Composition- and calcination-engineered ZnNi ferrites /hematite multiphase powders for tunable magnetic and visible-light photocatalytic performance

Zn-substituted nickel ferrites/hematite multiphase powders with nominal compositions Zn x Ni 1-x Fe 2 O 4 (x = 0–1.0) were synthesized by a co-precipitation route to investigate the combined effects of composition and calcination temperature on phase constitution, morphology, optical response, magnetic behavior, and photocatalytic activity. Quantitative Rietveld refinement of the XRD patterns revealed hematite-rich products containing 74.68–93.25 wt% α-Fe 2 O 3 and 6.75–25.32 wt% cubic spinel ferrite, depending on nominal Zn content. Representative EDS analyses confirmed the expected Fe, O, and Ni signals and the appearance of Zn in the Zn-containing samples, with the Zn/(Zn + Ni) atomic fraction increasing systematically with nominal Zn substitution. Among the investigated compositions, Zn 0.3 Ni 0.7 Fe 2 O 4 exhibited the highest saturation magnetization (4.1705 emu.g −1 ) and the highest apparent pseudo-first-order rate constant for Rhodamine B degradation (k = 0.0335 min −1 ). The temperature-dependent study of the x = 0.3 sample showed that calcination at 800 °C produced the strongest magnetic response and the highest RhB degradation efficiency (95.2% after 60 min) within the investigated temperature range. The combined results demonstrate that photocatalytic performance is not governed by apparent particle size or the fraction of a single crystalline phase alone, but is associated with coupled changes in phase constitution, local structure, morphology, and optical response.