Effect of Nickel Ions Substitution on the Magnetic and Optical Properties of a Nanosized Lithium-Iron Ferrite

This paper reports on the successful synthesis of fine nanoparticles of nickel-substituted lithium-iron ferrites of composition Li0.5-x/2NixFe2.5-x/2O4 (0.2< x <1.0) by the sol-gel autocombustion method. It has been found that the alternating current (AC) and direct current (DC) conductivity is preferably tuned due to their dependence on temperature and nickel doping. Analysis of the Arrhenius dependences also confirms the appearance of more than one conduction mechanism upon substitution. The predominance of one type of conductivity over another depends on the concentration of the substituting element. Measurement of the magnetic properties has shown that the substitution of Ni2+ can significantly change the saturation and residual magnetization. Samples of composition Li0.4Ni0.2Fe2.4O4 have the highest saturation magnetization (84.08 emu/g), residual magnetization (15.85 emu/g), and the lowest coercive force (0.18 kOe). All the obtained results indicate a significant effect of the substitution of Ni2+ ions on the structure and properties of Li0.5-x/2NixFe2.5-x/2O4 ferrite nanoparticles. Photocatalytic properties have been obtained by the degradation of Methylene Blue dye under illumination with a halogen lamp. It is shown that an increase in the content of nickel ions leads to a change in the type of conductivity: from n-type (unsubstituted lithium pentaferrite) to p-type (with substitution x = 0.8 and higher). These systems are characterized by hopping conduction realized by octa-positions according to the mechanisms Fe3++e-<-> Fe2+, and Ni3+<-> Ni2++h+. The predominance of one or another mechanism depends on the content of nickel ions. The optical band gap ranges from 1.4 to 2.25 eV. Samples with nickel content x = 0.4 and x = 0.8 have shown the best degradation ability, which is 97% within 160 min for Methylene Blue.