Claudia Wessel

Ab Initio Thermochemistry of Solid‐State Materials

In this contribution we introduce an electronic-structure-theory-based approach to a quantum-chemical thermochemistry of solids. We first deal with local and collective atomic displacements and explain how to calculate these. The fundamental importance of the phonons, their dispersion relations, their experimental determination as well as their calculation is elucidated, followed by the systematic construction of the thermodynamic potentials on this basis. Subsequently, we provide an introduction for practical computation as well as a critical analysis of the level of accuracy obtainable. We then show how different solid-state chemistry problems can be solved using this approach. Among these are the calculation of activation energies in perovskite-like oxides, but we also consider the use of theoretical vibrational frequencies for determining crystal structures. The pressure and temperature polymorphism of elemental tin which has often been classically described is also treated, and we energetically classify the metastable oxynitrides of tantalum. We also demonstrate, using the case of high-temperature superconductors, that such calculations may be used for an independent evaluation of thermochemical data of unsatisfactory accuracy. Finally, we show the present limits and the future challenges of the theory.

Electronic structure and thermodynamics of V2O3 polymorphs

A metastable bixbyite‐type polymorph of vanadium sesquioxide, V2O3, has recently been synthesized, and it transforms to the corundum‐type phase at temperatures around 550 °C. The possibility of a paramagnetic to canted antiferromagnetic or even spin‐glass‐like transition has been discussed. Quantum‐chemical calculations on the density‐functional theory level including explicit electronic correlation confirm the metastability as well as the semiconducting behavior of the material and predict that the bixbyite‐type structure is about 0.1 eV less stable than the well‐known corundum‐type phase. Nonetheless, quasiharmonic phonon calculations manifest that bixbyite‐type vanadium sesquioxide is a dynamically stable compound. Other possible V2O3 polymorphs are shown to be even less suitable candidates for the composition V2O3.