Iron oxides occur ubiquitously in environmental, geological, planetary, and technological settings. They exist in a rich variety of structures and hydration states. They are commonly fine-grained (nanophase) and poorly crystalline. This review summarizes recently measured thermodynamic data on their formation and surface energies. These data are essential for calculating the thermodynamic stability fields of the various iron oxide and oxyhydroxide phases and understanding their occurrence in natural and anthropogenic environments. The competition between surface enthalpy and the energetics of phase transformation leads to the general conclusion that polymorphs metastable as micrometer-sized or larger crystals can often be thermodynamically stabilized at the nanoscale. Such size-driven crossovers in stability help to explain patterns of occurrence of different iron oxides in nature.

Goethite, R-FeOOH, and hematite, R-Fe 2 O 3 , have high affinity for water, especially when particle size is small. To determine the enthalpy of different types of adsorbed water, we performed water adsorption calorimetry on goethite and hematite with surface areas of 60-270 and 2-150 m 2 /g, respectively, using a new calorimetric technique combining a microcalorimeter and an automated gas dosing system. Several types of strongly bound water can be distinguished on hematite, depending on heating temperature and surface area. These have enthalpies of adsorption relative to liquid water (∆H ads ) equal to -67.1 ( 4.9, -48.6 ( 1.8, and -25.5 ( 4.4 kJ/mol. The last value corresponds to water adsorption on very fine grained hematite and is very close to the water adsorption enthalpy for goethite, ∆H ads ) -19.4 ( 4.2 kJ/mol. Surface enthalpies for anhydrous surfaces of goethite (0.91 ( 0.09 J/m 2 ) and hematite (1.9 ( 0.3 J/m 2 ) determined experimentally are reported for the first time. The significant difference in surface enthalpies of goethite and hematite creates an energy crossover and makes fine-grained hematite metastable relative to goethite.

Akaganeite, β-FeOOH, is a commonly occurring ferric mineral in the environment and is a sorbent, ion exchanger, and catalyst. It is often fine-grained (nanophase) and frequently contains excess water. Its enthalpy of formation was studied by solution calorimetry in aqueous HCl. The enthalpy of water adsorption was studied by a new calorimetric technique combining a Calvet microcalorimeter and an automated gas dosing system, used for surface adsorption measurements. Akaganeite samples with surface areas of 30−280 m2/g were used. Sample characterization was performed by X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer−Emmett−Teller method, scanning electron microscopy, and transmission electron microscopy. Surface enthalpy and enthalpy of water adsorption are reported for the first time. By adsorbing water, akaganeite decreases its effective surface enthalpy from 0.44 J/m2 to 0.34 J/m2. The enthalpy of formation of akaganeite can vary by 10−12 kJ/mol as a function of the surface area. The standard enthalpy of formation of akaganeite with zero surface area was refined and is −554.7 ± 1.9 kJ/mol. Thus, the standard enthalpy of formation and surface enthalpy of akaganeite are between those of goethite and lepidocrocite. The more metastable the polymorph, the lower its surface energy.

Cathodoluminescence real-color imaging and spectroscopy were employed to study the properties of Ga(2)O(3) nanowires grown with different Sn/Ga ratios. The structures grown under Sn-rich conditions show large spectral emission variation, ranging from blue to red, with a green transition zone. Spectral emission changes correlate with changes in the chemical composition and structure found by energy dispersive spectroscopy and electron diffraction. A sharp transition from green to red emission correlates with a phase transition of beta-Ga(2)O(3) to polycrystalline SnO(2). The origin of the green emission band is discussed based on ab initio calculation results.

High-temperature oxide-melt solution calorimetry and acid-solution calorimetry were used to determine the heat of dissolution of synthetic goethite with particle sizes in the range 2–75 nm and measured surface areas of 30–273 m2/g (27–240 × 103 m2/mol). Sample characterization was performed using X-ray diffraction, Fourier transform infrared spectroscopy, the Brunauer, Emmett and Teller method and thermogravimetric analysis. Water content (structural plus excess water) was determined from weight loss after firing at 1100°C. Calorimetric data were corrected for excess water assuming this loosely adsorbed water has the same energetics as bulk liquid water. The enthalpy of formation was calculated from calorimetric data using enthalpies of formation of hematite and liquid water as reference phases for high-temperature oxide-melt calorimetry and using enthalpy of formation of lepidocrocite for acid-solution calorimetry. The enthalpy of formation of goethite can vary by 15–20 kJ/mol as a function of surface area. The plot of calorimetric data vs. surface area gives a surface enthalpy of 0.60±0.10 J/m2 and enthalpy of formation of goethite (with nominal composition FeOOH and surface area = 0) of −561.5±1.5 kJ/mol. This surface enthalpy of goethite, which is lower than values reported previously, clarifies previous inconsistencies between goethite-hematite equilibrium thermodynamics and observations in natural systems.