The geometry, electronic, and structural properties of an unusually stable boron cage made of 80 boron atoms are studied, using ab initio calculations. The shape of this cluster is very similar to that of the well-known ${\mathrm{C}}_{60}$ fullerene, but in the ${\mathrm{B}}_{80}$ case, there is an additional atom in the center of each hexagon. The resulting cage preserves the $I_h$ symmetry, has a relatively large highest occupied and lowest unoccupied energy gap ($\sim 1\mathrm{eV}$) and, most importantly, is energetically more stable than boron double rings, which were detected in experiments and considered as building blocks of boron nanotubes. To our knowledge, this is the most stable boron cage studied so far.

A family of unusually stable boron cages was identified and examined using first-principles local-density functional method. The structure of the fullerenes is similar to that of the B₁₂icosahedron and consists of six crossing double-rings. The energetically most stable fullerene is made up of 180 boron atoms. A connection between the fullerene family and its precursors, boron sheets, is made. We show that the most stable boron sheets are not necessarily precursors of very stable boron cages. Our finding is a step forward in the understanding of the structure of the recently produced boron nanotubes.

A first-principles density functional theory\char21{}generalized gradient approximation DFT-GGA+U study of the doping of bulk GaN and its surface with Cr, Mn, and Fe confirms a strong tendency of these magnetic ions, occupying Ga sites of the wurtzite and zinc blende phases of GaN, to form embedded clusters. Our study reveals that the tendency for aggregation is larger in the case of Cr and Mn ions than for Fe ions. In contrast to previous DFT-GGA calculations, we predict a ferromagnetic ordering of the Cr and Mn clusters consisting of more than two atoms, whereas it turns out that the Fe clusters are all antiferromagnetic. We have also investigated the magnetic ordering of the nearest-neighbor ionic pairs that substitute gallium atoms at the (0001) wz-GaN surface. We find that Fe ions tend to aggregate, whereas there is a mutual repulsive interaction in the case of Cr and Mn. The nearest neighbor Mn and Fe pairs are coupled antiferromagnetically, whereas the Cr pair is coupled ferromagnetically. Further, the relevance of our findings to recent experimental results is discussed.

Electronic structure of GaAs with a 3% addition of B and N is studied by ab initio molecular dynamics. We find that the influence of B on the band gap and on its pressure dependence is weaker than that of N. Hydrostatic pressure strongly reduces the oscillator strength of the fundamental transition in GaAs:B, but not in GaAs:N. Group-theoretical analysis shows that the differences are symmetry induced, and stem from the different substitution sites of B (cation sublattice) and N (anion sublattice). For both alloys we find an additional optical transition at about 0.5 eV above the band gap involving a state derived from $L_{1c};$ its features agree with these of the $E_{+}$ transition in GaAs:N.

First-principles theory is used to calculate the interactions between interstitial iron or the iron-boron pair and a vacancy, between interstitial iron and a divacancy, oxygen-vacancy pair, self-interstitial, and interstitial carbon, as well as substitutional carbon, interstitial oxygen, and the oxygen dimer in silicon. The structures, charge and spin states, binding energies, and, in some cases, vibrational spectra are predicted. The gap levels are estimated using the marker method. The strongest interactions involve vacancies, result in the formation of pairs of metastable defects, and profoundly change the electrical activity of the component species. Iron also traps at self-interstitial-type defects, but not at interstitial oxygen or substitutional carbon. Several observed but incompletely characterized defects are identified and trends in the behavior of Fe in Si are discussed.

Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and ab initio studies, that the codoping of GaN:Mn with Mg allows to control the Mnⁿ⁺ charge and spin state in the range 3≤n≤5 and 2≥S≥1. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mg_k, where the number of ligands k is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.