Geometric parameterization of the YBaCo4O7 structure type: implications for stability of the hexagonal form and oxygen uptake.

We explore the stability of the hexagonal form of MBaCo4O7 cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M3+/2+ and the three-dimensional network of CoO4 tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo4O7 structure. © 2010, Elsevier Ltd

Mossbauer spectroscopy analysis of Fe-57-doped YBaCo4O7+delta: effects of oxygen intercalation.

Mossbauer spectroscopy of layered YBaCo3.96Fe0.04O7+delta (delta=0.02 and 0.80), where 1% cobalt is substituted With 57 Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe3+ fractions in the alternating triangular and Kagome layers in oxidized YBaCo3.96Fe0.04O7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K. © 2008, Elsevier Ltd

Transitions between P2¹, P6³(√3A), and P6³22 modifications of SrAl²O4 by in situ high-temperature X-ray and neutron diffraction.

The results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl2O4. The material undergoes two reversible phase transitions P2(1) P6(3)(√3A) and P6(3)(√3A) P6(3)22 at similar to 680 and similar to 860°C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the P6(3)(√3A) P6(3)22 transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al-O-Al angle to the ideal value 180°C. The analysis of the literature suggests that it is a common feature of the P6(3)22 phases of stuffed tridymites. © 2007, Elsevier Ltd