Nucleogenic iron charge states in CoO studied by Mössbauer spectroscopy Original Research Article

Single crystal of CoO was doped by about 40-at. ppm of 57Co. Mössbauer spectra were measured vs. sample orientation, temperature and under reduced oxygen pressure, i.e. for a low cobalt vacancy concentration. Distinctly different iron charge states coexist till about 866 K. A mixed-valent state is observed above this temperature. It evolves gradually towards the Fe2+ state above 1255 K due to the decreasing mobility of holes, as compared to the mobility of the electrons. The ratio of the mobility of holes to the mobility of electrons increases with the increasing vacancy concentration, i.e. with the increasing oxygen pressure. Defects order into clusters below 866 K leaving behind almost perfect structure. A recoilless fraction exhibits some anharmonicity above 923 K.