Phosphorus displays fascinating structural diversity and the discovery of new modifications continues to attract attention. In this work, a complete stability range of known and novel crystalline allotropes of phosphorus is described for the first time. This includes recently discovered tubular modifications and the prediction of not-yet-known crystal structures of [P12] nanorods and not-yet-isolated [P14] nanorods. Despite significant structural differences, all P allotropes consist of covalent substructures, which are held together by van der Waals interactions. Their correct reproduction by ab initio calculations is a core issue of current research. While some predictions with the established DFT functionals GGA and LDA differ significantly from experimental data in the description of the P allotropes, consistently excellent agreement with the GGA-D2 approach is used to predict the solid structures of the P nanorods.

In this work the electronic structure and mechanical properties of the phases X(2)BC with X =Ti, V, Zr, Nb, Mo, Hf, Ta, W (Mo(2)BC-prototype) were studied using ab initio calculations. As the valence electron concentration (VEC) per atom is increased by substitution of the transition metal X, the six very strong bonds between the transition metal and the carbon shift to lower energies relative to the Fermi level, thereby increasing the bulk modulus to values of up to 350 GPa, which corresponds to 93% of the value reported for c-BN. Systems with higher VEC appear to be ductile as inferred from both the more positive Cauchy pressure and the larger value of the bulk to shear modulus ratio (B/G). The more ductile behavior is a result of the more delocalized interatomic interactions due to larger orbital overlap in smaller unit cells. The calculated phase stabilities show an increasing trend as the VEC is decreased. This rather unusual combination of high stiffness and moderate ductility renders X(2)BC compounds with X = Ta, Mo and W as promising candidates for protection of cutting and forming tools.

The newly synthesized boride Ti(1+x)Os(2-x)RuB(2) (x = 0.6) has a novel structure featuring one-dimensional chains of titanium atoms, one-dimensional strings of face-sharing empty tetrahedral and square pyramidal clusters and, most importantly, trigonal planar and strongly bonded B4 units with a B-B distance of 1.89 A.

Selected allotropes of phosphorus are investigated by different levels of density functional theory (DFT) calculations to evaluate the relative stability orders with a special focus on the role of van der Waals interactions. Phosphorus is an excellent reference system with a large number of allotropes. Starting from low-dimensional molecular (0D, white P) and polymer structures (1D, P nanorods) to layered (2D, black P) and tubular structures (2D and 3D, crystalline forms of red P), covalent structure motifs are interconnected by van der Waals interactions. They are a key factor for the correct energetic description of all P allotropes. A comparative study is carried out within the local density approximation (LDA) and the generalized gradient approximation (GGA), with and without implementation of a dispersion correction by Grimme (GGA-D2). Our intention is to achieve a reasonable agreement of our calculations with experimental data, the plausibility of energy values, and the treatment of long-range interactions. The effect of van der Waals interactions is exemplified for the interlayer distances of black phosphorous and its electronic structure.

Predictions confirmed: The phase RhFe3N (see unit cell), which was recently predicted by total‐energy density functional calculations and proposed to have exciting magnetic properties, was synthesized for the first time. The experimental lattice parameter is in good agreement with the theoretical prediction, and the phase appears to be a semihard itinerant ferromagnet. The atomic magnetic saturation moment, ${{\mu {{{\rm {\rm s}}\hfill \atop {\rm {\rm a}}\hfill}}}}$, is 8.3 μB per formula unit, and the Curie temperature, TC, is 505(25) K.