Electron Paramagnetic Resonance in non-stoichiometric cobaltite Na$_x$CoO$_2$ and study of the influence of degradation processes on properties of Na$_x$CoO$_2$ system

Electron Paramagnetic Resonance (EPR) measurements of non-stoichiometric cobaltite Na$_x$CoO$_2$ in the sodium concentration range $x$ = 0.53; 0.58; 0.64; 0.75; 0.76; 0.79 were carried out and the effect of degradation processes on the properties of Na$_x$CoO$_2$ system were studied. ESR signal was detected in sample with maximum sodium concentration Na$_{0.79}$CoO$_2$. Analysis of the obtained results on EPR allows us to say that the observed resonance signal is due to the magnetic ions Co$^{4+}$ the appearance of which in the system under study is associated with sodium non-stoichiometry and possible presence of anti-structural defects caused by the localisation of cobalt ions in sodium positions. At low temperatures a sharp decrease in the width of the resonance line, $g$-factor and integral intensity of resonance signal was observed that indicates that in the Na$_{0.79}$CoO$_2$ sample the magnetic phase transition takes place. It has been established that after long-term storage under ambient conditions EPR signal in the sample Na$_{0.79}$CoO$_2$ disappear. The structural and electron microscopic studies carried out show that as a result of long-term storage of samples, the essential changes of phase composition take place.