Ab Initio Structure Solution of BaFeO2.8−δ, a New Polytype in the System BaFeOy (2.5≤y≤3.0) Prepared from the Oxidative Thermal Decomposition of BaFe[(CN)5NO]·3H2O

BaFeO2.8−δ with a crystal structure different from any of those previously reported in the system BaFeOy (2.5≤y≤3.0) was prepared by a low-temperature method of synthesis, based on the oxidative thermal decomposition of BaFe[(CN)5NO]·3H2O. The structure was solved ab initio by high-resolution synchrotron X-ray powder diffraction and refined by Rietveld analysis (RP=7.78, Rwp=11.3, Rexp=3.76, RBragg=6.67, χ2=9.03). The compound crystallizes in the hexagonal crystal system, space group P63/mmc, Z=10, unit cell parameters a=5.77944(1), c=24.60871(6) Å. The structure consists of 10H close packed (hchch)2 stacking of BaOn layers (eight BaO3 layers and two oxygen-deficient BaO2 layers). Additional oxygen deficiencies are randomly distributed on the h BaO3 layers. Six iron ions occupy octahedral sites sharing faces between them along the c axis and four occupy tetrahedral sites (2 T+ and 2 T−) sharing faces with the octahedra and sharing corners between them.