Geometrical, electronic, and magnetic properties of small AunSc (n=1-8) clusters

Geometrical, electronic, and magnetic properties of the Sc-doped gold clusters, Au_nSc (n=1-8), have been studied using the density-functional theory within the generalized gradient approximation. An extensive structural search shows that the Sc atom in low-energy Au_nSc isomers tends to occupy the most highly coordinated position. The substitution of a Sc atom for an Au atom in the Au_n+1 cluster markedly changes the structure of the host cluster. Moreover, we confirm that the ground-state Au₆Sc cluster has a distortion to a lower D_2h symmetry. The relative stabilities and electronic properties of the lowest-energy Au_nSc clusters are analyzed based on the averaged binding energies, second-order energy differences, fragmentation energies, chemical hardnesses, and HOMO-LUMO gaps. It is found that the magic Au₃Sc cluster can be perceived as a superatom with high chemical stability and its HOMO-LUMO gap is larger than that of the closed-shell Zr@Au₁₄ cluster. The high symmetry and spin multiplicity of the Au₃Sc and Au₆Sc clusters are responsible for their large vertical ionization potential and electron affinity. The magnetism calculations indicate that the magnetic moment of the Sc atom in the ground-state Au_nSc (n=2-8) clusters gradually decreases for even n and is completely quenched for odd n.     

Structures, stabilities and electronic properties of FePbn (n=1-14) clusters: Density-functional theory investigations

The structures, stabilities and electronic properties of FePb_n (n=1-14) clusters have been studied using the density-functional theory (DFT). Extensive search of the ground-state structures has been carried out by considering a larger number of structural isomers for each cluster size. The Fe atom gradually falls into the interior of the Pb framework as the number of Pb atom increases from 1 to 14. The FePb_n clusters at n=3, 5, 10, 12 have relatively higher stability by analyzing the averaged binding energy and the second-order energy difference. Especially, FePb₁₂ is more stable, owing to its highest symmetrical icosahedron structure. The magnetic moments of FePb_n clusters do not quench when Fe atom is encapsulated in the Pb framework and mostly originate from 3d state of Fe atom.     

Ab-initio calculations of Co-based diluted magnetic semiconductors Cd1−xCoxX (X=S, Se, Te)

Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic semiconductors composed of II-VI compounds Cd_1−xCo_xX (X=S, Se, Te) at x=0.25. From the calculated results of band structure and density of states, the half-metallic character and stability of ferromagnetic state for Cd_1−xCo_xS, Cd_1−xCo_xSe and Cd_1−xCo_xTe alloys are determined. It is found that the tetrahedral crystal field gives rise to triple degeneracy t_2g and double degeneracy e_g. Furthermore, we predict the values of spin-exchange splitting energies Δ_x(d) and Δ_x(p−d) and exchange constants N₀α and N₀β produced by the Co 3d states. Calculated total magnetic moments and the robustness of half-metallicity of Cd_1−xCo_xX (X=S, Se, Te) with respect to the variation in lattice parameters are also discussed. We also extend our calculations to x=0.50, 0.75 for S compounds in order to observe the change due to increase in Co.     

Structural and electronic properties of Bin (n = 2-14) clusters from density-functional calculations

The structural and electronic properties of Bi_n (n = 2-14) clusters have been systematically studied using gradient-corrected density-functional theory. For each cluster size, a number of structural isomers were constructed and optimized to search for the lowest-energy structure. The competition of several structural patterns such as cages, superclusters, and layered structures leads to the alternating appearance of these configurations as global minima. Although the tendency of Bi to form puckered-layer structures is already well-known, the electronic states of Bi_n clusters are still far from that of the bulk. As well, a remarkable even-odd atom number oscillation is observed in the structural and electronic properties of the clusters, implying that the stability of Bi_n clusters is mainly dominated by the electron shell effect rather than by geometrical packing. The theoretically calculated values for electron affinities agree well with available experimental data.     

Planar nano-block structures Tin+1Al0.5Cn and Tin+1Cn (n = 1, and 2) from MAX phases: Structural,electronic properties and relative stability from first principles calculations

Structural, electronic properties and relative stability of quasi-two-dimensional (2D) free-standing planar nano-block (NBs) structures Ti_n+1Al_0.5C_n and Ti_n+1C_n (n = 1 and 2), which can be prepared using the recently developed procedure of exfoliation of corresponding NBs from MAX phases, were examined within first principles calculations in comparison with parent MAX phases Ti₃AlC₂ and Ti₂AlC. We found that in general Ti_n+1C_n and Ti_n+1Al_0.5C_n NBs retain the atomic geometries of the corresponding blocks of the MAX phases, but some structural distortions for the NBs occur owing to the lowering of the coordination number for atoms in the external Ti sheets of the nano-block structures. Our analysis based on their cohesive and formation energies reveals that the stability of the nano-block structures increases with index n (or, in other words, with a growth of the number of Ti–C bonds), the Al-containing NBs becoming more stable than the “pure” Ti–C NBs. Our data show that the magnetization of the simulated planar nano-block structures can be expected; so, for the Ti₃C₂ nano-block the most stable will be the spin configuration, where within each external Ti sheet the spins are coupled ferromagnetically together with antiferromagnetic ordering between opposite external titanium sheets of this nano-block.     

Geometries, stabilities and electronic properties of small Nb-doped gallium clusters: A density functional theory study

The host Ga_n+1 and doped Ga_nNb (n=1-9) clusters with several spin configurations have been systematically investigated by a relativistic density functional theory (DFT) with the generalized gradient approximation. The optimized equilibrium geometries tend to prefer the close-packed configurations for small Nb-doped gallium clusters up to n=9. The average binding energies per atom (E_b/atom), second-order differences of total energies (Δ₂E), fragmentation energies (E_f) and HOMO-LUMO gaps of Ga_n+1 and Ga_nNb (n=1-9) clusters are studied. The results indicate the doping of Nb atom in gallium clusters improves the chemical activities. In particular, the clusters with sizes of Ga₄Nb and Ga₇Nb are found to be more stable with respect to their respective neighbors. Our calculated vertical ionization potentials (VIPs) exhibit an obvious oscillating behavior with the cluster size increasing, except for Ga₃ and Ga₄Nb, suggesting the Ga₃, Ga₅, Ga₇, GaNb, Ga₃Nb, Ga₆Nb and Ga₈Nb clusters corresponding to the high VIPs. In the case of vertical electron affinities (VEAs) and chemical hardness η, VEAs are slightly increasing whereas chemical hardness η decreasing as Ga_nNb cluster size increases. Besides, the doping of Nb atom also brings the decrease as the cluster sizes increases for atomic spin magnetic moments (μ_b).     

Structural and magnetic properties in Bi1−xRxFeO3 (x=0-1, R=La, Nd, Sm, Eu and Tb) polycrystalline ceramics

A series of rare-earth doped BiFeO₃ samples, Bi_1−xR_xFeO₃ (x=0-1, R=La, Nd, Sm, Eu and Tb), were prepared in this work. X-ray diffraction analysis showed that the structure of rare-earth doped BiFeO₃ was transformed from rhombohedral lattice to orthorhombic one by increasing x. The lattice constants and unit-cell volume decreased with the increasing of the doping content, while both the Néel temperature and magnetization were enhanced. A magnetic phase transition was observed at about 35 K for BiFeO₃. The variation of the magnetization with temperature depended on applied field strength and magnetizing history, which was explained according to the antiferromagnetic exchange interaction between Fe and R sites in Bi_1−xR_xFeO₃(x>0). The magnetocrystalline anisotropy contributed by Fe sublattice gave rise to a large coercivity in Bi_xNd_1−xFeO₃ with an orthorhombic structure.     

Structural, electronic and magnetic properties of Mn, Co, Ni in Gen for (n=1-13)

The structural, electronic and magnetic properties of TMGe_n (TM=Mn, Co, Ni; n=1-13) have been investigated using spin polarized density functional theory. The transition metal (TM) atom prefers to occupy surface positions for n<9 and endohedral positions for n≥9. The critical size of the cluster to form endohedral complexes is at n=9, 10 and 11 for Mn, Co and Ni respectively. The binding energy of TMGe_n clusters increases with increase in cluster size. The Ni doped Ge_n clusters have shown higher stability as compared to Mn and Co doped Ge_n clusters. The HOMO-LUMO gap for spin up and down electronic states of Ge_n clusters is found to change significantly on TM doping. The magnetic moment in TMGe_n is introduced due to the presence of TM. The magnetic moment is mainly localized at the TM site and neighbouring Ge atoms. The magnetic moment is quenched in NiGe_n clusters for all n except for n=2, 4 and 8.     

Density functional theory study of AunMn(n=1-8) clusters

Equilibrium geometries, relative stabilities, and magnetic properties of small Au_nMn (n=1-8) clusters have been investigated using density functional theory at the PW91P86 level. It is found that Mn atoms in the ground state Au_nMn isomers tend to occupy the most highly coordinated position and the lowest energy structure of Au_nMn clusters with even n is similar to that of pure Au_n+1 clusters, except for n=2. The substitution of Au atom in Au_n+1 cluster by a Mn atom improves the stability of the host clusters. Maximum peaks are observed for Au_nMn clusters at n=2, 4 on the size dependence of second-order energy differences and fragmentation energies, implying that the two clusters possess relatively higher stability. The HOMO-LUMO energy gaps of the ground state Au_nMn clusters show a pronounced odd-even oscillation with the number of Au atoms, and the energy gap of Au₂Mn cluster is the biggest among all the clusters. The magnetism calculations indicate that the total magnetic moment of Au_nMn cluster, which has a very large magnetic moment in comparison to the pure Au_n+1 cluster, is mainly localized on Mn atom.     

Geometries, stabilities, and electronic properties of gold-magnesium (AunMg) bimetallic clusters

The geometrical structures, relative stabilities, and electronic properties of bimetallic Au_nMg (n=1-8) clusters have been systematically investigated by means of first-principle density functional theory. The results show that the ground-state isomers have planar structures for n=1-7. Here, the calculated fragmentation energies, the second-order difference of energies, the highest occupied-lowest unoccupied molecular orbital energy gaps, and the hardness exhibit a pronounced odd-even alternation, manifesting that the clusters, especially Au₂Mg, with even-number gold atoms have a higher relative stability. On the basis of natural population analysis, the charge transfer and magnetic moment are also discussed.     

Structural,optical, FTIR and photoluminescence properties of Zn0.96−xCo0.04CuxO (x=0.03, 0.04 and 0.05) nanopowders

Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn_0.96−xCo_0.04Cu_xO, x=0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The synthesized samples have been characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–Visible spectrophotometer and Fourier transform infrared spectroscopy. The calculated average crystalline size increases from 37.3 to 50.6 nm for un-hydrogenated samples from x=0.03 to 0.05 and it changes from 29.4 to 34.9 nm for hydrogenated samples. The change in lattice parameters, micro-strain, a small shift of X-ray diffraction peaks towards lower angles and reduction in energy gap reveal the substitution of Cu²⁺ ions into Zn–Co–O lattice. The hydrogenation effect reduces the particle size and induces the more uniform distribution of particles than the un-hydrogenated samples which is confirmed by SEM micrographs. Photoluminescence spectra of Zn_0.96−xCo_0.04Cu_xO system shows that red shift in near band edge ultraviolet emission from 393 to 403 nm with suppressing intensity and a blue shift in green band emission from 537 to 529 nm with enhancing intensity confirms the substitution of Cu into the Zn–Co–O lattice.     

Surface effects on the magnetic properties of Co2FeAl(0 0 1): An ab initio study

Electronic structures of the Co₂FeAl(0 0 1) surface are studied theoretically via first-principles calculations based on density functional theory. It is found that the minority spin band gap at the Fermi level in bulk Co₂FeAl disappears at the surface due to space localization of the states. However, beneath the surface, the density of states of individual atoms shows a trend of minority spin gap opening at the Fermi level, which indicates that the electronic structures become close to that of bulk Co₂FeAl. The termination of FeAl is more favorable for spin polarization of Co₂FeAl films than that of Co. Accordingly, we present a composite tri-layer model to illustrate the fading of the half-metallic property in Co₂FeAl films against the ideal character in bulk materials.     

First-principles study of electronic structure and elasticity of UAlx (x=1,2,3) system

A detailed theoretical study of structural, electronic, and elastic properties of cubic UAl_x (x=1,2,3) is presented employing the pseudopotential plane-wave method based on density-functional theory. The structure parameters of these three compounds have been calculated within generalized gradient approximation (GGA) and local density approximation (LDA). The calculated results were compared with the experimental data and previous research. With the GGA approximation, the elastic constants, shear modulus, Young's modulus, and Poisson's ratio of UAl_x (x=1,2,3) are derived. According to the generalized mechanical stability criteria for cubic crystals, our calculation suggested that C15 UAl₂ and L12 UAl₃ are stable substance under hydrostatic pressures, but B2 UAl might be expected as a metastable compound, which is not reported in previous literature, and future experimental confirmation is needed. Furthermore, the calculated energy band structure and density of state (DOS) are found to be in good agreement with the theoretical values. Additionally, the charge density of these compounds have also been worked out and analyzed.     

First-principles calculations for titanium monoxide clusters TinO （n=1-9）

Based on the density-functional theory, this paper studies the geometric and magnetic properties of TinO （n=1-9） clusters. The resulting geometries show that the oxygen atom remains on the surface of clusters and does not change the geometry of Tin significantly. The binding energy, second-order energy differences with the size of clusters show that Ti7O cluster is endowed with special stability. The stability of TinO clusters is validated by the recent time-of-flight mass spectra. The total magnetic moments for TinO clusters with n=1-4, 8-9 are constant with 2 and drop to zero at n=5-7. The local magnetic moment and charge partition of each atom, and the density of states are discussed. The magnetic moment of the TinO is clearly dominated by the localized 3d electrons of Ti atoms while the oxygen atom contributes a very small amount of spin in TinO clusters.     

Structural,electronic and optical properties of CdxZn1 − xS alloys from first-principles calculations

Structural, electronic and optical properties as well as structural phase transitions of ternary alloy Cd_xZn_1 − xS have been investigated using the first-principles calculations based on the density functional theory. We found that the crystal structure of Cd_xZn_1 − xS alloys transforms from wurtzite to zinc blende as Cd content of x=0.83$x=0.83$. Effect of Cd content on electronic structures of Cd_xZn_1 − xS alloys has been studied. The bandgaps of Cd_xZn_1 − xS alloys with wurtzite and zinc blende structures decrease with the increase of Cd content. Furthermore, dielectric constant and absorption coefficient also have been discussed in detail.     

Structural, electronic, magnetic and elastic properties of tetragonal layered diselenide KCo2Se2 from first principles calculations

The structural, elastic, magnetic and electronic properties of the layered tetragonal phase KCo₂Se₂ have been examined in details by means of the first-principles calculations and analyzed in comparison with the isostructural KFe₂Se₂ as the parent phase for the newest group of ternary superconducting iron-chalcogenide materials. Our data show that KCo₂Se₂ should be characterized as a quasi-two-dimensional ferromagnetic metal with highly anisotropic inter-atomic bonding owing to mixed ionic, covalent, and metallic contributions inside [Co₂Se₂] blocks, and with ionic bonding between the adjacent [Co₂Se₂] blocks and K sheets. This material should behave in a brittle manner, adopt enhanced elastic anisotropy rather in compressibility than in shear, and should show very low hardness.     

Stabilities,electronic and magnetic properties of Cu-doped nickel clusters: a DFT investigation

In this paper, we investigate the electronic and magnetic properties of Cu-doped nickel clusters by means of density functional theory. The stabilities of these clusters have also been studied in terms of the binding energies, second-order difference of energies, fragmentation energies and HOMO–LUMO energy gaps. The obtained results reveal that the N₄Cu, N₅Cu and Ni₇Cu clusters are found to be more stable that than all other clusters. Higher HOMO–LUMO gap was observed for Ni₅Cu cluster (2.265 eV), indicating its higher chemical stability. A half-metallic behaviour has also been observed for the Ni_nCu clusters, which suggests that these clusters can be employed as nanocatalysts for several catalytic processes, particularly for hydrogenation and dehydrogenation reactions. The magnetism calculations show that the magnetic moment is mostly located on the Ni atoms, and the contribution of the Cu atom to the total magnetic moment in the Ni_nCu clusters is very small. Furthermore, partial density of states analysis indicates that the 3d orbitals in Ni atoms are mostly responsible for the magnetic behaviour of these clusters, and the s orbitals have a very little contribution to the total magnetic moment.     

Stability and electronic properties of Rh-doped ruthenium clusters and their interaction with NH3 molecule

The stability and electronic properties of the Rh-doped ruthenium clusters and their reactivity towards NH₃ molecule have been studied using DFT calculations with the BLYP-D3/SDD level of theory. The results show that the doping of Ru clusters with Rh atom improves the catalytic performances of pure Ru clusters, and the Ru₅Rh and Ru₇Rh clusters are assumed to be less reactive than their neighbours. The interaction of NH₃ with clusters exhibits that the Ru atoms are preferred adsorption sites for the NH₃ molecule, and the adsorption takes place between the Ru atom of clusters and the N atom of NH₃ molecule. The adsorption energies of NH₃ on Ru_nRh clusters are in the range of ?101.5 to ?218.4?kJ?mol^?1, suggesting a strong adsorption between both species. Upon adsorption process, the electronic properties of the Ru_nRh clusters were substantially changed. The variation of E_g (ΔE_g) for the Ru_nRh (n?≥?7) clusters is very important (ΔE_g?≥?55%), suggesting that these clusters are very sensitive to the NH₃ molecule. Hence, these clusters can be employed as nanosensors for the detection of the NH₃ gas.     

Geometries, stabilities, and electronic properties of Be-doped gold clusters: a density functional theory study

We have systematically investigated the geometrical structures, relative stabilities and electronic properties of small bimetallic AunBe (n = 1, 2, . . . , 8) clusters using a density functional method at BP86 level. The optimized geometries reveal that the impurity beryllium atom dramatically affects the structures of the Aun clusters. The averaged binding energies, fragmentation energies, second-order difference of energies, the highest occupied-lowest unoccupied molecular orbital energy gaps and chemical hardness are investigated. All of them exhibit a pronounced odd-even alternation, manifesting that the clusters with even number of gold atoms possess relatively higher stabilities. Especially, the linear Au2Be cluster is magic cluster with the most stable chemical stability. According to the natural population analysis, it is found that charge-transferring direction between Au atom and Be atom changes at the size of n = 4.     

Structural, elastic, electronic and magnetic properties of ThCr2Si2 from first-principles calculations

The tetragonal (s.g. I4/mmm; #139) ThCr₂Si₂ is widely known as a structural type of the broad family of the so-called 122-like ternary phases which includes now more than 800 members. Among them the superconducting iron-pnictides (discovered in 2008, -earth metals) and the newest superconducting iron-chalcogenides (discovered in 2010, metals) have attracted recently enormous interest in this class of materials. Meanwhile, the data about the electronic, magnetic, and elastic properties of the ThCr₂Si₂ phase itself are still practically absent. Here, by means of first-principles calculations, the optimized structural parameters, spin ordering of the magnetic ground state, independent elastic constants, bulk, shear, and Young’s moduli, elastic anisotropy indexes, total and partial densities of states, and inter-atomic bonding picture for ThCr₂Si₂ were obtained for the first time and analyzed in comparison with the aforementioned most popular 122-like systems and .