Eur. Phys. J. B 46, 363–366 (2005) DOI: 10.1140/epjb/e2005-00265-5 THE EUROPEAN PHYSICAL JOURNAL B Optical investigations of the clathrate α-Eu 8 Ga 16 Ge 30 J. Sichelschmidt 1 , a , V. Voevodin 1 , V. Pacheco 1, b , Yu. Grin 1 , F. Steglich 1 , T. Nishi 2 , and S. Kimura 2,3 1 Max-Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany 2 Department of Structural Molecular Science, The Graduate University for Advanced Studies, Okazaki 444-8585, Japan 3 UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan Received 4 February 2005 / Received in final form 7 June 2005 Published online 18 August 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. We performed measurements of the optical reflectivity in the energy range 0.007–30 eV on the clathrate-VIII type compound α-Eu8Ga16-x Ge30x in order to investigate its electronic band structure. The very low charge carrier concentration as well as ferromagnetic ordering of the divalent Eu ions below 10.5 K characterize the spectra at photon energies below 0.4 eV in accordance with the results of band structure calculations. Disorder induced bound states have been identified to affect the optical conductivity at energies between 10 and 100 meV. PACS. 78.20.-e Optical properties of bulk materials and thin films – 72.15.-v Electronic conduction in metals and alloys 1 Introduction Intermetallic clathrates, evolved from metal-stabilized sil- icon clusters [1], are open framework, novel, tunable elec- tronic materials based on group-IV semiconductors and with the potential to form magnetic semiconductors. They are also believed to be basically new materials for effective thermoelectric devices as their properties combine glass- like heat conduction (‘phonon glasses’) with good electri- cal conduction (‘electron crystals’) [2]. The Eu-containing clathrate Eu 8 Ga 16 Ge 30 is made up of large cages of randomly distributed Ge and Ga atoms. The cages are fully occupied by Eu 2+ ‘guest’ ions which or- der ferromagnetically at low temperatures and thus make this clathrate a local-moment ferromagnet. The structure of Eu 8 Ga 16 Ge 30 belongs in its α-modification to the cubic clathrate-VIII type with the space group I ¯ 43m. Contrary to the β-modification the α-phase has only one type of cages centered by Eu atoms [3]. α-Eu 8 Ga 16 Ge 30 shows a Curie temperature as low as T C = 10.5 K and an extraor- dinarily low charge carrier concentration [3]. Although not realized for the clathrates yet, the properties of the Eu 8 Ga 16 Ge 30 clathrates seem to indicate a route towards new kinds of so-called Kondo-Insulators [4,5]. From band-structure calculations α-Eu 8 Ga 16 Ge 30 and β-Eu 8 Ga 16 Ge 30 with an ideal Zintl stoichiometry are ex- pected to be semiconductors [6]. However, the exact stoi- chiometry sensitively depends on the annealing conditions [7]. It was shown that the physical properties are closely a e-mail: sichelschmidt@cpfs.mpg.de b Present address : Instituto de F´ısica “Luis Rivera Terrazas”, Universidad Aut´onoma de Puebla, Aparto Postal J-48, Puebla, Pue. 72570, M´exico related to changes in the (Ga,Ge)–composition [8,9]. α-Eu 8 Ga 16 Ge 30 could be prepared only with a slight de- viation from the ideal Ga/Ge ratio α-Eu 8 Ga 16−x Ge 30+x [7,9]. As inferred from the normal Hall effect for x> 0, n-type electrical conductivity is established together with a low charge carrier density n. For x = 0.48 n = 0.461 e − /(unit cell) (for unit cell volumes see Ref. [7]). A small anomalous contribution to the Hall effect, typi- cal for ferromagnetic semiconductors, is found [3,7,9]. The electrical conductivity σ(T ) of α-Eu 8 Ga 16−x Ge 30+x shows metallic behavior (negative dσ/dT ) in a large temperature range [3]. Above the ferromagnetic phase transition tem- perature a narrow anomaly with a positive dσ/dT is found which may be due to scattering from critical spin fluctua- tions. The knowledge of the electronic structure helps to un- derstand the interplay of crystal structure, doping, mag- netism and transport properties. Optical investigations of clathrates, focusing on the electronic band structure, have already been performed [10] and found to strongly sup- port band structure calculations [11] for the chiral type clathrate Ba 6 Ge 25 . To our knowledge, analogous optical investigations of clathrate-VIII type structures are still lacking although their electronic band structure has been calculated [6]. This paper presents optical investigations on α-Eu 8 Ga 16−x Ge 30+x which provide not only results consistent with the calculated band structure but also in- sight into its metal-like behavior. 2 Experimental In this paper we focus on the optical properties of α-Eu 8 Ga 16−x Ge 30+x whose preparation is described in