Mössbauer study of Na$_{0.82}$CoO$_2$ (doped by 1% $^{57}$Fe)

Abstract

We studied by Mössbauer spectroscopy the Na$_{0.82}$CoO$_2$ compound using 1% $^{57}$Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe$^{3+}$ and Fe$^{4+}$. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co$^{+3}$, Co$^{+4}$ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe$^{4+}$ and all of Fe$^{3+}$, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.