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-delta with a crystal structure different from any of those previously 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 structure was solved ab initio by high-resolution synchrotron X-ray powder diffraction 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 parameters 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 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. (C) 2001 Academic Press.