Ab initio structure solution of BaFeO2.8-delta a new polytype in the system BaFeOy (2.5 <= y <= 3.0) prepared from the oxidative thermal decomposition of BaFe[(CN)(5)NO]center dot 3H(2)O

BaFeO2.8-delta with a crystal structure different from any of those previou sly reported in the system BaFeOy (2.5 less than or equal to y less than or equal to 3.0) was prepared by a low-temperature method of synthesis, based on the oxidative thermal decomposition of BaFe [(CN)(5)NO] . 3H(2)O. The s tructure was solved ab initio by high-resolution synchrotron X-ray powder d iffraction and refined by Rietveld analysis (R-P = 7.78, R-wp = 11.3, R-exp = 3.76, R-Bragg = 6.67, chi (2) = 9.03). The compound crystallizes in the hexagonal crystal system, space group P6(3)/mmc, Z = 10, unit cell paramete rs a = 5.77944(1), c = 24.60871(6) Angstrom. 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 sha ring 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 betwee n them. (C) 2001 Academic Press.