Hyperfine Interactions 156/157: 321–325, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 321 Exploring the Verwey-Type Transition in GdBaFe 2 O 5+w Using 57 Fe Mössbauer Spectroscopy J. LINDÉN 1,* , P. KAREN 2 , H. YAMAUCHI 3 and M. KARPPINEN 3 1 Åbo Akademi, Physics Department, FIN-20500 Turku, Finland; e-mail: jlinden@abo.fi 2 Department of Chemistry, University of Oslo, N-0315 Oslo, Norway 3 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan Abstract. 57 Fe Mössbauer spectroscopy was used to study the double perovskite GdBaFe 2 O 5+w , which exhibits mixing of the integer valence states of iron. The valence mixing/separation process Fe 2+ + Fe 3+ ↔ 2Fe 2.5+ was investigated as a function of temperature. For nearly stoichiometric compositions of w ≈ 0, a two-step Verwey-type transition is registered that separates Fe 2.5+ into in- termediate valence- and spin states Fe 2.5-ǫ and Fe 2.5+ǫ and then into the integer valences Fe 2+ and Fe 3+ . Both steps are accompanied by a decrease in electrical conductivity, altogether by two orders of magnitude. Seebeck measurements identify holes as dominating charge carriers, with activation energy for hopping of ∼0.10 eV in the valence-mixed state. It is inferred that the mixing electrons are not simply delocalized over the lattice, but rather form bridges connecting pairs of adjacent Fe atoms along the c axis. Key words: 57 Fe Mössbauer spectroscopy, Verwey-type transition, charge ordering, valence mixing, double perovskite. 1. Introduction GdBaFe 2 O 5+w belongs to the rare-earth (RE) based series REBaFe 2 O 5 , where ordering of the large Ba and small RE atoms stabilizes oxygen vacancies in the double-cell perovskite-type structure, in which Fe atoms reside in pyramidal oxy- gen coordinations similar to Cu(2) site in YBa 2 Cu 3 O 7 [1]. With the RE atom being trivalent [2] a mixed-valence situation is imposed upon iron, manifested by valence mixing at high temperatures and charge separation and -ordering at low temperatures. The phase transition between these states [3] is of the first order, ac- companied by changes in entropy and electrical conductivity of the type described for the first time by Verwey [4] on magnetite. At temperatures around 300 K and above, a single Fe 2.5+ state is observed in the 57 Fe Mössbauer spectrum [5]. The Verwey-type transition into two integer valence states proceeds in two steps. Upon cooling, Fe 2.5+ separates into two intermedi- ate valence states Fe 2.5-ǫ and Fe 2.5+ǫ , then the second step leads to a full charge separation and long-range ordering of the integer valences Fe 2+ and Fe 3+ [3, 6]. * Author for correspondence.