Trends in structural, electronic properties and Fermi surface topology of 15 tetragonal ThCr 2 Si 2 -type phases AFe 2 Ch 2 , where A = Li, Na, K, Rb, and Cs; Ch = S, Se, and Te, as parent systems of new ternary iron–chalcogenide superconductors V.V. Bannikov, I.R. Shein, A.L. Ivanovskii ⇑ Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Russia article info Article history: Received 7 April 2011 Received in revised form 2 March 2012 Accepted 10 March 2012 Available online 6 April 2012 Keywords: Ternary iron chalcogenides Structural, electronic properties Fermi surface FLAPW-GGA calculations abstract By means of the first principles FLAPW-GGA approach, a systematic study of the structural and electronic properties of full series of 15 synthesized and hypothetical tetragonal ThCr 2 Si 2 -type phases in the system A–Fe–Ch, where A = Li, Na, K, Rb, and Cs; Ch = S, Se, and Te, was performed. We find that for the tuning of the structural parameters of conducting [Fe 2 Ch 2 ] blocks the replacements of Ch 2 ions will be most effec- tive, whereas for the tuning of the inter-blocks separation [Fe 2 Ch 2 ]–[Fe 2 Ch 2 ] the replacements of the alka- line ions seem more useful. The trends in the electronic properties and Fermi surface topology of AFe 2 Ch 2 phases are analyzed in relation to their structural parameters to explore whether these materials can be as the parent systems for new ternary iron–chalcogenide superconductors. For all these systems, similar multi-sheet Fermi surfaces consist exclusively of electronic-like sheets, two of which are 2D-like sheets in the corners of the Brillouin zone, and the third one is a closed pocket around Z. The total DOSs at the Fermi level are related with the type of anion Ch 2 and their values grow in the sequence AFe 2 S 2 ? AFe 2 Se 2 ? AFe 2 Te 2 . Thus we assume that AFe 2 Se 2 and AFe 2 Te 2 phases may be considered as favorable parent systems for the search of new ternary iron–chalcogenide SCs. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Recently, the first ThCr 2 Si 2 -type ternary iron–chalcogenide superconductor K 0.8 Fe 2 Se 2 with enhanced T C  31 K was discov- ered [1]. This finding has stimulated much activity in search for related materials and triggered intense studies of their properties. Very soon superconductivity (T C  28–32 K) was also found in the series of related ternary systems (so-called 122 phases) such as Cs x Fe 2y Se 2 , Rb x Fe 2y Se 2 , and (TlK) x Fe 2y Se 2 (TlRb) x Fe 2y Se 2 , see [2–9], which formed a new group of superconducting iron-based materials without toxic As, review [10]. On the other hand, a quite limited number of A–Fe–Ch systems (where A are alkaline metals, and Ch are chalcogens) is involved in current research as possible ternary iron–chalcogenide super- conducting materials; among them are selenides: K x Fe 2y Se 2 , Rb x Fe 2y Se 2 , and Cs x Fe 2y Se 2 [1–10], and very recently the first data about series of related sulfides AFe 2 S 2 (where A = K, Rb and Cs) be- came available [11]. As far as we are aware, only one related ThCr 2 Si 2 -type phase with tellurium (K 1.00 Fe 0.85 Ag 1.15 Te 2.0 [12]) was synthesized. From theoretical side, the electronic properties of this Ag-con- taining phase have been examined recently [13]; besides, the first attempt to find some trends in structural and electronic properties of three related tetragonal phases KFe 2 Ch 2 , depending on chalco- gen type (Ch = S, Se, and Te) was undertaken by means of DFT method [14]. We have found [14] that anion replacements (S M Se M Te) produce no critical changes in the electronic struc- ture of KFe 2 Ch 2 phases. On the other hand, our analysis [14] of structural and electronic parameters for hypothetical KFe 2 Te 2 al- lows us to propose this system as a possible parent phase for search of new iron–chalcogenide superconducting materials. The main ambition of this paper is to find the general trends in the structural, electronic properties and Fermi surface topology for full series of 15 synthesized and hypothetical tetragonal ThCr 2 Si 2 - type phases in the system A–Fe–Ch (where A = Li, Na, K, Rb, and Cs; Ch = S, Se, and Te) depending on the both anion (Ch 2 ) and cation (A 1+ ) substitutions, which can be useful for designing of new super- conducting materials. 2. Models and computational aspects We have examined the ternary iron–chalcogenide phases with the nominal composition AFe 2 Ch 2 (A = Li, Na, K, Rb, and Cs; Ch = S, Se, and Te) and the tetragonal ThCr 2 Si 2 -type structure (space group I4/mmm; #139). Here, the Fe atoms form a square lattice, whereas the Ch atoms are located at the apical sites of 0927-0256/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2012.03.019 ⇑ Corresponding author. Tel.: +7 343 374 5331; fax: +7 343 374 4495. E-mail address: ivanovskii@ihim.uran.ru (A.L. Ivanovskii). Computational Materials Science 60 (2012) 1–6 Contents lists available at SciVerse ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci