Hyperfine Interact (2013) 219:33–40 DOI 10.1007/s10751-012-0697-1 Possible cage motion of interstitial Fe in α-Al 2 O 3 H. P. Gunnlaugsson · K. Johnston · H. Masenda · R. Mantovan · T. E. Mølholt · K. Bharuth-Ram · H. P. Gislason · G. Langouche · M. B. Madsen · D. Naidoo · S. Ólafsson · G. Weyer · the ISOLDE Collaboration Published online: 7 November 2012 © Springer Science+Business Media Dordrecht 2012 Abstract In addition to spectral components due to Fe 2+ and Fe 3+ , a single line is observed in emission Mössbauer spectra following low fluence (<10 15 cm −2 ) implan- tation of 57 Fe*, 57 Mn and 57 Co in α-Al 2 O 3 . For the 57 Co and 57 Mn implantations, the intensity of the single line is found to depend on the emission angle relative to the crystal symmetry axis. This angular dependence can be explained by a non-isotropic f -factor and/or motion of the Fe ion between sites in an interstitial cage. It is argued H. P. Gunnlaugsson (B ) · G. Weyer Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark e-mail: hpg@phys.au.dk K. Johnston · the ISOLDE Collaboration Physics Department, ISOLDE/CERN, 1211 Geneva 23, Switzerland H. Masenda · D. Naidoo School of Physics, University of the Witwatersrand, WITS 2050, South Africa R. Mantovan Laboratorio MDM, IMM-CNR, Via Olivetti 2, 20864 Agrate Brianza (MB), Italy T. E. Mølholt · H. P. Gislason · S. Ólafsson Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland K. Bharuth-Ram School of Physics, University of KwaZulu-Natal, Durban 4001, South Africa K. Bharuth-Ram iThemba LABS, P.O. Box 722, Somerset West 7129, South Africa G. Langouche Instituut voor Kern- en Stralingsfysica, University of Leuven, 3001 Leuven, Belgium M. B. Madsen Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark