Experimental heat capacities, excess entropies, and magnetic properties of bulk and nano Fe₃O₄-Co₃O₄ and Fe₃O₄-Mn₃O₄ spinel solid solutions

We have measured the heat capacities of several well-characterized bulk and nanophase Fe₃O₄-Co₃O₄ and Fe₃O₄-Mn₃O₄ spinel solid solution samples from which magnetic properties of transitions and third-law entropies have been determined. The magnetic transitions show several features common to effects of particle and magnetic domain sizes. From the standard molar entropies, excess entropies of mixing have been generated for these solid solutions and compared with configurational entropies determined previously by assuming appropriate cation and valence distributions. The vibrational and magnetic excess entropies for bulk materials are comparable in magnitude to the respective configurational entropies indicating that excess entropies of mixing must be included when analyzing entropies of mixing. The excess entropies for nanophase materials are even larger than the configurational entropies. Changes in valence, cation distribution, bonding and microstructure between the mixing ions are the likely sources of the positive excess entropies of mixing.