The structure of defects, the electron energy-band structure, and the semiconductor–metal transition in PrBaCo$_{2}$O$_{5.5}$ cobaltite: ab initio PAW approach

The calculations of electronic band structure of the cobaltite PrBaCo$_{2}$O$_{5+\delta}$ for the content of oxygen near 5.5 have been performed using the first-principle PAW method. It has been shown that the semiconductor-metal transition near 5 + $\delta$ = 5.5 is associated with the conversion of cobalt atoms in octahedral oxygen surrounding from high to low spin state and the similar atoms in pyramidal surrounding from the low to high spin state. The metal conductivity appears due to raising of the energy of pyramidal Co $e_g$ antibonding states. As a result these states turn up at the Fermi level thus defining the conductivity. The effect of oxygen content deviation from 5.5 on the band structure and conductivity has been studies. It is shown that the semiconductor-metal transition can be observed in the narrow range of 5 + $\delta$ below 5.5.