X-ray spectra and electronic structure of the Ca 3 Ga 2 Ge 3 О 12 compound I.D. Shcherba a, b, * , L.V. Kostyk b , H. Noga a , L.V. Bekenov c , D. Uskokovich d , B.M. Jatsyk e a Institute of Technology, Pedagogical University, Podchoranzych Str. 2, 30-084 Cracow, Poland b Ivan Franko National University, Kyryla & Mefodiya Str. 8, 79-005 Lviv, Ukraine c Institute for Metal Physics of the N.A.S. of Ukraine, Vernadskogo Str. 36, 03142 Kyiv, Ukraine d Institute of Technical Sciences of SASA, Knez Mihailova 35/IV, PO Box 377,11000 Belgrade, Serbia e Lviv National University of Veterinary Medicine and Biotechnologies, Lviv, Ukraine article info Article history: Received 3 February 2017 Received in revised form 26 June 2017 Accepted 29 June 2017 Available online 30 June 2017 Keywords: Garnet X-ray emission spectra Electronic structure abstract The band structure of Ca 3 Ga 2 Ge 3 О 12 with the garnet structure has been determined for the first time by X-ray emission and photoelectron spectroscopy. It has been established that the bottom of the valence band is formed by Ge d states, which are not dominant in the chemical bonding. Strong hybridization of oxygen 2s states with 4p states of Ga and Ge revealed by the presence of an extra structure in the X-ray emission spectra has been found. The middle of the valence band has been demonstrated to be occupied by d states of Ga, while Ga and Ge 4рstates with a considerable admixture of oxygen 2p states form the top of the valence band. © 2017 Elsevier Masson SAS. All rights reserved. 1. Introduction Compounds with the garnet structure (space group Ia3d [1]) are receiving heightened interest due to their wide applications in laser technology. A large number of papers is devoted to studies of the effect of various impurities on their optical properties [2e4]. The Ca-Ga-Ge garnet doped with rare earth ions is a promising material for diode pumped solid-state lasers and optical pressure sensors [5]. Among the advantages of this materialcompared to other gar- netsareits lower melting point (T melt ¼ 1370  С), high thermal conductivity, and broadened absorption and luminescence lines of rare earth ions [6,7]. Nevertheless, the electronic structure of the base compound, namely Ca 3 Ga 2 Ge 3 О 12 , has remained unexplored so far. One of the most fundamental techniques uniquely describing the density of electronic states over the whole valence band is certainly the high-energy spectroscopy. 2. Experimental detail Single crystals of Ca 3 Ga 2 Ge 3 О 12 were obtained by the Czo- chralski technique using a platinum crucible in an argon-oxygen atmosphere. The initial components CaCO 3 , Ga 2 O 3 , GeO 2 with a purity of not less than 99.99% were taken in stoichiometric pro- portion with 0.5% excess of GeO 2 [8]. The samples were 0.1e0.9 mm-thick platescut along the (100) plane. The X-ray emission spectra of gallium and germanium in the Ca 3 Ga 2 Ge 3 О 12 garnet were obtained by a tube-spectrometer with the RKD-01-1 X-ray coordinate detector of original design [9]. Quartz with the (1011) reflecting plane was used as a crystal- analyzer for the components of К spectra; while for those of La spectra muscovite with the (001) reflecting plane was used. The spectra of the core levels and valence band of the Ca 3 Ga 2 Ge 3 О 12 garnet were measured by the “Kratos” X-ray photo- electron spectrometer in the Institute for Metal Physics of the N.A.S. of Ukraine. The Кa line of magnesium with the energy of 1253.6 eV served as a photon source. The spectrometer resolution was 0.9 eV, and the precision of positioning the core levels maxima was 0.1 eV. 3. Discussion The X-ray emission spectra of gallium and germanium were matched with the valence band photoelectron spectrum on a uni- fied energy scale using the energies of the Ga and Ge2р 3/2 core levels and the energies of their X-ray Кa 1 photons (Fig. 1). As we showed in paper [10], the X-ray emission Кb 2.5 band of Ga has fine structure lines, which are normally absent in intermetallic com- pounds [9]. We found that the Кb 2 subband of gallium (4р/1s * Corresponding author. Institute of Technology, Pedagogical University, Pod- choranzych Str. 2, 30-084 Cracow, Poland. E-mail address: ishcherba@gmail.com (I.D. Shcherba). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie http://dx.doi.org/10.1016/j.solidstatesciences.2017.06.014 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved. Solid State Sciences 71 (2017) 1e2