| Summary: | The physical properties of the cubic and ferrimagnetic spinel ferrite LiFe<sub>5</sub>O<sub>8</sub> has made it an attractive material for electronic and medical applications. In this work, LiFe<sub>5</sub>O<sub>8</sub> nanosized crystallites were synthesized by a novel and eco-friendly sol-gel process, by using powder coconut water as a mediated reaction medium. The dried powders were heat-treated (HT) at temperatures between 400 and 1000 °C, and their structure, morphology, electrical and magnetic characteristics, cytotoxicity, and magnetic hyperthermia assays were performed. The heat treatment of the LiFe<sub>5</sub>O<sub>8</sub> powder tunes the crystallite sizes between 50 nm and 200 nm. When increasing the temperature of the HT, secondary phases start to form. The dielectric analysis revealed, at 300 K and 10 kHz, an increase of <inline-formula><math display="inline"><semantics><mrow><msup><mi>ε</mi><mo>′</mo></msup></mrow></semantics></math></inline-formula> (≈10 up to ≈14) with a <inline-formula><math display="inline"><semantics><mrow><mi>t</mi><mi>a</mi><mi>n</mi></mrow></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mi>δ</mi></semantics></math></inline-formula> almost constant (≈0.3) with the increase of the HT temperature. The cytotoxicity results reveal, for concentrations below 2.5 mg/mL, that all samples have a non-cytotoxicity property. The sample heat-treated at 1000 °C, which revealed hysteresis and magnetic saturation of 73 emu g<sup>−1</sup> at 300 K, showed a heating profile adequate for magnetic hyperthermia applications, showing the potential for biomedical applications.
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