Thermochemistry of iron manganese oxide spinels

Abstract

Oxide melt solution calorimetry has been performed on iron manganese oxide spinels prepared at high temperature. The enthalpy of formation of (Mn x Fe 1−x) 3 O 4 at 298 K from the oxides, tetragonal Mn 3 O 4 (hausmannite) and cubic Fe 3 O 4 (magnetite), is negative from x=0 to x=0.67 and becomes slightly positive for 0.67<x<1.0. Relative to cubic Mn 3 O 4 (stable at high temperature) and cubic Fe 3 O 4 (magnetite), the enthalpy of formation is negative for all compositions. The enthalpy of formation is most negative near x=0.2. There is no significant difference in the trend of enthalpy of formation versus composition for cubic (x<0.6) and tetragonal (x>0.6) spinels of intermediate compositions. The enthalpies of formation are discussed in terms of three factors: oxidation-reduction relative to the end-members, cation distribution, and tetragonality. A combination of measured enthalpies and Gibbs free energies of formation in the literature provides entropies of mixing. ΔS mix , consistent with a cation distribution in which all trivalent manganese is octahedral and all other ions are randomly distributed for x>0.5, but the entropy of mixing appears to be smaller than these predicted values for x<0.4.

Thermodynamics of Fe3O4–Co3O4and Fe3O4–Mn3O4spinel solid solutions at the bulk and nanoscale

“…The values of enthalpy, entropy, and free energy of mixing calculated by Navrotsky are comparable to Muan's experimental values. 25 The more recent experimental determination of the mixing enthalpies of Fe 3 O 4 -Mn 3 O 4 solid solution via high temperature oxide melt solution calorimetry by Guillemet-Fritsch et al 31 is in a reasonable agreement with the earlier calculated mixing properties. 11,21 Despite these earlier studies, separating the effects of enthalpy and entropy on the Gibbs energy of mixing in these complex solid solutions is difficult without independent calorimetric measurements of the enthalpy of mixing, which has only been done for the bulk Mn 3 O 4 -Fe 3 O 4 system.…”

“…appears first order6 , there does not appear to be a well defined two phase region for coexistence of cubic and tetragonal spinel in the solid solutions, rather the tetragonality appears to diminish gradually31 . Thus at intermediate compositions, the transition may be higher order and a single curve representing enthalpy of mixing may be appropriate.…”

Progress and New Directions in Calorimetry: A 2014 Perspective

“…As long as the flow rate through both calorimetric chambers is constant, the baseline remains stable. In addition to controlling oxygen fugacity (bubbling with oxygen, air, nitrogen, or argon), the bubbling stirs the melt, enhancing dissolution and oxidation rates, dispersing the solute particles in the melt, and preventing local saturation). This has greatly extended the range of oxides which can be dissolved to include titania, zirconia, hafnia, and lanthanide and actinide oxides.…”

A Highly Crystalline Manganese‐Doped Iron Oxide Nanocontainer with Predesigned Void Volume and Shape for Theranostic Applications

“…The hollow nanocontainer composition was identified as (Mn 0.2 Fe 0.8 ) 3 O 4 by energy‐dispersive X‐ray spectroscopy (EDAX) analysis (Figure S5,S6, Supporting information). It was previously shown that the enthalpy of formation of (Mn x Fe 1– x ) 3 O 4 ( x = 0 to x = 0.67) from Mn 3 O 4 and Fe 3 O 4 is negative at 298 K, with the minimum enthalpy value near x = 0.2 27. The XPS analysis indicated that both Mn 2+ and Mn 3+ ions were present, implying that the Mn and iron atoms randomly occupied the divalent sites in the spinel structure (Figure S10, Supporting information).…”