Solid State Communications 150 (2010) 1241–1244 Contents lists available at ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc The local structure around the Nd impurity incorporated into the Bi 4 Ge 3 O 12 crystal matrix: An ab initio study S.A.S. Farias, M.V. Lalic ∗ Universidade Federal de Sergipe, Departamento de Física, P.O. Box 353, 49100-000, São Cristóvão, SE, Brazil article info Article history: Received 4 March 2010 Accepted 2 April 2010 by F. Peeters Available online 23 April 2010 Keywords: A. Bi 4 Ge 3 O 12 crystal C. Rare-earth impurity D. Structural relaxation E. Density-functional theory abstract The local structure around the Nd impurity introduced at the Bi site in the Bi 4 Ge 3 O 12 compound is theoretically investigated by density-functional theory based augmented plane wave method. A set of parameters that completely describes the Nd neighborhood is calculated by treating the Nd 4f electrons either as localized or as delocalized. In both cases the local structure was dominantly determined by the Nd departure from the ideal host position along the trigonal axis. Displacement occurs in the direction towards the center of the Oxygen octahedron around the Nd. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Bismuth Ortho-Germanate Bi 4 Ge 3 O 12 (BGO) is a well known scintillator with a wide variety of applications in high energy physics, nuclear medicine and non-linear optical devices [1–3]. When doped with a transition metal and rare-earth ions, it shows potential for use as a solid state laser host [4,5]. Nd is an especially interesting dopant, because the BGO:Nd system presents good electro-optical properties such as second harmonic generation and optical rotation [6]. It also exhibits medium laser amplification characteristics [7], being recently investigated as a possible laser system which can be pumped by laser diode [8,9]. The optical properties of the BGO:Nd system were experimen- tally studied by Kaminskii et al. [7] and more recently by Loro et al. [10]. In order to understand and manipulate these proper- ties, it is important to know the position of the Nd impurity within the host crystal matrix, as well as the local structure around it, i.e. the positions of its neighboring atoms. As the Nd and the Bi ions have the same valence (3+) and very similar ionic radii (0.104 and 0.096 nm respectively), it is expected that the Nd 3+ ion substi- tutes the Bi 3+ ion. This fact has been confirmed by electron para- magnetic resonance (EPR) investigation, in which the determined EPR parameters were associated to the Nd 3+ impurity located ex- actly at the octahedral Bi 3+ site with trigonal symmetry [11]. Us- ing the perturbation formulas of the EPR parameters for a 4f 3 ion ∗ Corresponding address: Departamento de Física, Universidade Federal de Sergipe, Marechal Rondon s/n, 49100-000, São Cristóvão, SE, Brazil. Tel.: +55 79 2105 6809; fax: +55 79 2105 6807. E-mail address: mlalic@ufs.br (M.V. Lalic). in a trigonally-distorted crystal field, Wu and Dong concluded that the Nd impurity does not occupy exactly the host Bi site, but dis- locates from it along the C 3 symmetry axis [12]. The authors, how- ever, stress that their semi-empirical calculations contain various approximations and need to be verified either experimentally or by using more accurate calculations based on density functional theory (DFT). The DFT description of the structural and, especially, electronic properties of f-electron systems, however, faces serious difficulties due to a strong correlation between f-electrons which causes their localization. This fact is not properly described by LDA or GGA exchange-correlation potentials, which apply much better to delocalized s-, p- or d-states [13]. Most f-electron systems are characterized by the simultaneous presence of itinerant (delocalized) and localized f states and the interaction between them [14]. The situation of delocalized f-electrons refers to the extended f-shell which overlaps with the electronic orbitals of neighboring atoms and participates in chemical bonding. DFT calculations simulate this situation by treating the f-electrons in the same manner as the other valence electrons. On the other hand, the localized f-electrons are confined within the shell which is screened by outer shells (free-atom-like) and do not participate in chemical bonding. This situation is approximated in the DFT calculations by the so-called open-core treatment, in which the f-electrons are removed from the valence band and treated as the core electrons. A more sophisticated approach to this problem is provided by applying the LDA + U[15] or LDA + SIC [16] schemes. The objective of the present study is to theoretically investi- gate the local structure around the Nd impurity incorporated into the BGO crystal matrix. To achieve this objective we employed first-principles DFT calculations on a BGO compound doped with 0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.04.005