Narrow Results

The structural, electronic and magnetic properties of the neutral and cationic cobalt oxide clusters (CoO)${}_n^q$ ($n=3-10$, $q=0,+1$) have been studied using a modified basin-hopping Monte Carlo (BHMC) algorithm refined by spin-polarized DFT. A systematic search of global minimum structures predicts new global minima of (CoO)${}_9^{0∕+}$ and reproduced other minima that are in excellent agreement with previous works. For most low-spin and high-spin states, the structural transition from planar-like to compact structure occurs at (CoO)${}_5^{0∕+}$, which is in contrast with the general notion that the structural changes at (CoO)${}_6^{0∕+}$. Supported by the results of the binding energy, second-order total energy difference, chemical hardness, chemical potential and HOMO-LUMO gap confirms the stability of (CoO)₄. Results of the spin magnetic moments for the global minima show that (CoO)₄ and (CoO)₈ spin configurations exhibit a fully antiferromagnetic (AFM) ordering, while (CoO)₉ spin displays the highest ferromagnetic (FM) ordering. Interestingly, elongation of Co–Co bond in (CoO)₄ causes O being polarized by the neighboring Co atoms that accordingly follows the Goodenough-Kanamori-Anderson rule of FM super-exchange coupling for the Co-O-Co structural rearrangement to 90^∘ ($D_{4h}$ structure) in order to accommodate the spin magnetic ordering changes. This rearrangement is a result of the valence band being shifted away from the Fermi level to lower energy causing high population of the spin-up density of state and leading to the asymmetrical polarization of the whole (CoO)₄ structure. As far as the dissociation energy surfaces are concerned, the first ever such surfaces are constructed corresponding to ${(\mathrm{\text{CoO}})}_n^q\to {(\mathrm{\text{CoO}})}_m^q+{(\mathrm{\text{CoO}})}_{(n-m)}$, which identify a complete dissociation pathway linking the cationic and neutral clusters and finally confirm (CoO)${}_4^{0∕+}$ as the most stable cluster compared to the rest.