Structures of Mn clusters

Abstract: Subnanometric transition-metal (TM) clusters have attracted great attention due to their unexpected physical and chemical properties, leastwise compared to their bulk counterparts. An in-depth understanding of the evolution of the properties as a function of the number of atoms for such systems is a basic prerequisite to leverage countless applications, from catalysis to magnetic storage, as well as to answer fundamental questions related to their intrinsic stability. Here, we reported a systematic density functional study to investigate the structural, electronic properties and stability of all TMn (30 elements) unary clusters as a function of the number of atoms (n = 2-15). We provided the complete structural patterns for all TM periodic table groups, considering the growth evolution as well as the main trends of the structural and electronic properties. The combination of the occupation of the bonding/anti-bonding d-states and the s-d hybridization is found to be the main stabilization mechanism, helping in the understanding of the structural patterns. Most TMn clusters have a magic number of atoms, for which there are peaks in s-d hybridization and null electric dipole moments. Thus, our extensive and comparative study addresses size effects along with the evolution of d-orbital occupation for the TMn gas-phase cluster properties.

Abstract: Using first-principles density-functional-theory-based calculations, we analyze the structural stability of small clusters of 3d late transition metals. We consider the relative stability of the two structures: layer-like structures with hexagonal closed packed stacking and more compact structures of icosahedral symmetry. We find that the Co clusters show an unusual stability in hexagonal symmetry compared to the small clusters of other members, which are found to stabilize in icosahedral-symmetry-based structure. Our study reveals that this is driven by the interplay between the magnetic-energy gain and the gain in covalency through the s-d hybridization effect. Although we have focused our study primarily on clusters with 19 atoms, we find this behavior to be general for clusters with between 15 and 20 atoms.

Abstract: We present the results of first-principles molecular orbital calculations describing the interaction of metallic nanoparticles, represented by Mn13, Ag13, and Al13 atomic clusters, with a biologically active molecule, dopamine The interaction strength, determined in terms of the nanoparticle−molecule complex binding energy, is found to be higher for Mn than either Ag or Al and can be explained in terms of the degree of the hybridization of the (metal) atomic orbitals with the molecular orbitals in the complex Furthermore, smaller interaction strength of these metallic nanoparticles with water compared to that with dopamine predicts the preference of forming a complex of dopamine with the metallic nanoparticles in the aqueous solution The calculated results may therefore suggest that the presence of these metallic nanoparticles could induce different levels of dopamine depletion in solution

Abstract: Using first-principles density-functional-theory--based calculations, we analyze the structural stability of small clusters of 3$d$ late transition metals. We consider the relative stability of the two structures: layer-like structures with hexagonal closed packed stacking and more compact structures of icosahedral symmetry. We find that the Co clusters show an unusual stability in hexagonal symmetry compared to the small clusters of other members, which are found to stabilize in icosahedral-symmetry--based structure. Our study reveals that this is driven by the interplay between the magnetic-energy gain and the gain in covalency through the $s$-$d$ hybridization effect. Although we have focused our study primarily on clusters with 19 atoms, we find this behavior to be general for clusters with between 15 and 20 atoms.

Abstract: With the goal of achieving an understanding of the properties of bimetallic alloy clusters having atoms of two isoelectronic elements, we have studied the structural, electronic and magnetic properties of MnmTcn, MnmRen and TimZrn clusters with m?+?n?=?13 (n?=?0, 1, 4, 6, 9, 12, 13), using first-principles density functional calculations. MnmTcn and MnmRen represent clusters of isoelectronic series with a half-filled d shell, while TimZrn represents an isoelectronic cluster series of early transition metals. Mn-rich alloy clusters are found to prefer compact structures and isoelectronic Tc-rich or Re-rich alloy clusters are found to adopt open structures. In contrast, TimZrn clusters are all found to stabilize in compact structures, irrespective of being Ti-rich or Zr-rich. This change in behavior between two isoelectronic series is found to be driven by differences in hybridization effects, due to differences in the evolution of the relative energy positions of the d level with respect to the s and p levels upon moving from 3d to 4d or 5d elements. This effect further competes with the magnetization effect to decide the morphology of the alloy clusters. Focusing on the magnetic properties of the studied clusters, we find that the single Tc atom substituted alloy cluster exhibits markedly improved magnetic properties compared to that of pure Mn clusters.

Abstract: We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order Natoms2 scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special ‘preconditioning’ optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.

Abstract: We propose a simple analytic representation of the correlation energy ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$ for a uniform electron gas, as a function of density parameter ${\mathit{r}}_{\mathit{s}}$ and relative spin polarization \ensuremath{\zeta}. Within the random-phase approximation (RPA), this representation allows for the ${\mathit{r}}_{\mathit{s}}^{\mathrm{\ensuremath{-}}3/4}$ behavior as ${\mathit{r}}_{\mathit{s}}$\ensuremath{\rightarrow}\ensuremath{\infty}. Close agreement with numerical RPA values for ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$,0), ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$,1), and the spin stiffness ${\mathrm{\ensuremath{\alpha}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$)=${\mathrm{\ensuremath{\partial}}}^{2}$${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{c}}$(${\mathit{r}}_{\mathit{s}}$, \ensuremath{\zeta}=0)/\ensuremath{\delta}${\mathrm{\ensuremath{\zeta}}}^{2}$, and recovery of the correct ${\mathit{r}}_{\mathit{s}}$ln${\mathit{r}}_{\mathit{s}}$ term for ${\mathit{r}}_{\mathit{s}}$\ensuremath{\rightarrow}0, indicate the appropriateness of the chosen analytic form. Beyond RPA, different parameters for the same analytic form are found by fitting to the Green's-function Monte Carlo data of Ceperley and Alder [Phys. Rev. Lett. 45, 566 (1980)], taking into account data uncertainties that have been ignored in earlier fits by Vosko, Wilk, and Nusair (VWN) [Can. J. Phys. 58, 1200 (1980)] or by Perdew and Zunger (PZ) [Phys. Rev. B 23, 5048 (1981)]. While we confirm the practical accuracy of the VWN and PZ representations, we eliminate some minor problems with these forms. We study the \ensuremath{\zeta}-dependent coefficients in the high- and low-density expansions, and the ${\mathit{r}}_{\mathit{s}}$-dependent spin susceptibility. We also present a conjecture for the exact low-density limit. The correlation potential ${\mathrm{\ensuremath{\mu}}}_{\mathit{c}}^{\mathrm{\ensuremath{\sigma}}}$(${\mathit{r}}_{\mathit{s}}$,\ensuremath{\zeta}) is evaluated for use in self-consistent density-functional calculations.

Abstract: A molecular beam of manganese clusters, Mn (n) (n = 11-99), produced at 68 K is deflected toward high field by a gradient field magnet. These results indicate that Mn (n) clusters in this size range are superparamagnetic species whose intrinsic moments can be determined within the framework of the Langevin model of paramagnetism. Local minima in per-atom magnetic moments are observed for Mn (13) and Mn (19), suggestive of an icosahedral growth sequence for the smaller size range. For larger clusters, broad oscillations in the per-atom moments are observed, with a minimum near Mn (32-37) and a maximum around Mn (50-56).

Abstract: Theoretical electronic structure studies on ${\mathrm{Mn}}_{n}$ $(n=2--8)$ clusters have been carried out using a linear-combination-of-atomic-orbitals--molecular-orbital approach within the density-functional formalism It is shown that ${\mathrm{Mn}}_{2}$ and ${\mathrm{Mn}}_{3}$ have energetically close ferromagnetic and antiferromagnetic or frustrated antiferromagnetic solutions ${\mathrm{Mn}}_{4},$ ${\mathrm{Mn}}_{5},$ ${\mathrm{Mn}}_{6},$ ${\mathrm{Mn}}_{7},$ and ${\mathrm{Mn}}_{8}$ are all ferromagnetic with moments of 20, 23, 26, 29, and 32${\ensuremath{\mu}}_{B}$ The appearance of ferromagnetic character is shown to be accompanied by bonding between minority d states The relation between geometry and multiplicity and the possibility of closely spaced multiplet states are discussed

Abstract: The equilibrium geometries, electronic structure and magnetic properties of small Mn clusters consisting of up to five atoms have been calculated self-consistently using first principles molecular orbital theory. The electron-electron interaction has been accounted for using the local spin density and generalized gradient approximation to the density functional theory. The atomic orbitals forming the molecular orbital have been represented separately by Gaussian and numerical basis sets. Two different computer codes (Gaussian 94 and DMOL) were used to check the numerical consistency of our calculations. is found to be a weakly bound van der Waals molecule and its binding energy depends sensitively on the choice of basis set as well as the form of the exchange-correlation potential. The binding energies are less sensitive to these approximations in larger clusters. The binding improves with cluster size, but remains significantly lower than those in other transition metal clusters. The equilibrium geometries are fairly compact and symmetric although other isomers with distorted geometries and with nearly the same energy as that of the ground state do exist for . The clusters also exhibit a variety of low-lying spin multiplicities, but the ground state spin configuration is ferromagnetic with a magnetic moment of . This not only contrasts with its bulk behaviour which is antiferromagnetic, but also differs from the behaviour in other transition-metal clusters where the magnetic moments/atom are always less than the free-atom value. The results are compared with available experiments on matrix isolated Mn clusters.