ABSTRACT
We present a combined experimental and theoretical investigation targeted at the thermochemical properties of a series of alkaline-earth metal carbodiimides. Their Gibbs energies and decomposition temperatures were calculated on the basis of phonons derived from density functional theory. The theoretical decomposition temperatures arrive at 1270, 1224, and 1185 K for CaNCN, α-SrNCN, and tetragonal BaNCN, respectively. Only the melt of tetragonal BaNCN is maintained at â¼1173 K, which is slightly below its calculated decomposition temperature. Experimentally, the melt of BaNCN did not decompose below 1273 K. On the contrary, both CaNCN and α-SrNCN partially decompose by forming a mixture of their carbides, metals, and nitrogen. The calculated Gibbs energies also show that the tetragonal phase of BaNCN is more stable than the rhombohedral one. We conclude that the melt of BaNCN is useful in the crystal growth of oxynitride perovskites such as BaTaO2N.
ABSTRACT
The high-pressure and low-temperature behaviour of the GeSexTe1-x system (x = 0, 0.2, 0.5, 0.75, 1) was studied using a combination of powder diffraction measurements and first-principles calculations. Compounds in the stability field of the GeTe structure type (x = 0, 0.2, 0.5) follow the high-pressure transition pathway: GeTe-I (R3m) â GeTe-II (f.c.c.) â GeTe-III (Pnma). The newly determined GeTe-III structure is isostructural to ß-GeSe, a high-pressure and high-temperature polymorph of GeSe. Pressure-dependent formation enthalpies and stability regimes of the GeSexTe1-x polymorphs were studied by DFT calculations. Hexagonal Ge4Se3Te is stable up to at least 25â GPa. Significant differences in the high-pressure and low-temperature behaviour of the GeTe-type structures and the hexagonal phase are highlighted. The role of Ge...Ge interactions is elucidated using the crystal orbital Hamilton population method. Finally, a sketch of the high-pressure phase diagram of the system is provided.
ABSTRACT
A computational study of the possibilities of contemporary theoretical chemistry as regards calculated thermodynamic properties for molecular crystals from first-principles is presented. The study is performed for a testing set of 22 low-temperature crystalline phases whose properties such as densities of phonon states, isobaric heat capacities, and densities are computed as functions of temperature within the quasi-harmonic approximation. Electronic structure and lattice dynamics are treated by plane-wave based calculations with optPBE-vdW functional. Comparison of calculated results with reliable critically assessed experimental data is especially emphasized.
ABSTRACT
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.