Journal of Alloys and Compounds 481 (2009) 28–34 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Electronic structure of cadmium selenogallate CdGa 2 Se 4 as studied using ab initio calculations and X-ray photoelectron spectroscopy A.A. Lavrentyev a , B.V. Gabrelian a , I.Ya. Nikiforov a , O.V. Parasyuk b , O.Yu. Khyzhun c,∗ a Department of Physics, Don State Technical University, Gagarin Sq. 1, 344010 Rostov-on-Don, Russian Federation b Department of General and Inorganic Chemistry, Volyn National University, 13 Voli Ave, UA-43025 Lutsk, Ukraine c Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, UA-03142 Kyiv, Ukraine article info Article history: Received 18 December 2008 Received in revised form 5 March 2009 Accepted 11 March 2009 Available online 21 March 2009 Keywords: Cadmium selenogallate CdGa2Se4 Electronic structure Band-structure calculations X-ray photoelectron spectroscopy abstract Electronic properties of cadmium selenogallate CdGa 2 Se 4 , a very promising electro-optical material, were studied both from theoretical and experimental points of view employing the ab initio band-structure augmented plane wave + local orbitals (APW + LO) method with the WIEN2k code and X-ray photoelectron spectroscopy (XPS). The present APW + LO calculations reveal that the Se p-like states are the dominant contributors into the top of the valence band of CdGa 2 Se 4 , while the bottom of the band is dominated by contributions of the Cd d-like states. Additionally, the bottom of the conduction band of the compound under consideration is dominated by contributions of the Ga s-like states. The present calculations render that the valence-band maximum and conduction band minimum in CdGa 2 Se 4 are located at  resulting in a direct energy gap. The imaginary part of the electronic dielectric function ε(ω) was also calculated for the compound under consideration based on its band-structure data. The ε xx (ω) and ε zz (ω) components of the imaginary part of the electronic dielectric function reveal a considerable anisotropy in CdGa 2 Se 4 . The XPS valence-band spectrum of CdGa 2 Se 4 has been derived and the binding energies of core-level electrons of the constituting atoms of the compound have been measured. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Cadmium selenogallate CdGa 2 Se 4 is a well-known semicon- ductor, which is widely used in nonlinear optics (NLO) [1,2]. The CdGa 2 Se 4 compound crystallizes in a defect chalcopyrite structure (I ¯ 4, Z = 2), in which one of the cation sites is unoccupied [3–6]. The CdGa 2 Se 4 crystal structure is considered as a three-dimensional zinc-blende type superstructure: the cadmium selenogallate struc- ture can be constructed by the doubling of the zinc-blende unit cell along the c axis [7]. At room temperature, under influence of high pressures, the CdGa 2 Se 4 semiconductor reveals (at 21 GPa) the pressure-induced order–disorder phase transition to the NaCl- type metallic polymorph [8]. Furthermore, as Grzechnik et al. [8] have established, under decompression the metallic CdGa 2 Se 4 rock- salt polymorph transforms into the metastable semiconducting phase with the zinc-blende structure (Fm3m, Z = 4) at pressures of 7.5–4.0 GPa. A first-order phase transition was detected at 20.2 GPa by Mitani et al. [9] when measuring the Raman scattering spectra of CdGa 2 Se 4 under pressures up to 20.9 GPa. ∗ Corresponding author. Tel.: +380 44 424 33 64; fax: +380 44 424 21 31. E-mail address: khyzhun@ipms.kiev.ua (O.Yu. Khyzhun). As a representative member of defect chalcopyrite-type materials belonging to NLO family, CdGa 2 Se 4 possesses high pho- tosensitivity and strong luminescence in the visible range [10–12]. Additionally, cadmium selenogallate CdGa 2 Se 4 is a promising material for its application as a narrow-band optical filter [13]. Therefore, different routes are applied for obtaining high-quality bulk crystals of CdGa 2 Se 4 , e.g. chemical vapour transport (CVT) reactions in which iodine is used as a transport agent [14–16], chemical vapour deposition (CVD) [17], the flux and solution-melt methods [18–20]. In the present paper we aim at a comprehensive study of the energy distribution of electronic states of different symmetries of the constituent atoms of cadmium selenogallate CdGa 2 Se 4 . The band-structure model of this semiconductor was constructed by Syrbu and Tezlevan [21] using the experimental data on optical reflectivity spectra, but band-structure calculations of some semi- conductors crystallizing within the same chalcopyrite structure were performed in Refs. [22,23]. To the best of our knowledge, no theoretical first-principles band-structure calculations have been made so far for CdGa 2 Se 4 . With this purpose, we employ possi- bilities of the augmented plane wave + local orbitals (APW + LO) method as incorporated in the WIEN2k code [24] in order to study total density of states (DOS) and partial densities of states of the CdGa 2 Se 4 compound. The imaginary part of the electronic dielectric 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.057