We explored how switching the transition metal (T = Cu, Sn, Co, Ni) in T0.4Mn0.6Fe2O4 nanocomposites (NCs)
controls their structure and functionality. The structure changes from a monoclinic T2Mn3O8 phase for Cu and Co
to a cubic TMn2O4 phase for Sn and Ni. This structural difference underlies a major property divergence. Cu and
Sn NCs possess higher surface area (up to 67.86 m2/g), greater charge carrier concentration, and lower band gaps
(Eg2 ≈ 1.5–1.65 eV), leading to superior photocatalytic degradation of methylene blue (62.78 % and 60.73 %
efficiency). Conversely, Co and Ni NCs, though poorer photocatalysts, display a high q-factor (5 × 105 for Co)
and higher oscillator energies, marking them as promising candidates for energy storage applications. The
photoluminescence spectra for all NCs show violet, blue, and green emissions, with lower intensity for Co and Ni,
suggesting more defect-related recombination. The collective evidence confirms that non-magnetic Cu and Sn
ions enhance photocatalytic activity for environmental cleanup, while magnetic Co and Ni ions favor properties
needed for electronic charge storage, providing a definitive guide for tailoring these materials.