Electronic structure and half-metallicity in Heusler alloys Fe2YB (Y=Ti, V, Mn, Cr)

The electronic structure and magnetic properties of B-based Heusler alloys Fe₂YB (Y=Ti, V, Cr and Mn) have been studied theoretically. These alloys are all ferrimagnets except for Fe₂VB. The latter has 24 valence electrons and is a paramagnetic semimetal. Fe₂CrB is predicted to be half-metals at equilibrium lattice constant. The spin polarization of Fe₂MnB is also quite high. The calculated total moments are 1.00 μ_B for Fe₂CrB and 2.04 μ_B for Fe₂MnB. In Fe₂CrB and Fe₂MnB, the total moments are mainly determined by the partial moment of Cr or Mn. The Fe moment is relatively small and antiparallel to that of Cr or Mn. Under uniform lattice distortion, the half-metallicity of Fe₂CrB is more stable than Fe₂MnB, which is related to the detailed DOS structure of them near E_F.     

First-principles study of the electronic structure and magnetism of SrFe0.5Ru0.5O2

We study the electronic structures and magnetism of SrFe_0.5Ru_0.5O₂ by first-principles calculations in the framework of density functional theory (DFT) with generalized gradient approximation plus on site repulsion (GGA+U). The DFT calculations were carried out with ten kinds of Fe-site doping form. Calculations show that the d-orbital electronic configurations of Fe²⁺ and Ru²⁺ ions are (d_z²)²(d_yzd_xz)²(d_xy)²(d_x²_−y²)¹ and (d_z²)²(d_yzd_xz)³(d_xy)¹(d_x²_−y²) , respectively, which are independent of the doping form. The degenerated (d_xzd_yz) orbitals of Ru²⁺ ions are occupied by three electrons, so it gives rise to the Jahn–Teller distortion. The calculated magnetic moments of Fe²⁺ and Ru²⁺ ions are 3.7 μ_B and 1.6 μ_B, respectively. The exchange parameters including nearest neighbor (NN) ions and next nearest neighbor (NNN) ions are calculated by using Heisenberg model and the magnetic frustration in the ordered structure is explained by the competition of the exchange parameters. We also study the external pressure effect on the compound. A pressure-induced orthorhombic to tetragonal structure transition accompanied by an insulator to half-metal transition and an antiferromagnetic (or spin glass) to ferromagnetic state transition is observed.     

Electronic,magnetic and Fermi properties investigates on quaternary Heusler NiCoCrAl,NiCoCrGa and NiFeCrGa

Using the full-potential local-orbital minimum-basis method within the framework of density functional theory, we study the electronic, magnetic and Fermi properties of three quaternary Heusler compounds: NiCoCrAl, NiCoCrGa and NiFeCrGa. Results identify that these compounds are half-metallic ferromagnets with integer spin magnetic moment, and their spin moments follow the Slater–Pauling rule. Accordingly, the origin of gap and magnetic moment are also discussed. In addition, the Fermi surface is further plotted to explore the behavior of electronic states in the vicinity of Fermi level for these compounds. Finally, we argue the influence of tetragonal deformation on electronic and magnetic properties. Meanwhile, the possible L2₁ disorder is also discussed for NiCoCrAl and NiCoCrGa.     

Electronic structures,magnetic properties and half-metallicity in Heusler alloys Zr2IrZ (Z = Al,Ga, In)

The electronic structures, magnetic properties and half-metallicity in Zr₂IrZ (Z = Al, Ga, In) alloys with the Hg₂CuTi-type structure were systematically investigated by using the first-principle calculations. Zr₂IrZ (Z = Al, Ga, In) alloys are predicted to be half-metallic ferrimagnets which are quite robust against hydrostatic strain and tetragonal deformation. The total magnetic moment of Zr₂IrZ (Z = Al, Ga, In) alloys mainly originates from the 4d electrons of Zr atoms and follows the conventional Slater-Pauling rule: M_t = Z_t−18. (M_t is the total magnetic moment per unit cell and Z_t is the valence concentration). The origin of the band gap for Zr₂IrZ (Z = Al, Ga, In) alloys is also well studied. Unconventionally, Zr₂Ir-based alloys contain element with 5d valence electrons, which implies a wider field to search for new half-metallic materials.     

Effect of Si substitution on electronic structure and magnetic properties of Heusler compounds Co2TiAl1−xSix

The electronic structures of Co-based Heusler compounds CoTiAl_1−xSi_x (x=0, 0.25, 0.5, 0.75 and 1) are calculated by first-principles using the full potential linearized augmented plane wave (FP-LAPW) method within GGA and LSDA+U scheme. Particular emphasis was put on the role of the main group elements. In recent years, the GGA calculations of Co₂TiAl (x=0) and Co₂TiSi (x=1) indicated that they are half-metallic, but the electronic structure of this compound with x=0.25, 0.5 and 0.75 has not been reported yet, neither theoretically nor experimentally. The calculated results reveal that these are half-metallic and exhibit an energy gap in the minority spin state and also show 100% spin polarization. The substitution of Al by Si leads to an increase in the number of valence electrons, with increasing x. Our calculated results clearly show that with the Si doping, the lattice parameter linearly decreases; bulk modulus increases, and the total magnetic moment increases. The calculated energy gap in the minority spin state, using GGA scheme, was smaller than that obtained by using LSDA+U scheme. The outcomes of this research also show that the Co-3d DOS and therefore, the magnetic properties of compounds are dependent on electron concentration of the main group elements and it will affect the degree of p-d orbital occupation.     

Effect of low-valent atom substitution on electronic structure and magnetic properties of Fe1.5M0.5CoSi (M=V, Cr, Mn, Fe) Heusler alloys

Quaternary Heusler alloys Fe_1.5M_0.5CoSi with M=V, Cr, Mn and Fe have been investigated theoretically and experimentally. All of these samples crystallize in the ordered Heusler-type structure. The calculated electronic structure shows a pseudogap around E_F in the minority spin states of Fe₂CoSi. With the substitution of low-valent atoms for Fe, the majority antibonding peak is shifted to higher energy and a minority gap around the Fermi level is opened. High spin polarization ratio is obtained in Fe_1.5M_0.5CoSi (M=V, Cr, Mn) alloys. The calculated total spin moments decrease with decreasing number of valence electrons and follow the Slater-Pauling curve, which agree with the experimental results well. The Curie temperature decreases as M atom varies from Fe to V, but is always higher than 650 K, which is suitable for technical applications.     

Lithium absorption on single-walled boron nitride,aluminum nitride,silicon carbide and carbon nanotubes: A first-principles study

Using the DFT-B3LYP calculations we investigate the adsorption of Li atom on CNT, BNNT, AlNNT and SiCNT. We found that Li atom can be chemisorbed on zig-zag SiCNT with binding energy of −2.358 eV and charge transfer of 0.842 |e|, which are larger than the results of other nanotubes. The binding energy of Li on SiCNT is foun to be stronger than activation energy barrier indicating that Li metal could be well dispersed on SiCNTs. Furthermore, the average voltage caused by the lithium adsorption on SiCNT demonstrated that SiCNTs could exhibit as a stable anode similar to the lithium metal anode. The binding nature has been rationalized by analyzing the electronic structures. Our findings demonstrate that Li-BNNT, Li-SiCNT and Li-AlNNT systems exhibit spin polarized behaviors and can fascinating potential application in future spintronics. Also, Li-SiCNT system with rather small band gap might be a promising material for optical applications and active molecule in its environment.     

Electronic and pressure dependent vibrational properties of silicide Sr(Ni0.5Si0.5)2

We report a detailed ab initio study of the structural, electronic, and volume dependent elastic and lattice dynamical properties of Sr(Ni_0.5Si_0.5)₂. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The phonon dispersion curves along the high-symmetry directions and phonon frequencies with their Grüneisen parameters at the Brillouin zone center are computed by using density functional perturbation theory while the elastic constants are calculated in metric-tensor formulation. The band structure, and partial densities of states and Fermi surface topology are also discussed.     

First-principle calculations of structural,electronic and magnetic investigations of Mn2RuGe1-xSnx quaternary Heusler alloys

First-principles calculations were used to calculate the structural, electronic and half-metallic ferromagnetism of Mn₂RuGe_1-xSn_x (x?=?0, 0.25, 0.50, 0.75, 1) Heusler alloys. The Hg₂CuTi-type structure is found to be energetic more than Cu₂MnAl-type structure for both Mn₂RuGe and Mn₂RuSn compounds. The calculated lattice constants for Mn₂RuGe and Mn₂RuSn are 5.91?Å and 6.17?Å, respectively. The electronic band structures and density of states of Mn₂RuGe show a half metallic character with total magnetic moments, 2 μ_B per formula unit that are in good agreement with Slater-Pauling rule with indirect band gap, 0.31?eV along the direction Γ –X. It is observed that the total magnetic moment per cell increases as Sn concentration increases in the Heusler alloys.     

LiMgPbSb型四元Heusler合金CoFeTiSb的半金属性及其无序效应

基于密度泛函理论的GGA计算,我们研究了LiMgPbSb型Heusler合金CoFeTiSb电子结构,发现CoFeTiSb在费米面处存在100%的自旋极化率,并且具有2μB的原胞总磁矩.此外,我们考虑了Co-Fe、Co-Ti和Fe-Ti交换无序对CoFeTiSb合金电子结构的影响,发现这三种交换无序均使得CoFeTiSb完全丧失了半金属性,在Co-Fe交换无序下,CoFeTiSb合金具有85%的自旋极化率,而在Co-Ti无序下,CoFeTiSb合金的自旋极化率只有5%.     

Effect of oxygen content and charge on the structure,stability and optoelectronic properties of yttrium oxide clusters

The electronic and geometrical structures of neutral and charged YOn (n=2–12) clusters have been investigated using density functional theory (DFT) with generalized gradient approximation. The oxygen atom in YOn has been found to be in oxo, peroxo and in superoxo forms. The geometrical structures and topologies of small size anionic clusters resemble that of neutral clusters. Yttrium showed higher coordination number than scandium. Computed results reveal the existence of YO₁₀ cluster to have a penta-peroxo oxygen with a homoleptic Y(η² –O₂)₅ geometrical configuration. The HOMO–LUMO gaps decrease with increasing n due to the increase in 2p orbital population of oxygen atoms. It has been shown that in these clusters bonding are predominantly ionic in nature and anions are thermodynamically more stable, due to the charge delocalization between the metal atom and oxygen ligands. YO₁₀⁺ and YO₁₂⁺ were found to be highly exothermic to release one and two oxygen molecules, while YO₁₁⁺ dissociates though the ozonide dissociation channel. Computed absorption spectra of small clusters are mainly contributed by yttrium metal d and s valence orbitals. The absorbance spectra, shifts towards lower energy with cluster size increase, while charge has no substantial effect on the absorption spectrum.     

First principles study of half-metallic ferromagnetism in Zn1−xEuxS

Density functional theory calculations by using both generalized gradient approximation (GGA) method and the GGA with considering strong correlation effect (GGA+U) for various Eu concentrations x (=0.00, 0.25, 0.50, and 0.75). It is found that after the Europium incorporation, a new localized band appears between the valence and conduction bands, which corresponds to the majority spin of Eu-4f states, the strong correlation effects is very important for the 4f orbit of the Eu atom in ZnEuS. We find that Zn_1−xEu_xS exhibits a half-metallic characteristic, and the ferromagnetic state is more favorable in energy than the antiferromagnetic state. Structural properties are determined from the total energy calculations, and we discuss the electronic structures, total and partial densities of states and local moments.     

Trends in stability, elastic and electronic properties of cubic Rh, Ir, Pd and Pt carbides depending on carbon content: MC versus M4C from first-principles calculations

First principles FLAPW-GGA calculations have been performed to understand the peculiarities of stability, elastic, electronic properties and chemical bonding for cubic carbides of four noble metals M=Rh, Pd, Ir and Pt depending on carbon stoichiometry: MC versus M₄C. Our main findings are as follows: (i) in contrast to mono-carbides MC with positive formation energies E_form>0, carbon-deficient sub-carbides M₄C are stable (E_form<0), thus carbon stoichiometry is one of the major factors determining successful synthesis of these materials, and (ii) as distinct from the majority of other 3d-5d metals (including Pd and Pt examined here), an unusual effect of Rh and Ir “metallization” and the increasing of ductility for these metals owing to the introduction of carbon has been established.     

Ga2基Heusler合金Ga2XCr(X = Mn,Fe, Co,Ni, Cu)的四方畸变、电子结构、磁性及声子谱的第一性原理计算

运用基于密度泛函理论的第一性原理的方法, 对Ga₂基Heusler合金Ga₂XCr (X = Mn, Fe, Co, Ni, Cu)的四方畸变、电子结构、磁性及声子谱特性进行了系统的研究. 结果表明, 在保持体积不变的四方畸变中, 五种合金的磁矩主要由Cr元素提供; Ga₂FeCr, Ga₂CoCr和Ga₂CuCr保持稳定的立方相, 而在Ga₂MnCr和Ga₂NiCr 中观察到能量更低的四方相, 且其能量最低点对应的c/a分别位于1.28和1.11处, 而对应的能量差ΔE 分别为-8.26 meV和-6.14 meV. 电子结构显示, Ga₂MnCr和Ga₂ NiCr的费米能级附近存在尖锐的电子态密度峰, 导致3d电子能级杂化向宽能量范围扩展, 以消除体系的高能量不稳态, 这个过程导致结构转变的发生. 基于适度的畸变度和能量差, 本文认为Ga₂MnCr有存在铁磁马氏体相变的可能, 其声学支虚频的出现, 也进一步表明体系有声子模软化的行为.     

Adhesion and bonding of Pt/Ni and Pt/Co overlayers: Density functional calculations

The electronic and energetic properties of bimetallic surfaces Pt/Ni(111) and Pt/Co(111) are examined using the FP-LAPW (Full-PotentialLinearized Augmented Plane Wave) method by means of spin-polarized and non-polarized calculations. We present both the results of the shifts in the d-band centers when one metal (Pt) is pseudomorfically deposited on another with smaller lattice constant (Ni, Co) and those corresponding to the surface and adhesion energies. The surface is modeled by a seven layer slab separated in z direction by a vacuum region of six substrate layers. The results obtained for pure Ni, Co and Pt surfaces are presented in order to compare with experimental and theoretical data reported in the literature     

Electronic structure and magnetic properties of Co- and Mn-based Heusler alloys and thin films

The full-potential linearized augmented-plane-wave method is used to investigate the electronic structure of several Co- and Mn-based ferromagnetic Heusler alloys. It is shown that calculated lattice constants and spin magnetic moments are in good agreement with experimental values. Electronic structure of Ni₂MnGa(001) surface as well as Ni₂MnGa thin film on GaAs(001) substrate is also investigated. The changes of electronic structure and magnetic properties at surface are analyzed.     

Theoretical study of the transition metal ring functionalized tips of open-ended (5,5) boron nitride nanotube (BNNT)

The nanotube with open edges is an excellent candidate for designing efficient tip for atomistic scanning probes or field emission display (FED) devices. In the present work, we have studied the functionalization of an open-ended boron nitride nanotube (BNNT) with a series of transition metal rings and the effects on the properties of open-ended BNNT through density functional theory (DFT) calculations. The results show that the TM-BNNT complexes are energetically favorable. Moreover, it is found that the functionalization (a) significantly decreases the band gap of BNNT to different degrees, which might effectively modify the electronic properties of the open-ended BNNT; and (b) efficiently lowers the work function, which might improve the field emission properties. Our results might be helpful not only to design specific BNNT-based tips but also to further discuss the chemical vapor deposition (CVD) growth of BNNT on nanoparticles.     

Structural and magnetic properties of Ti12M clusters (M=Sc to Zn)

The geometries, electronic, and magnetic properties of the 3d atom doped icosahedron (ICO) Ti₁₂M (M=Sc to Zn), where a dopant atom replaces either the centra l(Ti₁₂M^c) or surface (Ti₁₂M^s) Ti atom in ICO Ti₁₃ cluster, have been systematically investigated by using the density functional theory. The structures of all the optimized Ti₁₂M^c and Ti₁₂M^s clusters are distorted ICO. Sc, Ni, Cu, and Zn atoms prefer to displace surface Ti atom, V, Cr, Mn, and Fe atoms prefer to displace central Ti atom. The position of impurity atom depends on the strength of the interaction between the central atom and the surface atoms. As compared to the pure Ti₁₃ cluster, Ti₁₂M^c and Ti₁₂M^s (M=V, Fe, Co, and Ni) clusters are more stable, Ti₁₂M^c and Ti₁₂M^s (M=Sc, Cr, Mn, Cu, and Zn) are less stable. Both Ti₁₂Ni^s and Ti₁₂Ni^c are magic clusters, which originate from their electronic as well as geometric closed shells. Because the exchange interaction prevails over the crystal field in Ti₁₂M clusters, the valence electrons fill molecular orbitals in terms of Hund’s rule of maximum spin.     

Electronic structure and related properties of the ferroelectric crystal triglycine sulfate

The electronic band structure and related physical properties of the crystal triglycine sulfate (TGS), (NH₂CH₂COOH)₃·H₂SO₄, in the ferroelectric phase P2₁ have been calculated using the first principles Linear Combination of Atomic Orbitals (LCAO) code CRYSTAL’06 at the B3LYP level of theory. The interactions between the quasi-separate three glycine groups and the sulfate complex have been studied by the analysis of the density of states of the crystal. The glycine 2 group was found to be the zwitterion in the optimized structure. Ten coefficients of the elastic stiffness tensor c_kl, four coefficients of the tensor of the elastoelectrical effect e_ki, and the spontaneous polarization P_s of TGS have been calculated for the first time and have been found to agree well with experimental data. Extrema are found in the elastoelectric coefficient e₂₂ as a function of the strain ε₂, and in the spontaneous polarization P_s as a function of the unit cell volume.