Journal of Alloys and Compounds 496 (2010) 61–68 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Electronic structure of FeWO 4 and CoWO 4 tungstates: First-principles FP-LAPW calculations and X-ray spectroscopy studies S. Rajagopal a , V.L. Bekenev b , D. Nataraj a , D. Mangalaraj c , O.Yu. Khyzhun b,∗ a Thin Film & Nanomaterials Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, India b Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, UA-03142 Kyiv, Ukraine c Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641 046, India article info Article history: Received 2 February 2010 Accepted 10 February 2010 Available online 18 February 2010 Keywords: Iron tungstate FeWO4 Cobalt tungstate CoWO4 Electronic structure Band structure calculations X-ray emission spectroscopy X-ray photoelectron spectroscopy abstract Total and partial densities of states of the constituent atoms of iron tungstate, FeWO 4 , and cobalt tungstate, CoWO 4 , have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. The results obtained reveal that the O 2p-like states are the dominant contributors into the valence band of the tungstates under consideration, whilst the bottom of the conduction band of FeWO 4 and CoWO 4 is dominated by contributions of the empty Fe 3d- and Co 3d-like states, respectively. The FP-LAPW data indicate that the O 2p-like states contribute mainly into the top of the valence band, with also significant contributions throughout the whole valence-band region, of FeWO 4 and CoWO 4 compounds. Other significant contributors into the valence-band region are the Fe 3d- and W 5d-like states in FeWO 4 and the Co 3d- and W 5d-like states in CoWO 4 . All the above d-like states contribute throughout the whole valence-band region of the tungstates under con- sideration, however maximum contributions of the W 5d-like states occur in the lower, whilst the Fe (Co) 3d-like states in the upper portions of the valence band, respectively. To verify the above FP-LAPW data, the X-ray emission bands representing the energy distributions of mainly the valence O p-, Fe (Co) d-, Fe (Co) p- and W d-like states were measured and compared on a common energy scale with the X-ray photoelectron valence-band spectrum of the corresponding tungstate. The experimental data were found to be in good agreement with the theoretical FP-LAPW results for the electronic structure of FeWO 4 and CoWO 4 compounds. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Iron tungstate, FeWO 4 , and cobalt tungstate, CoWO 4 , belong to a fascinating family of wolframite-type materials which have highly potential and technological applications in many areas such as scintillation detectors, optical fibers, humidity sensors, photoan- odes, phase-change optical recording devices, laser hosts, catalysts, pigments, etc. [1–7]. Additionally, wolframite-type tungstates are considered to be novel and commercially important materials due to their several encouraging properties, mainly high values of ther- mal stability, refractive indexes and X-ray absorption coefficients [8,9]. In particular, among wolframite-type tungstates, FeWO 4 and CoWO 4 have been intensively studied. FeWO 4 and CoWO 4 tungstates are well-known p-type semiconductors [10–12]. From experimental measurements [10,13], the energy band gap, E g , equals 2.8 and 2.0 eV for CoWO 4 and FeWO 4 , respectively. Estima- tions by Lacomba-Perales et al. [14] based on correlations between the ionic radius of the A 2+ cation in a number of AWO 4 tungstates ∗ Corresponding author. Tel.: +380 044 424 33 64; fax: +380 044 424 21 31. E-mail address: khyzhun@ipms.kiev.ua (O.Yu. Khyzhun). reveal values of E g = 2.43 eV for CoWO 4 and E g = 2.35 eV for FeWO 4 . Iron and cobalt tungstates were studied in Refs. [15–18] to explore potential applications of these materials in magnetic fields. As it has been established by Weitzel [15], the magnetic unit cell (2a,b,c) of CoWO 4 is twice of the chemical unit (a,b,c) and identical with that of FeWO 4 . Additionally, the magnetic properties of CoWO 4 are proved to be symmetric with respect to three orthogonal twofold axes x, y, and z: the ‘magnetic’ axes x and z are inclined to the vectors a and c of the crystal unit cell at an angle of 40 ◦ in the ac-plane, whilst the magnetic axis y lies along the crystal (true) two-fold axis b [16]. FeWO 4 and CoWO 4 tungstates are isostructural compounds crystallizing in a monoclinic structure belonging to the P2/a space group, with unit cell parameters a = 4.753 Å, b = 5.720 Å, c = 4.968 Å, ˇ = 90.1 ◦ for FeWO 4 [19] and a = 4.670 Å, b = 5.687 Å, c = 4.951 Å, ˇ = 90.0 ◦ for CoWO 4 [20]. The findings by the authors [19,20] are consistent with those derived by other investigators [21–25], as data listed in Table 1 reveal. In the structure of FeWO 4 and CoWO 4 tungstates, with two formula per unit cell, every metal atom is surrounded by six oxygen atoms: zigzag chains of oxygen octa- hedra coordinating the metal ions are aligned along the c axis [10]. Iron(cobalt), tungsten and oxygen atoms occupy the 2f, 2e and 4g sites, respectively. As an example, Fig. 1 shows the crystal struc- 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.107