The influence of lanthanum content in the Fe₂O₃-Li₂O-La(OH)₃ system on phase formation and properties of composite material

Objectives. To study the effect of varying lanthanum content in the Fe₂O₃–Li₂O–La(OH)₃ system on phase formation and corresponding structural and electromagnetic properties of a lithium-ferrite composite material obtained using high-temperature ceramic technology.

Methods. Following the addition of lanthanum occurred at the initial stage of mixing the Fe₂O₃/Li₂CO₃/La(OH)₃ components in a certain weight ratio, the obtained samples were sent for preliminary synthesis at a temperature of 900°C for 240 min in an air atmosphere and sintering in a dilatometer at a temperature of 1100°C for 120 min. The microstructure and properties of the sintered composite samples were studied using X-ray phase analysis (XRD), thermogravimetry (TG), differential scanning calorimetry (DSC), and scanning electron microscopy.

Results. XRD analysis confirmed the formation of a two-phase structure following solid-phase synthesis, consisting of the magnetic phase of lithium ferrite Li_0.5Fe_2.5O₄ and the perovskite-like phase LaFeO₃. XRD carried out after sintering showed that the hightemperature heating process did not affect the changes in the phase composition of the sample phases. Dilatometric shrinkage curves obtained after sintering showed that the addition of La reduces the rate of compaction of the samples at the stage of their heating. The sintered samples were characterized by a density of 4.34, 3.84, 3.93 g/cm³ and a porosity of 0.7, 16, and 18%, respectively, having an increased mass content of La(OH)₃ at the synthesis stage. A decrease in the grain sizes was also observed. An increase in the amount of lanthanum hydroxide La(OH)₃ additive from 0 to 4.4 and 13.9 wt % led to an increase in the concentration of the synthesized LaFeO₃ phase in the samples to 4.2 and 16.6 wt %, resulting in decreased specific saturation magnetization values from 59.4 to 58.2 and 49.7 G∙cm³/g and the initial magnetic permeability from 41.6 to 22.8 and 19.5, respectively. TG and DSC showed that high-temperature sintering of lithium ferrite without additives leads to the predominant formation of the disordered b-phase Li_0.5Fe_2.5O₄, which has a reduced Curie temperature of 626°C. This process is associated with a violation of the stoichiometric composition of the samples for lithium and oxygen due to the release of these elements from the samples during high-temperature sintering.

Conclusions. The high values of the Curie temperature of 631°C confirm that the addition of lanthanum during the production of lithium ferrite prevents the violation of the stoichiometric composition of the ferrite during sintering due to the construction of an additional LaFeO₃ lattice. The addition of lanthanum was also found to lead to a significant increase in specific electrical resistance from 5 ∙ 10² to 6 ∙ 10⁹ and 1 ∙ 10¹² Ohm∙cm, which may be associated with both a change in the microstructure of the samples and a change in their phase composition.

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