Defining the sonochemical threshold for phase-pure Fe3O4 nanoparticle synthesis

This study systematically examines the sonochemical synthesis of Fe₃O₄ nanoparticles as a function of precursor concentration, alkalinity, reagent-addition mode, and effective irradiation time. The central result is that complete Fe(OH)₂ conversion into Fe₃O₄ during sonication determines phase purity and reproducibility. When this threshold is not reached, residual precursor continues to transform during post-synthesis handling, so washing acts not as a passive purification step but as a secondary conversion stage in incompletely converted systems. For phase-pure samples obtained after complete sonochemical conversion, faceted Fe₃O₄ nanoparticles with tunable sizes from 18(6) to 60(20) nm were reproducibly obtained. In these samples, SLP follows the expected size dependence under fixed AMF conditions. In contrast, for incomplete sonochemical conversion systems, the evolving Fe₃O₄ fraction makes SLP normalization unstable, and the absorbed power normalized by total iron (SPI) provides a more robust metric. Across these dynamic systems, SPI correlates with XRD microstrain, indicating that structurally defective secondary magnetite formed during post-irradiation conversion correlates to heating efficiency. These results define the process window required for reproducible sonochemical Fe₃O₄ synthesis and clarify how incomplete precursor conversion biases both structural and hyperthermia analyses.