(CaO)(FeSe): A Layered Wide Gap Oxychalcogenide Semiconductor
  A new iron-oxychalcogenide (CaO)(FeSe) was obtained which crystallizes in the orthorhombic space group Pnma (No. 62) with a = 5.9180(12) {\AA}, b = 3.8802(8) {\AA}, c = 13.193(3) {\AA}. The unique structure of (CaO)(FeSe) is built up of a quasi-two-dimensional network of corrugated infinite layers of corner-shared FeSe2O2 tetrahedra that extend in the ab-plane. The corrugated layers composed of corner-shared FeSe2O2 tetrahedra stack along the c-axis with Ca2+ cations sandwiched between the layers. Optical spectroscopy and resistivity measurements reveal semiconducting behavior with an indirect optical band gap of around 1.8 eV and an activation energy of 0.19(1) eV. Electronic band structure calculations at the density function level predict a magnetic configuration as ground state and confirm the presence of an indirect wide gap in (CaO)(FeSe). 
Fei Han, Di Wang, Christos D. Malliakas, Mihai Sturza, Duck Young   Chung, Xiangang Wan, and Mercouri G. Kanatzidis
cond-mat.mtrl-sci cond-mat.supr-con