To provide a complete picture of the energy landscape of Al 2 O 3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al 2 O 3 ). a-Al 2 O 3 nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al 2 O 3 targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 °C to provide powders with surface areas of 670−340 m 2 /g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3−5 nm nanoparticles that remain amorphous to temperatures as high as 600 °C. The structure consists of a network of AlO 4 , AlO 5 , and AlO 6 polyhedra, with AlO 5 being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al 2 O 3 was determined to be 0.97 ± 0.04 J/m 2 . We conclude that the lower surface energy of a-Al 2 O 3 compared with crystalline polymorphs γand α-Al 2 O 3 makes this phase the most energetically stable phase at surface areas greater than 370 m 2 /g.

This work brings insights into the defect chemistry of YBZ solid solutions by measuring enthalpies of formation. We find a correlation between the obtained thermodynamic data and the known trend of the proton conductivity of YBZ solid solutions. This study is important for informed thermodynamic history-based materials selection and processing for specific applications.

The surface enthalpies of nanocrystalline CaTiO 3 and SrTiO 3 perovskites were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The nanocrystalline samples were synthesized by a hydrothermal method and characterized using powder X-ray diffraction, FTIR spectroscopy, and Brunauer-Emmett-Teller surface area measurements. The integral heats of water vapor adsorption on the surfaces of nanocrystalline CaTiO 3 and SrTiO 3 are À78.63 AE 4.71 kJ/mol and À69.97 AE 4.43 kJ/mol, respectively. The energies of the hydrous and anhydrous surfaces are 2.49 AE 0.12 J/m 2 and 2.79 AE 0.13 J/m 2 for CaTiO 3 and 2.55 AE 0.15 J/m 2 and 2.85 AE 0.15 J/m 2 for SrTiO 3 , respectively. The stability of the perovskite compounds in this study is discussed according to the lattice energy and tolerance factor approach. The energetics of different perovskites suggest that the formation enthalpy becomes more exothermic and surface energy increases with an increase in ionic radius of the "A" site cation (Ca, Sr, and Ba), or with the tolerance factor. PbTiO 3 shows a lower surface energy, weaker water binding, and a less exothermic enthalpy of formation than the alkaline-earth perovskites.

Water adsorption on the surface of LiCoO2 nanoparticles was investigated. As the water coverage increases the adsorption enthalpy decreases reaching the enthalpy of water condensation (−44 kJ mol−1). The experimentally observed average surface energy corresponding to all facets agree well with those reported from DFT calculations. The observed low surface energy is attributed to the surface Co3+ spin transition in nanophase LiCoO2.

To explore the surface properties of perovskites with ions of different bond character, the surface and interface enthalpies of nanocrystalline PbTiO 3 and BaTiO 3 perovskites were determined for the first time by a combination of calorimetric, morphological, and structural analyses. PbTiO 3 and BaTiO 3 nanocrystalline samples of varying surface areas and degrees of agglomeration were synthesized by solvothermal and hydrothermal methods, respectively. All synthesized samples were characterized using X-ray diffraction and Raman spectroscopy. Interface areas were estimated by comparing the surface areas measured by N 2 adsorption to the crystallite sizes refined from X-ray diffraction data. The integrated heats of water vapor adsorption on the surfaces of the nanocrystalline phases are À62 ± 4 kJ/mol for PbTiO 3 , which is less exothermic than the value À72 ± 9 kJ/mol for the isostructural BaTiO 3 , both phases having the same chemisorbed water coverage. Similar behavior is observed for the surface and interface enthalpies. The energies of the hydrous and anhydrous surfaces are 1.97 ± 0.67 J/m 2 and 1.11 ± 0.23 J/m 2 for PbTiO 3 , and 3.69 ± 0.22 J/m 2 and 3.99 ± 0.28 J/m 2 for BaTiO 3 , respectively. The interface energies of the hydrous and anhydrous surfaces are 0.55 ± 0.74 J/m 2 and 0.73 ± 0.27 J/m 2 for PbTiO 3 , and 1.11 ± 0.13 J/m 2 for BaTiO 3 . These observations suggest that PbTiO 3 has lower surface energy and lower affinity for water adsorption on the surface than BaTiO 3 and that surface energy and hydrophilicity of the surface decrease with increasing covalent character of the ions, as was seen previously in comparing TiO 2 and SnO 2 .

Pyrochlore, an ordered derivative of the defect fluorite structure, shows complex disordering behavior as a function of composition, temperature, pressure, and radiation damage. We propose a thermodynamic model to calculate the disordering enthalpies for several RE2Zr2O7 (RE = Sm, Eu, Gd) pyrochlores from experimental site distribution data obtained by in situ high temperature synchrotron X-ray diffraction. Site occupancies show a gradual increase in disorder on both cation and anion sublattices with increasing temperature and even greater disorder is achieved close to the phase transition to defect fluorite. The enthalpy associated with cation disorder depends on the radius of the rare earth ion, while the enthalpy of oxygen disordering is relatively constant for different compositions. The experimental data support trends predicted by ab initio calculations, but the obtained enthalpies of disordering are less endothermic than the predicted values. Thermal expansion coefficients are in the range (8.6–10.8) × 10−6 K−1. These new experimental determinations of defect formation energies are important for understanding the stability of pyrochlore oxides and their disordering mechanisms, which are essential in the context of their potential applications in nuclear waste management and other technologies.

The accurate determination of structure and thermal expansion of refractory materials at temperatures above 1500°C is challenging. Here, for the first time, we demonstrate the ability to reliably refine the structure and thermal expansion coefficient of oxides at temperatures to 2200°C using in situ synchrotron diffraction coupled with aerodynamic levitation. Solid solutions in the Eu 2 O 3 -ZrO 2 binary system were investigated, including the high-temperature order-disorder transformation in Eu 2 Zr 2 O 7 . The disordered fluorite phase is found to be stable above 1900°C, and a reversible phase transition to the pyrochlore phase is noticed during cooling. Site occupancies in Eu 2 Zr 2 O 7 show a gradual increase in disorder on both cation and anion sublattices with increasing temperature. The thermal expansion coefficients of all cubic solid solutions are relatively similar, falling in the range 8.6-12.0 3 10 -6 C -1 . These studies open new vistas for in situ exploration of complex structural changes in high-temperature materials.

The formation enthalpies of perovskite-structured lithium lanthanum titanate (LLTO) Li3xLa0.67−xTiO3 (compositions x = 0.04 to 0.15) are exothermic and subject to two regimes with structural transformation.