Resonance photoelectron spectroscopy of TiX2 (X = S, Se, Te) titanium dichalcogenides

The photoelectron valence band spectra of TiS₂, TiSe₂, and TiTe₂ dichalcogenides are investigated in the Ti 2p-3d resonance regime. Resonance bands in the vicinity of the Fermi energy are found for TiS₂ and TiTe₂. The nature of these bands is analyzed based on model calculations of the density of electronic states in TiS₂, TiSe₂, and TiTe₂ compounds intercalated by titanium atoms. Analysis of experimental data and their comparison with model calculations showed that these bands have different origins. It is found that the resonance enhancement of an additional band observed in TiS₂ is explained by self-intercalation by titanium during the synthesis of this compound. The resonance enhancement in TiTe₂ is caused by occupation of the 3d band in Ti.     

Structure,electrical transport,and optical properties of a new ordered iron intercalated dichalcogenide,Fe0.34TiSe2

The compound Fe_0.34TiSe₂ crystallizes in space group D³_3d?P3&#x0304;ml of the trigonal system with 8 formula units in a cell of dimensions a = 7.148(4), c = 11.880(7) Å at 117 K. The structure of Fe_0.34TiSe₂ can be derived from that of TiSe₂ by insertion of Fe atoms into some of the octahedral holes between the layers. No two adjacent octahedral holes along c contain Fe atoms, that is no two Fe atoms are related by a translation of $\text{c}\text{2}$. The compound Fe_xTiSe₂, x = 0.34, exhibits physical properties remarkably different from those found in either the host material, TiSe₂, or in low iron concentration (x < 0.2) intercalated materials. The electrical resistivity of this compound exhibits a sharp decrease below 120 K which is coincident with a well defined peak in the Hall coefficient. These data have been interpreted as the onset of antiferromagnetic ordering as suggested by results of magnetic susceptibility measurements. Optical reflectance measurements indicate a simple Drude-like free carrier behavior and are consistent with the interpretation of iron as an electron donor in this material.     

Hierarchy of percolation thresholds and the mechanism for reduction of magnetic moments of transition metals intercalated into TiSe<Subscript>2</Subscript>

The concentration dependences of the effective magnetic moment of transition metal atoms intercalated into TiSe₂ are analyzed in the framework of the percolation theory. It is shown that, depending on the degree of localization of impurity states, the effective magnetic moment is determined by the overlap of 3d orbitals of transition metals or orbitals of titanium atoms coordinated by impurity atoms.     

Structure and physical properties of titanium diselenide intercalated with nickel

The structure, electrical resistivity, thermopower, and magnetic susceptibility of titanium diselenide intercalated with nickel (N_xTiSe₂) are studied systematically in the nickel concentration range x=0–0.5. In accordance with a model proposed earlier, strong hybridization of the Ni3d/Ti3d states is observed, giving rise to suppression of the magnetic moment because of delocalization of the nickel d electrons. It is shown that the strain caused by the Ni3d/Ti3d hybridization does not change the local coordination of a titanium atom.     

Ab initio study of structural and magnetic properties of cubic Fe4N(0 0 1) surface

First-principles calculations are employed to study the structural and magnetic properties of fully-relaxed cubic Fe₄N(0 0 1) surfaces with both Fe₂- and Fe₂N-termination. The results of surface stability calculations show that the (0 0 1) surface of Fe₄N is most possibly existing with Fe₂N-termination. Slab structures have more localized features in the density of states especially for the Fe₂N-terminated surface due to structure relaxation. The average magnetic moments of Fe atoms increase with increasing thickness of slabs. The calculated interlayer distances indicate that the decreases of d₁₂ and d₂₃ result in stronger hybridization and shorter bond distances between Fe2 atom in the second layer and other atoms in surface or the third layers, which lead to variation of magnetic moments with different slab thicknesses.     

Magnetic state and properties of the Fe0.5TiSe2 intercalation compound

The Fe_0.5TiSe₂ compound with a monoclinic crystal structure has been prepared by intercalation of Fe atoms between Se-Ti-Se sandwiches in the layered structure of TiSe₂. The crystal and magnetic structures, electrical resistivity, and magnetization of the Fe_0.5TiSe₂ compound have been investigated. According to the neutron diffraction data, the Fe_0.5TiSe₂ compound has a tilted antiferromagnetic structure at temperatures below the Néel temperature of 135 K, in which the magnetic moments of Fe atoms are antiferromagnetically ordered inside layers and located at an angle of approximately 74.4° with respect to the layer plane. The magnetic moment of Fe atoms is equal to (2.98 ± 0.05)μ_B. The antiferromagnetic ordering is accompanied by anisotropic spontaneous magnetostrictive distortions of the crystal lattice, which is associated with the spin-orbit interaction and the effect of the crystal field.     

Influence of the chalcogen substitution on the character of magnetic ordering in Fe<Subscript>0.5</Subscript>TiS<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intercalated compounds

The crystal structure and magnetic properties of layered Fe_0.5TiS_{2 − x} Se _x (0 ≤ x ≤ 2) compounds intercalated by iron atoms have been investigated. It has been shown that the substitution of selenium for sulfur is accompanied by an increase in the unit cell volume, a transition from the ferromagnetic to antiferromagnetic behavior, and a nonmonotonic variation in the paramagnetic Curie temperature. The intercalated iron atoms are characterized by lower values of the effective moment (3.4–4.0μ_B) as compared to the predicted value (4.89μB) for the Fe²⁺ ion at g = 2. The results obtained have been discussed under the assumption that there are the hybridization of 3d electronic states of intercalated Fe atoms with the electronic states of the TiS_{2 − x} Se _x host compounds and the competition of exchange interactions of different types.     

Electrical and Magnetic Properties of Hafnium Disulfide Intercalated with Iron Atoms

Simultaneous study of the dependences of the structural parameters, electrical, and magnetic properties of hafnium disulfide intercalated iron atoms in the dependence on the intercalate concentration and temperature has been performed for the first time. The temperature dependences of the electrical resistance are shown to exhibit the activation character with the activation energies characteristic of impurity conduction. The effective magnetic moments of iron ions in Fe _x HfS₂ is found to be significantly lesser than the values of free iron ions and to decrease as the iron content increases. The character of the temperature dependences of the effective magnetic moments and negative values of the paramagnetic Curie temperatures indicate possible interactions of the antiferromagnetic type between intercalated atoms. However, the dependences of the magnetization on field for Fe_0.33HfS₂ and Fe_0.5HfS₂ obtained at T = 2 K demonstrate the hysteresis phenomenon characteristic of the ferromagnetic state. The results are discussed assuming the existence of hybridization 3d electron states of intercalated iron atoms with the electronic states of HfS₂ matrices and the competition of various exchange interaction.     

Computer simulation of the energy gap in ZnO- and TiO2-based semiconductor photocatalysts

Ab initio calculations of the electronic structures of binary ZnO- and TiO₂-based oxides are performed to search for optimum dopants for efficient absorption of the visible part of solar radiation. Light elements B, C, and N are chosen for anion substitution. Cation substitution is simulated by 3d elements (Cr, Mn, Fe, Co) and heavy metals (Sn, Sb, Pb, Bi). The electronic structures are calculated by the full-potential linearized augmented plane wave method using the modified Becke-Johnson exchange-correlation potential. Doping is simulated by calculating supercells Zn₁₅D₁O₁₆, Zn₁₆O₁₅D₁, Ti₁₅D₁O₃₂, and Ti₈O₁₅D₁, where one-sixteenth of the metal (Ti, Zn) or oxygen atoms is replaced by dopant atoms. Carbon and antimony are found to be most effective dopants for ZnO: they form an energy gap ΔE = 1.78 and 1.67 eV, respectively. For TiO₂, nitrogen is the most effective dopant (ΔE = 1.76 eV).     

Volume dependence of electronic structure and magnetic properties of Fe16N2

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of Fe₁₆N₂ system and their unit cell volume dependence. It has been found that total magnetic moment increases as increasing unit cell volume of Fe₁₆N₂. In addition, it also has been found that the d electron number on Fe I, Fe II and Fe III atoms decreases as increasing unit cell volume and the local magnetic moment on Fe atoms increases with the decrease of d electron number. The present study provides a clear insight into the numerous conflicting experimental results on the magnetic properties of Fe₁₆N₂ system.     

Co和稳定元素对Nd3(Fe,Co,M)29(M=Ti,V,Cr) 化合物结构和磁性的影响

利用x射线衍射和磁测量研究了不同稳定元素Co以及Ti,V和Cr替代对Nd₃Fe_29-x-yCo_xM_y(M=Ti,V,Cr)化合物结构和磁性的影响.研究发现:每一个稳定元素都有一替代量极限,在此极限以内所有化合物均为Nd₃(Fe,Ti)₂₉型结构,A2/m空间群.不同稳定元素的溶解极限不同.Co的替代量与稳定元素有关,当以Cr作为稳定元素时,Cr的替代量随着Co含量的提高而提高 关键词： 3(Fe')" href="#">Nd₃(Fe Co 29')" href="#">M)₂₉ 结构 磁性     

Kinetics of reactions in interlayer space of titanium diselenide intercalated with iron

Decomposition of a homogeneous intercalation compound with the formation of inclusions in the interlayer space of a matrix lattice has been directly observed. In full compliance with previously advanced theoretical concepts, it has been shown that the decomposition is accompanied by metallic iron extraction, which then gradually transforms into iron selenide due to the interaction with gaseous selenium.The hierarchy of diffusion mobilities of various defects in Fe_xTiSe₂ intercalation compound has been determined. It has been found that the largest mobility is inherent to intrinsic defects of the TiSe₂ lattice, i.e., vacancies in Ti and Se sublattices. The phenomenon of dissociation pressure oscillations as a function of time has been detected. This phenomenon has been explained by the existence of a slow decomposition stage, i.e., intercalated iron diffusion during the formation and dissolution of its inclusions as an individual phase.     

Study on the oscillatory behaviour of the lattice parameter in ternary iron–nitrogen compounds

The structural properties of the XFe₃N (X=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) cubic ternary iron based nitrides as well as the preferential occupation site of X in the structure were studied using Full Potential Linearized Augmented Plane Wave method, within the Density Functional Theory formalism, Wien2k code, the exchange-correlation potential described with the Perdew–Burke–Ernzerhof expression, based in the Local Spin Density Approximation and Generalized Gradient Approximation. According the calculations, the Sc, Co, Ni, Cu and Zn, atoms preferred the corner sites of the cubes, while Ti, V, Cr and Mn occupy the centre of the faces of the equilibrium structures. The equilibrium structure lattice parameters have an oscillatory behaviour with the atomic number of X, with decreasing amplitude as the atomic number of X increases. This trend do not correlated with the atomic radii of X.     

Electronic, magnetic and transport properties of Co2TiZ (Z=Si, Ge and Sn): A first-principle study

We have studied the electronic structure, magnetic and transport properties of some Co based full Heusler alloys, namely Co₂TiZ (Z=Si, Ge and Sn), in the frame work of first-principle calculations. The calculations show that Co₂TiZ (X=Si, Ge and Sn) are to be half-metallic compounds with a magnetic moment of 2 μ_B, well consistent with the Slater-Pauling rule. The electronic structure results reveal that Co₂TiZ has the high density of states at the Fermi energy in the majority-spin state and show 100% spin polarization. Our results also suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the electron splitting of Z (Z=Si, Ge and Sn) and 3d Co atoms and also the d-d hybridization between the Co and Ti atoms. The transport properties of these materials are discussed on the basis of Seebeck coefficients, electrical conductivity coefficients and thermal conductivity coefficients.     

Structures magnetiques des composes MFe2Se4 (M = Ti,V, Cr,Fe, Co,Ni)

Ternary selenides MFe₂Se₄ [M = Ti, V, Cr, (Fe), Co, Ni] are studied by neutron diffraction, electrical conductivity and Mössbauer effect. Several new common features appear in these monoclinic isomorphous compounds (space groupe I2/m), with a M₃□X₄, ordering of vacancies derived from B8 type. Half of the iron atoms are located in (00l) vacancy planes. Other iron and M atoms are statistically distributed in full planes. These compounds are all ferrimagnetic and show a metallic type conductivity. Magnetic moments are weak and the moments of the atoms located in full planes are strongly reduced (50–70%). In all cases, we must assume a delocalization of the electrons of the cations.     

Site preference and magnetism of Fe3−xCrxAl0.5Si0.5

In order to gain better insight into the origin of the observed differences between Fe_3−xCr_xAl and Fe_3−xCr_xSi, alloys of Fe_3−xCr_xAl_0.5Si_0.5 (x=0, 0.125, 0.250, 0.375 and 0.5) were prepared and studied by means of X-ray and neutron diffraction as well as by magnetization measurements. Electronic structure calculations of these alloys have been performed by means of TB-LMTO-ASA method. It was expected, and experimentally verified, that the presence of silicon and aluminum atoms in 1:1 proportion will result in the independence of the lattice parameter on the iron/chromium concentration. All samples have been proved to be a single phase of the DO₃-type of structure. Theoretical and experimental results indicate that chromium atoms locate preferentially in B sublattice. Cr magnetic moments are oriented antiparallel to Fe magnetic moments. Neutron measurements show a linear dependence of the magnetic moments of Fe(A,C), Fe(B) and Cr(B) as a function of Cr concentration. However the calculated total magnetic moment decreases faster with chromium content than indicated by the experiment.     

铬和镍的添加对Fe3Al合金力学性能影响的DFT研究

利用第一原理研究了过渡金属元素 Cr 或 Ni 在 Fe₃Al合金中的优先占位行为及其合金化效应. 计算结果表明: Cr 或 Ni 的取代有助于Fe₃Al 合金体系更稳定, Cr 优先占据 Fe_I 位, Ni 优先占据 Fe_II位. Fe₂NiAl-II 具有最小的剪切模量G, 杨氏模量E和G/B值, 因此Fe₂NiAl-II合金的韧性、延展性最佳. 态密度和电荷密度图表明, 过渡金属元素的取代提高了它们与近邻基体原子之间的相互作用, 削弱了Al和Fe的相互作用.     

Magnetic and crystallographic properties of substituted Pr2Fe14−xMxB compounds (M = Si,Ga, CrandCu)

Structural and magnetic properties of Pr₂Fe_14−xM_xB systems (M = Si, Ga, CrandCu) are investigated. These compounds crystallize in a tetragonal system of P4₂/mnm-type for a Si content up to x = 2, Cr content up to x = 3 and for Ga and Cu concentration up to x = 1. The Curie temperature increases when Fe is substituted by Si, Ga and Cu, decreases when Fe is replaced by Cr. The saturation magnetization decreases with increasing content of an M element. The rate of the decrease is larger than that expected by a simple dilution model. The influence of M atoms on the anisotropy field is discussed.     

Structural and phase stability of titanium dichalcogenides with isovalent and inovalent substitutions

Cu _x Ti(Se_{1 ? y} S _y )₂ compounds are synthesized and studied. All of the synthesized materials are isostructural to TiSe₂. None of the resistive anomalies typical of materials transitioning to a state with a chargedensity wave are observed; neither are any of the signs of a superconducting state transition (at least down to a temperature of 8 K). Band calculations confirm that states near the Fermi level are formed and reveal the structure of the valence and conduction bands.     

First-principles prediction of coexistence of magnetism and ferroelectricity in rhombohedral Bi2FeTiO6

First principles calculations based on the density functional theory within the local spin density approximation plus U(LSDA + U) scheme, show rhombohedral Bi₂FeTiO₆ is a potential multiferroic in which the magnetism and ferroelectricity coexist. A ferromagnetic configuration with magnetic moment of 4μB per formula unit has been reported with respect to the minimum total energy. Spontaneous polarization of 27.3 μC/cm², caused mainly by the ferroelectric distortions of Ti, was evaluated using the berry phase approach in the modern theory of polarization. The Bi-6s stereochemical activity of long-pair and the ‘d⁰-ness’ criterion in off-centring of Ti were coexisting in the predicted new system. In view of the oxidation state of Bi³⁺, Fe²⁺, Ti⁴⁺, and O²⁻ from the orbital-resolved density of states of the Bi-6p, Fe-3d, Ti-3d, and O-2p states, the valence state of Bi₂FeTiO₆ in the rhombohedral phase was found to be Bi³⁺₂Fe²⁺Ti⁴⁺O₆.