To better visualize if the tuned composition could result in a single or core/shell MNP structure as previously reported, we used STEM-Energy-Dispersive X-ray Spectroscopy for selected MnxFe3-xO4 MNPs with the lowest, medium, and highest content of Mn 2+ (xEmpiric = 0.14, 0.70 and 1.40 respectively) (Figure ). The chemical maps showed that the Mn and Fe signals were homogeneously distributed along the magnetic cores, indicating the absence of a core-shell structure and, thus, the successful synthesis of a unique MnxFe3-xO4 core in agreement with several reports . For further analysis, we selected samples with increasing Mn 2+ content, that is, xempiric = 0.14, 0.23, 0.37, 0.47, 0.70, and 1.40. Figure shows the evolution of the XRD patterns of these MNPs together with the positions of the peaks corresponding to the MnFe2O4 spinel-type phase (PDF file 96-230-0619 in the ICDD powder diffraction file database). Interestingly, the presence of additional reflections (denoted with an asterisk) corresponding to the Fe1-xO phase (PDF file 96-101-1199) appeared in the samples with Mn 2+ content x ≥ 0.70. The coexistence of both magnetic phases can have a high impact on the structural, magnetic, and heating properties of ferrite MNPs, as previously reported . As expected, we found a gradual shift toward lower angles while increasing the Mn 2+ content from 35.43 (xEmpiric = 0.14) to 35.26 (xEmpiric = 1.40), taking as reference the reflection (311). This corresponds to an increase in the cubic unit cell parameter due to the substitution of host ions by Mn 2+ and probably to the appearance of strains inside the spinel structure . To further confirm this hypothesis, we calculated the strain (ε) component inside the spinel structure following the Williamson-Hall (W-H) method by XRD patterns and measured the lattice distance for selected samples by HRTEM . As shown in the HRTEM images (Figure ), the inverse FFT of the (220) plane distance increased from 0.286 nm (xEmpiric = 0.23) to 0.290 nm (xEmpiric = 0.37), in agreement with the XRD patterns. On the other hand, from the fitting of the W-H plot, we found that when Mn 2+ content increased from xEmpiric = 0.14 up to 1.40, the lattice strain increased from 0.20 x 10 -3 to 2.12 x 10 -3 (Table in the S.I.), indicating a larger lattice spacing due to the introduction of Mn 2+ . This ultimately leads to the appearance of uniform tensile strain , which corresponds to the left shift shown in the XRD patterns (Figure ). Although the lattice defects varied among the samples, the effect was larger for the sample with the highest content of Mn 2+ (Mn1.40Fe1.60O4).