Thermoanalytical, optical, and magnetic investigations on nanocrystalline Li0.5Fe2.5O4 and resulting ceramics prepared by a starch-based soft-chemistry synthesis

Nanocrystalline Li0.5Fe2.5O4 was prepared by a starch-based soft-chemistry synthesis. Calcining of the (LiFe)-gel between 350 and 1000 ​°C results in Li0.5Fe2.5O4 powders with crystallite sizes from 13 to 141 ​nm and specific surface areas between 35 and 7.1 ​m2 ​g−1. XRD investigations reveal the formation of ordered Li0.5Fe2.5O4. Sintering between 1050 and 1250 ​°C leads to ceramics with relative densities of 67–95% consisting of grains between 0.3 and 54 ​μm. As the sintering temperature increases a rising weight loss of the ceramic samples was observed due to the loss of Li2O. Temperature-dependent magnetic measurements indicate a superparamagnetic behaviour for the nano-sized samples. Field-dependent measurements at 3 ​K of ceramics sintered between 1050 and 1200 ​°C show increasing saturation magnetization values (Ms) of 70.0–73.0 emu g−1 most likely due to the formation of lithium vacancies and a decrease of the inversion parameter. The magnetization drops down to 67.7 emu g−1 after sintering at 1250 ​°C caused by the formation of hematite. Diffuse reflectance spectra reveal an indirect allowed band gap decreasing from 1.93 to 1.60 ​eV depending on thermal treatment. DSC measurements of the order ⇆ disorder phase transition on nano-sized powders and bulk ceramics exhibit transition temperatures between 734 and 755 ​°C and enthalpy changes (ΔtrsH) ranging from 5.0 to 13.5 ​J ​g−1. The linear thermal expansion coefficient was found to be 11.4⋅10−6 ​K−1. Nanocrystalline Li0.5Fe2.5O4 powders were prepared by a soft-chemistry method using starch and NH4NO3. The magnetic properties between 3 ​K and 300 ​K were investigated depending on particle size and heat treatment. The phase transitions were examined via DSC and dilatometry. The influence of Li-loss during sintering on the magnetic behaviour was studied. The optical band gaps and the type of transition were determined. [Display omitted] •Facile synthesis for nano Li0.5Fe2.5O4 using starch and NH4NO3 as polymerization agent and oxidizer.•Monitoring the phase evolution, crystallite growth, and Li-loss during heat treatment.•Determination of the optical band gap and transition mechanism.•Investigations of the magnetic behaviour of nano and bulk Li0.5Fe2.5O4 from 3 ​K to 300 ​K.•Examination of phase transitions via DSC and dilatometry depending on particle size.