γ-alumina $(\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3)$ derived from boehmite has historically been described as having a cubic spinel structure with $\mathrm{Fd}3\mathrm{}¯m$ symmetry, despite reports of tetragonal distortion in the structure. Based on neutron diffraction, transmission electron microscopy, and magic angle spinning NMR data, we propose a tetragonal model for the structure of boehmite-derived $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3$ with ${I4\mathrm{}}_1/amd$ space group symmetry, a maximal subgroup of $\mathrm{Fd}3\mathrm{}¯m.\mathrm{}$ It is also demonstrated that an accurate average structural model cannot be achieved if the cations are restricted to spinel positions.

We have performed an extensive computational study of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$, beginning with the geometric analysis of approximately 1.47 billion spinel-based structural candidates, followed by derivative method energy minimization calculations of approximately 122 000 structures. Optimization of the spinel-based structural models demonstrated that structures exhibiting nonspinel site occupancy after simulation were more energetically favorable, as suggested in other computational studies. More importantly, none of the spinel structures exhibited simulated diffraction patterns that were characteristic of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$. This suggests that cations of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ are not exclusively held in spinel positions, that the spinel model of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ does not accurately reflect its structure, and that a representative structure cannot be achieved from molecular modeling when the spinel representation is used as the starting structure. The latter two of these three findings are extremely important when trying to accurately model the structure. A second set of starting models were generated with a large number of cations occupying $c$ symmetry positions, based on the findings from recent experiments. Optimization of the new $c$ symmetry-based structural models resulted in simulated diffraction patterns that were characteristic of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$. The modeling, conducted using supercells, yields a more accurate and complete determination of the defect structure of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ than can be achieved with current experimental techniques. The results show that on average over 40% of the cations in the structure occupy nonspinel positions, and approximately two-thirds of these occupy $c$ symmetry positions. The structures exhibit variable occupancy in the site positions that follow local symmetry exclusion rules. This variation was predominantly represented by a migration of cations away from $a$ symmetry positions to other tetrahedral site positions during optimization which were found not to affect the diffraction pattern. This study has provided further insight of the defect structure of $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ which is necessary for the understanding and optimization of properties. This work also demonstrates the advantages of prior use of geometric analysis and interatomic potentials to assess a large number of structural possibilities, before striving to achieve high accuracy with DFT on promising cases.

We analyze the mechanisms of contrast formation in noncontact SFM imaging of ionic surfaces and calculate constant frequency shift scanlines of the perfect surfaces of NaCl, MgO, and LiF. The noncontact scanning force microscopy (SFM) operation is modeled by a perturbed oscillator using atomistic static and molecular-dynamics techniques for the force-field calculations. The electrostatic potentials of silicon tips contaminated by various atoms and that of a MgO tip are calculated using a periodic density-functional theory (DFT) method. Their analysis demonstrates that the presence of polar groups or chemisorbed species, such as oxygen atoms, makes the electrostatic forces acting on the surface ions from the Si tip one of the most important contributions to the image contrast. The $(\mathrm{MgO}{)}_{32}$ cube model of the nanotip was found to be representative of a wide class of polar tips and used in the image calculations. The results of these calculations demonstrate that the contrast in noncontact SFM imaging of ionic surfaces is based on an interplay of the electrostatic and van der Waals forces. The main contributions to the contrast formation result from the interaction of the tip with the alternating surface potential and with the surface polarization induced by the electric field of the tip. The results emphasize the importance of the tip-induced relaxation of the surface ions in the tip-surface interaction and in image contrast. The noncontact SFM image of the ${\mathrm{Mg}}^{2+}$-cation vacancy defect on the LiF surface is calculated using the same method.

Crystal structures of four new coumarin polymorphs were solved by crystal structure prediction method and their lattice and free energies were calculated by advanced techniques.