Application of X-ray absorption spectroscopy to the investigation of charge states of iron ions in iron borate nanoceramics

The method of X-ray absorption spectroscopy has been used for the investigation of charge states of iron ions in iron borate nanoceramics prepared by shear deformation under pressure. The experimental Fe 2$p$ X-ray absorption spectra have been presented in comparison with the calculation of atomic multiplets of iron ions taking into account the charge transfer from the 2$p$ orbitals of oxygen to the 3$d$ orbitals of iron and the crystal-field splitting of the 3$d$ orbitals of iron. Our results indicate that, in addition to iron ions in the ground charge state Fe$^{3+}$ , nanostructured FeBO$_3$ contains a few percent of Fe$^{2+}$ ions. It has been found that an increase in the degree of plastic deformation (the rotation angle of the anvils) leads to a decrease in the size of crystallites and to an increase in the concentration of Fe$^{2+}$ ions without the formation of new phases. The results of this work agree with the magnetic and optical measurements and confirm high defectness of FeBO$_3$ nanoceramics.