Thermoanalytical, optical, and magnetic investigations on nanocrystalline Li0.5Fe2.5O4 and resulting ceramics prepared by a starch-based softchemistry 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. Fielddependent measurements at 3 K of ceramics sintered between 1050 and 1200 °C show increasing saturation magnetization values (Ms) of 70.0 to 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