ИСТИНА

Layered transition metal compounds have attracted attention due to their wide range of magnetic and transport properties. Transition metal oxychalcogenides and oxypnictides often crystallize in a two-dimensional structure due to anion site orderings because of the large different sizes between the oxide and chalcogenide (pnictide) ions [1]. Recently, high-temperature superconductivity was discovered in layered iron oxypnictides with a general formula LnFeAsO (1111-structure) [2]. There exist a few oxychalcogenides of rare earths and iron as well as cobalt [3–5] with a composition Ln2T2O3Ch2 (T = Fe or Co, and Ch = S or Se) which share a common structure type somewhat different from those of quaternary “1111” Ln – Fe oxypnictides. In these compounds, the T2OCh2 monolayers and the Ln2O2 bilayers are stacked along the c-axis. The peculiarity of this structure is in transition metal residing in a multi-ligand environment, i.e. a trans-O2Ch4 octahedron (Ch = S or Se). We have synthesized four new compounds Ce2Fe2O3S2, Ce2Fe2O3Se2, Pr2Fe2O3S2, and Pr2Fe2O3Se2 in sealed quartz tubes. Crystal structures of the new compounds Ce2Fe2O3S2 and Pr2Fe2O3S2 were determined from powder X-ray diffraction and electron microscopy data. These compounds are isostructural to La2Fe2O3Se2. The crystal chemical boundaries of this structure type can be explained in terms of two oppositely directed deformations of the trans-TO2Ch4 octahedra (T – transition metal, Ch – chalcogen) condensed into the [T2OCh2]2- slabs, the stretching or compression of the axial T–O bonds and rectangular distortion of the equatorial TCh4 squares. Magnetic measurements performed for Ce2Fe2O3S2 sample revealed Curie – Weiss behavior above 130 K. Akin to known isostructural oxide chalcogenides and pnictides and opposed to other layered iron chalcogenides and pnictides, the new compounds do not exhibit superconducting properties. The support of the Russian Foundation for Basic Research is acknowledged (Grants No. 10-03-00681-a, 10-02-01281). References 1. S.J. Clarke et al, Inorg. Chem. 47 (2008) 8473. 2. Y. Kamihara et al, J. Am. Chem. Soc. 130 (2008), 3296. 3. J.M. Mayer et al, Angew. Chem. Int. Ed. Engl. 31 (1992) 1645. 4. Y. Fuwa et al, J. Phys.: Cond. Matt. 22 (2010) 346003. 5. Y. Fuwa et al, Solid State Commun. 150 (2010) 1698.