The iron oxides and iron oxyhydroxides exist as several different polymorphs, and a thermodynamic understanding of these polymorphs can provide us with an understanding of their relative stability and chemical reactivity. This study provides heat capacity measurements for lepidocrocite (γ-FeOOH) over the temperature range (0.8 to 38) K and akaganéite (β-FeOOH) over the range (0.7 to 302) K. Fits of the heat capacity of the two samples below T = 15 K showed similar behavior to previously published fits of goethite (α-FeOOH), which required a linear term and an anisotropic gap parameter to model accurately the antiferromagnetic spin–wave contributions. The akaganéite measurements were compared to previously reported measurements all of which showed significant disagreement. It is believed that the measurements reported here are the most reliable. Also, the presence of adsorbed water contributes significantly to the heat capacity of akaganéite, and the standard molar entropy at T = 298.15 K of the hydrated form was calculated to be (81.8 ± 2) J · mol−1 · K−1.

The Verwey transition in magnetite (Fe3O4) has been studied extensively with a wide assortment of experimental techniques to investigate the effects of impurities, oxygen stoichiometry, and crystal quality; however, no studies have tested the effects of crystal size on the Verwey transition. In this study, the heat capacity of a magnetite powder with an average crystallite size of 13 nm was measured in the temperature range of 0.5−350 K. No obvious anomaly was observed in the heat capacity in the vicinity of 120 K, yet the measurements exhibited unusual thermal behaviors between 50 and 90 K with relaxation times increasing from an average of 25 min to as much as 5 h in this temperature interval. The behavior is typical of a phase transition, so the lack of a distinct anomaly suggests either the phenomenon is spread out over a large temperature interval with little enthalpy or an insulating phase with different thermal conductivities appears. The heat capacity below 90 K was dependent on cooling rate with an inconsistent anomaly found below 4 K. The results suggest that the Verwey transition has a particle-size dependence. Additionally, theoretical fits below 15 K suggest that magnetite nanoparticles display anisotropic ferrimagnetic behavior and a superparamagnetic contribution to the heat capacity which are properties not observed in bulk magnetite.