Defect simulation and crystal field studies ofLn 3þ :LiCaAlF 6 and LiSrAlF 6 Jomar B. Amaral a , Mario E.G. Valerio a, * , Marcos A. Couto dos Santos a , Robert A. Jackson b a Departamento de F  ısica, Universidade Federal de Sergipe, S~ ao Crist ov~ ao SE 49100-000, Brazil b Lennard–Jones Laboratories, School of Chemistry and Physics, Keele University, Keele, Staffordshire ST5 5BG, UK Abstract LiCaAlF 6 (LiCAF) and LiSrAlF 6 (LiSAF) crystal hosts doped with trivalent positive lanthanide ions (Ln 3þ ) were studiedviadefectsimulationtechniquesandcrystalfieldparametercalculations.Dependingontheratioofionicradius ofthedopanttothehostdivalentcation,theluminescentionscanoccupytheSr 2þ (orCa 2þ )siteortheAl 3þ .Whenthe dopantentersatthedivalentcationsite,chargecompensationisneededthatchangesdrasticallythestartingsymmetry ofthissite.However,whentheAl 3þ siteispreferred,nochargecompensationschemesareinvolved,butneverthelessthe lattice is distorted since the sizes of the Ln 3þ ions are bigger than the Al 3þ ion. The actual positions of the first neighboursoftheLn 3þ ionwereinputinthecrystalfieldparametersequations,throughwhichtheconclusionsonsite symmetryoftheopticallyactiveionhavebeendrawnandtheseresultsallowsthepredictionofpossibleopticalactivity based on the possibility of the 4f–4f transitions. Ó 2003 Elsevier B.V. All rights reserved. Keywords: LiCaAlF 6 ; LiSrAlF 6 ;Ln 3þ ions; Defect modelling; Crystal field parameter 1. Introduction Research on laser crystals having tuneable monochromatic activity has received much atten- tion due to the applicability of lasers in human surgery, in laser technology, as well as due to fundamental knowledge [1–3]. The very narrow emission lines of the lanthanide trivalent ions (Ln 3þ )-doped crystals make such media of special interest [4–7]. In order to predict the reliability of producing thiskindofmaterial,structuralmodellinghasbeen developed in our group [8–12]. Recently, Ce-doped LiCaAlF 6 (LiCAF) and LiSrAlF 6 (LiSAF)appearedasgoodcandidatesfor tuneablesolid-statelasersintheUV-region[2].Our aiminthisworkistostudyLn 3þ (Ce,Nd,Eu,Yb) doping these structures by a combination of the defectsimulationtechniqueandcrystalfieldtheory. 2. Defect simulation technique The defect calculations were performed based on empirical potential fitting [13] and lattice * Corresponding author. Tel.: +55-21-79-212-6810; fax: +55- 21-79-212-6807. E-mail address: mvalerio@fisica.ufs.br (M.E.G. Valerio). 0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.12.027 Nuclear Instruments and Methods in Physics Research B 218 (2004) 232–235 www.elsevier.com/locate/nimb