Characterization of The Lithium−Manganese Ferrite LiFeMnO4 Prepared by Two Different Methods

Lithium−manganese ferrite LiFeMnO4 was prepared using two different methods: a conventional ceramic high-temperature solid-state reaction technique (Cer) and thermal decomposition of metal nitrate salts (NTD). The characterization of the compounds was carried out by SEM/EDX, XRD, XPS, Fe K- and Mn K-edge XANES, and Mössbauer spectroscopy. Both Cer and NTD LiFeMnO4 samples have the nominal expected Fe/Mn atomic ratio and show a homogeneous morphology, but they exhibit different particle sizes. Fe K-edge XANES and Mössbauer spectroscopy results show that the oxidation state of Fe ions is +3 in both samples, whereas the Mn K-edge XANES data indicate that the bulk average Mn oxidation state is +3 for the NTD sample and close to +4 for the Cer one. Thus, to maintain the charge neutrality, the NTD sample has to be nonstoichiometric in oxygen with a composition close to LiFeMnO3.5. The Mn 3s XPS data indicate that the average surface oxidation state of Mn is also lower in the NTD sample. The results suggest the occurrence of small Fe clusters inside the spinel-related structure in both samples. The Fe cluster occurrence can be due to the presence of diamagnetic Li+ ions, which share both tetrahedral and octahedral sites with the Fe3+ ions. This can result in different configurations around the Fe-occupied sites such that some of the Fe3+ ions located at the octahedral sites remain in a paramagnetic state at 298 K and, therefore, are responsible for the doublet observed in the corresponding Mössbauer spectrum. The neighboring Mn-rich regions containing Mn3+ or Mn4+ ions in octahedral positions also modify the existing magnetic interactions in such a way that they become more complex in the NTD sample than in the Cer one, which is reflected in a more marked superparamagnetic-like behavior. This suggests the existence in the NTD sample of a larger amount of Fe clusters, which can be explained by the different oxidation states of Mn, a larger number of oxygen vacancies, and a higher number of Fe sites with reduced coordination in this material. Finally, the differences found in the chemical and structural properties of Cer and NTD samples can be due not only to variations in thermal treatments used in each synthesis procedure but also to the different nature of the starting products in both methods.