Defining the Sonochemical Threshold for Phase-pure Fe3O4 Nanoparticle Synthesis

This study systematically examines the sonochemical synthesis of Fe 3 O 4 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 Fe 3 O 4 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 Fe 3 O 4 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 Fe 3 O 4 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 Fe 3 O 4 synthesis and clarify how incomplete precursor conversion biases both structural and hyperthermia analyses. • Complete sonochemical Fe(OH) 2 conversion is required for phase-pure Fe 3 O 4 . • Irradiation time and precursor amount control phase purity, size, and faceting. • Incomplete Fe(OH) 2 conversion causes post-synthesis wash Fe 3 O 4 formation. • In sonochemical phase-pure Fe 3 O 4 , heating follows size; in defect-rich ones, strain.