A. Svane

Effect of van der Waals interactions on the structural and elastic properties of black phosphorus

The structural and elastic properties of orthorhombic black phosphorus have been investigated using first-principles calculations based on density functional theory. The structural parameters have been calculated using the local density approximation (LDA), the generalized gradient approximation (GGA), and with several dispersion corrections to include van der Waals interactions. It is found that the dispersion corrections improve the lattice parameters over LDA and GGA in comparison with experimental results. The calculations reproduce well the experimental trends under pressure and show that van der Waals interactions are most important for the crystallographic b axis in the sense that they have the largest effect on the bonding between the phosphorus layers. The elastic constants are calculated and are found to be in good agreement with experimental values. The calculated C22 elastic constant is significantly larger than the C11 and C33 parameters, implying that black phosphorus is stiffer against strain along the a axis than along the b and c axes. From the calculated elastic constants, the mechanical properties, such as bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio are obtained. The calculated Raman active optical phonon frequencies and their pressure variations are in excellent agreement with available experimental results.

Understanding the valency of rare earths from first-principles theory

The rare-earth metals have high magnetic moments and a diverse range of magnetic structures. Their magnetic properties are determined by the occupancy of the strongly localized 4f electronic shells, while the outer s–d electrons determine the bonding and other electronic properties. Most of the rare-earth atoms are divalent, but generally become trivalent in the metallic state. In some materials, the energy difference between these valence states is small and, by changing some external parameter (such as pressure), a transition from one to the other occurs. But the mechanism underlying this transition and the reason for the differing valence states are not well understood. Here we report first-principles electronic-structure calculations that enable us to determine both the valency and the lattice size as a function of atomic number, and hence understand the valence transitions. We find that there are two types of f electrons: localized core-like f electrons that determine the valency, and delocalized band-like f electrons that are formed through hybridization with the s–d bands and which participate in bonding. The latter are found only in the trivalent systems; if their number exceeds a certain threshold, it becomes energetically favourable for these electrons to localize, causing a transition to a divalent ground state.

Electronic structures of normal and inverse spinel ferrites from first principles

We apply the self-interaction corrected local spin density approximation to study the electronic structure and magnetic properties of the spinel ferrites

First-principles study of elastic properties of CeO2, ThO2 and PoO2

\mathrm{Mn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}

High-pressure structural, elastic, and electronic properties of the scintillator host material KMgF3

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Half-metallic to insulating behavior of rare-earth nitrides

{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}

First principles study of rare-earth oxides

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Lanthanide contraction and magnetism in the heavy rare earth elements

\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}

Electronic structure and ionicity of actinide oxides from first principles

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Elastic properties of MgCNi3 - a superconducting perovskite

\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}