materials 36 (2014) 655–657 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat