Structural phase transitions in YBaCo_4O_(7 + x) cobaltate upon variations in oxygen content, according to X-ray diffraction data obtained using synchrotron radiation

摘要

According to thermogravimetric data, the equilibrium oxygen content in layered YBaCo_4O_(7 + x) cobaltate varies stepwise from x ≈ 1.4 to x ≈ 0 when temperature is varied in the range of 300-400 C. Detailed ex situ and in situ structural investigations using synchrotron radiation show that variation in the oxygen content in YBaCo_4O_(7 + x) is accompanied by substantial reordering of the crystal structure and has the character of a first-order phase transition between two rigorously determined phases that differ by the parameters and symmetry of the crystal lattice as well as by the oxygen content. In addition to uniquely high oxygen liberation upon heating, dx/dT, great mechanical stresses that lead to the material cracking and complicate its practical use are a consequence of the character of stepwise transitions in YBaCo_4O_(7 + x).