Structures and bonding characters of (MgO)_3n (n = 2–8) clusters

Abstract

The chemical bonds in clusters may be different to those in the conventional bulk. Bulk magnesium oxide (MgO) is a well-known ionic material with the rocksalt structure. However, the bonding characters of MgO clusters are not clear. (MgO) 3n are the magic-size clusters. The rocksalt structure and the hexagonal tube are two basic structural patterns. For large MgO clusters, the rocksalt structures are the most stable, but for small clusters, the hexagonal tubes are preferable in energy. It was suggested that the orbitals of oxygen in the hexagonal tubes hybrid in sp 2 and the Mg-O bonds possess considerable covalent characters, and the orbital interaction favours the hexagonal tubes. We investigate the geometric and electronic structures of (MgO) 3n (n = 2-8) clusters by ab initio methods. For n = 2-5, the n-stacked hexagonal rings and the 2 × 3 × n rocksalt isomers have the same topological structure and the hexagonal tube is lower in energy. For n = 6 and 8, the rocksalt structures change to 3 × 3 × 4 and 3 × 4 × 4 layers respectively, and they become the most stable configurations. The AIM (atoms in molecules) and NBO (natural bond orbital) analyses show the charge transfer from Mg to O is around 1.78e and 1.52e respectively, which means strongly ionic bonds in MgO clusters. The large charge transfer pushes up the 2p levels drastically in oxygen anions and the wide energy gap hinders the hybridization between the 2s and 2p orbitals of oxygen. In both the rocksalt structures and the hexagonal tubes, the overlap compositions between Mg 3s and O 2p are almost the same, and they do not vary obviously with the cluster size. For small MgO clusters, while there are more Mg-O bonds in the rocksalt structures, the average bond lengths are shorter in the hexagonal tubes, which can compensate for the smaller number of the bonds. For (MgO) 18,24 , many more Mg-O bonds form in the rocksalt structures, and this makes the rocksalt structures preferable to the hexagonal tubes.

Tuning the electronic and chemisorption properties of hexagonal MgO nanotubes by doping – Theoretical study

“…Moreover, due to a lower coordination number of the edge sites, the reduction of the Mg-O bond is more noticeable at the nanotubes edges, in agreement with Ref. [25]. The bond lengths obtained in PBE and PBE+D2 calculations compare well with those calculated using B3LYP with the localized basis set.…”

“…The theoretical investigation of small (MgO) 3k clusters shows that for k < 6, the hexagonal nanotubes are preferred over the rock salt structures [25]. Changes of the chemical properties of such MgO nanotubes (k = 4) was reported by Yang et al [26] who showed that substitutional doping by Ni, Pd and Pt enhanced the CO adsorption properties of hexagonal (MgO) 12 (k = 4) nanotubes.…”

Time-invariant discord in dynamically decoupled systems

“…The use of either DD techniques or non-Markovian effects prolongs the preservation of both entanglement and discord in presence of environmental noise [31][32][33][34][35]. However, studies of single qubit dynamics have recently shown that the simultaneous use of non-Markovian reservoir engineering and DD protocols is counterproductive for coherence preservation [7].…”

Structure Stability and Growth Strategies of the (MgO)n (n=1-13) Clusters

“…The small clusters also show a preference for the hexagonal tubes which are stacked by the (MgO) 3 hexagons, and they have been confirmed by infrared spectra for (MgO) 3n (n = 2-5) [19] and the mass spectrum of MgO clusters [20]. The covalent interaction in small MgO clusters is responsible for the preferable stability of the hexagonal tubes over the rock-salt structures [21]. …”