Gadolinium in lutetium ¯uorideÐan electron paramagnetic resonance study K.J. Guedes a , K. Krambrock a, * , J.Y. Gesland b a UFMG, Departamento de Fõ Âsica, ICEx, CP 702, 30.123-970 Belo Horizonte, Brazil b Equipe de Physique de l'Etat Condense Â, CNRS URA no 807 Avenue Messiaen BP, 535 72017 Le Mans Cedex, France Received 15 May 2000; accepted 19 May 2000 Abstract Gadolinium (Gd 31 ) in Czochralski-grown lutetium ¯uoride (LuF 3 ) has been investigated by electron paramagnetic resonance (EPR). Detailed analysis of the EPR angular dependencies reveals that Gd 31 entered substitutionally for Lu 31 in monoclinic C s site symmetry. The Lu site seems to be more distorted by the Gd ion as compared to the Y site in YF 3 . The zero-®eld splitting of the ground state b 2 0 is 21.8372 GHz which is more than twice compared to that of Gd 31 in host crystals of LaF 3 , in which gadolinium has the same co-ordination number. The results of the ®ne structure analysis are compared to a similar study in the iso-structural host YF 3 . The obtained results seem to be in contrast to the superposition model. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: D. Electron paramagnetic resonance 1. Introduction From a theoretical point of view, it is interesting to investi- gate the interaction of Gd 31 with its crystalline environment, because the zero-®eld splittings of Gd 31 with a pure S ground state have not been understood so far. It is clear that the point- charge model cannot give a satisfactory solution to the problem. However, the superposition model[1], which was used successfully to explain the interaction of Gd 31 in the LaF 3 iso-structural hosts[2], seems to be in contrast to our observations of the ®ne structure parameters of Gd 31 in the iso-structural hosts, YF 3 [3] and LuF 3 , presented in this paper. From a technological point of view, it is interesting to study ¯uoride materials since they can be used as active media for tunable solid state lasers or as fast scintillator materials. The lanthanide tri-¯uorides LnF 3 with Ln  Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb and Lu are particularly inter- esting in such applications because their room temperature space group is orthorhombic P nma D 16 2h  which favours appli- cations in non-linear optics. As examples, LuF 3 doped by Ce 31 ions has been shown to have very fast scintillating properties [4] and GdF 3 doped by Nd 31 has interesting laser properties in the near infrared[5]. Like YF 3 and GdF 3 , single crystal growth of lutetium ¯uoride by the Czochralski method is dif®cult because of a high temperature phase transition from rhombohedral to orthorhombic structure[6]. However, we succeeded to grow large single crystals of LuF 3 directly in the orthorhombic phase by using a mixture of LiF (0.45) and LuF 3 (0.55) in the melt. The crystals are colourless and non-hygroscopic, which is important for their use as active media in laser or scintillator technology. For the electron paramagnetic resonance (EPR) measure- ments samples with dimensions of 3 £ 2 £ 5 mm 3 were cut with faces normal to the crystallographic axes using the clea- vage plane (010). The orientation of the samples was con®rmed by Laue X-ray analysis. EPR spectra were obtained at low and room temperature and X-band frequencies with the usual 100 kHz ®eld modulation. EPR angular dependencies were recorded for rotations of the crystal in all three orthorhombic planes ab, ac and bc or alternatively (001), (010) and (100). 2. Experimental results and discussion Fig. 1 shows the EPR spectra of Gd 31 in LuF 3 single Journal of Physics and Chemistry of Solids 62 (2001) 485±489 0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(00)00190-6 www.elsevier.nl/locate/jpcs * Corresponding author. Tel.: 155-31-499-5625; fax: 155-31- 499-5600. E-mail address: klaus@®scia.ufmg.br (K. Krambrock).