Orbital-decomposed electronic and magnetic properties of the double perovskite Sr2FeReO6

The detailed orbital-decomposed electronic structures and magnetic properties of the double perovskite Sr₂FeReO₆ have been studied using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both occupied and unoccupied s and three p states of Fe³⁺ ion are located far away from the Fermi level, while all up-spin states and most down-spin states are completely filled for the s and three p states of Re⁵⁺ ion. The octahedral crystal field of the oxygen atoms around transition-metal (TM) sites splits the five-fold degenerate d states of the free TM atoms into triply degenerate t_2g states with smaller bonding-antibonding splitting and doubly degenerate e_g states with larger bonding-antibonding splitting. The Fe³⁺ and Re⁵⁺ ions are in the states (3d⁵, S=5/2) and (5d², S=1) with magnetic moments 3.70 and −0.86μ_B, respectively and thus antiferromagnetic coupling via oxygen between them. There are no direct interactions between two nearest Fe-Fe or Re-Re pairs, whereas along each Fe-O-Re-O-Fe or Re-O-Fe-O-Re chains, the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Re 5p, 5d and 6s orbitals are fairly significant.     

New ferromagnetic La3Co2TaO9 double perovskite: Structural and magnetic properties

The new double perovskite La₃Co₂TaO₉ has been prepared by a solid-state procedure. The crystal and magnetic structures have been studied from X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) data. Rietveld refinements were performed in the monoclinic space group P2₁/n. The structure consists of an ordered array of alternating B′O₆ and B″O₆ octahedra sharing corners, tilted along the three pseudocubic axes according to the Glazer notation a⁻b⁻c⁺. Rietveld refinements show that at RT the cell parameters are a=5.6005(7) Å, b=5.6931(7) Å, c=7.9429(9) Å and β=89.9539(7)°, and the refined crystallographic formula of this “double perovskite” can be written as La₂(Co)_2d(Co_1/3Ta_2/3)_2cO₆. Magnetization measurements and low-temperature NPD data show that the perovskite is a ferromagnet with T_C=72 K. At high T it follows the Curie–Weiss law with an effective magnetic moment of 3.82μ_B per Co ion which is very close to spin only Co²⁺ (HS).     

Structural and ac electrical properties of a newly synthesized single phase rare earth double perovskite oxide: Ba2CeNbO6

A single phase rare earth double perovskite oxide Ba₂CeNbO₆ (BCN) is synthesized by solid-state reaction technique for the first time. The X-ray diffraction pattern of the sample at room temperature shows monoclinic structure, with the lattice parameters, a=5.9763 Å, b=5.975 Å and c=8.48 Å and β=90.04°. Impedance spectroscopy is used to study the ac electrical behavior of this material as a function of frequency (10²-10⁶ Hz) at various temperatures (30-450 °C). A relaxation is observed in the entire temperature range. Conduction mechanism is investigated by fitting the complex impedance data to Cole-Cole equation. Complex impedance plane plots show only one semicircular arc, indicating only the grain contribution of dielectric relaxation. The scaling behavior of imaginary part of electric modulus (M″) and imaginary part of electrical impedance (Z″) suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependence of conductivity is interpreted in terms of the jump relaxation model and is fitted to Jonscher's power law. The values of dc conductivities extracted from the Jonscher power law varies from 2.79×10⁻⁷ to 3.5×10⁻⁵ Sm⁻¹ with the increase in temperature from 100 to 450 °C. The activation energies (0.37 eV) extracted from M″(ω) and Z″(ω) peaks are found to follow the Arrhenius law.     

Magnetic properties and structural characterization of Sr2RuHoO6 complex perovskite

We report an experimental study of the crystallographic lattice, morphologic characteristics and magnetic feature of Sr₂RuHoO₆ complex perovskite, which is used as a precursor in the fabrication process of the superconducting ruthenocuprate RuSr₂HoCu₂O₈. The samples were produced through the standard solid state reaction. A Rietveld refinement of experimental X-ray diffraction patterns shows that the material crystallizes in a monoclinic structure, which belongs to the P21/n (#14) space group, with lattice parameters a=5.7719(6) Å, b=5.8784(5) Å, c=8.1651(9) Å, and tilt angle β=90.200°. Magnetic susceptibility measurements reveal the occurrence of an antiferromagnetic ordering for a Néel temperature T_N=10.1 K. From the Curie-Weiss fitting of the paramagnetic regime we obtain an effective magnetic moment of 11.31 μ_B.     

First-principles study of the electronic and magnetic properties of a Nickel-Zinc ferrite: ZnxNi1−xFe2O4

Using first-principles density functional theory within the generalized gradient approximation method, the effect of Zn doping on electronic and magnetic properties of NiFe₂O₄ ferrite spinel has been studied. The crystal structure of the compounds is assigned to a pseudocubic structure and the lattice constant increases as the Zn concentration increases. Our spin-polarized calculations give a half-metallic state for NiFe₂O₄ and a normal metal state for Zn_xNi_1−xFe₂O₄ (0<x≤0.5). Based on the magnetic properties calculations, it is found that the saturation magnetic moment enhances linearly with increase in the Zn content in NiFe₂O₄. The Zn doping in NiFe₂O₄ also induces strong ferrimagnetism since it decreases the magnetic moment of A-sites.     

Dielectric relaxation and ac conductivity of double perovskite oxide Ho2ZnZrO6

Double perovskite oxide holmium zinc zirconate Ho₂ZnZrO₆ (HZZ) is synthesized by solid state reaction technique under a calcination temperature of 1100 °C. The crystal structure has been determined by powder X-ray diffraction, which shows monoclinic phase at room temperature. The variation of dielectric constant (ε′) and loss tangent (tan δ) with frequency is carried out assuming a distribution of relaxation times. The frequency corresponding to loss tangent peak is found to obey an Arrhenius law with activation energy of 89.7 meV. The frequency-dependant electrical data are analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary electric modulus shows the temperature-independent nature of the distribution of relaxation times. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions.     

Structural and magnetic properties of double-perovskite Ba2MnMoO6 by density functional theory

Perovskite-like materials which include magnetic elements have relevance due to the technological perspectives in the spintronics industry. In this work, we report the studies of Ba₂MnMoO₆ material by using the density functional theory. The interchange-correlation potential was included through the generalized gradient approximation. Our structural calculations are in agreement with the experimental results which show that the material crystallizes in the 225 space group (Fm3¯m) and has a lattice parameter of about 8070 Å. The density of states study was carried out by considering the up and down spin orientations. Results show that Ba₂MnMoO₆ has a conductor behavior due to dominant Mn spin-up and Mo spin-down contributions. The magnetic moment was calculated to be 2.9 μ_B.     

Structural, magnetic and transport properties of double perovskite compounds (Sr2−3xLa2xBax)FeMoO6

The crystal structure and magnetic properties of a series of ordered double perovskite oxides (Sr_2−3xLa_2xBa_x)FeMoO₆ (0x0.3) have been investigated. X-ray powder diffraction reveals that the crystal structure of the compounds changes from a tetragonal I4/m lattice to a cubic lattice around x=0.2. Though the nominal average size of the A site cation of (Sr_2−3xLa_2xBa_x)FeMoO₆ is designed to be almost independent of x, the refinements of the crystal structure show that the lattice constants increase with x in both the tetragonal and the cubic phase regions due to electron doping. As the x increases, the degree of cationic ordering on the B site is decreased pronouncedly, while the Curie temperature of the compounds is nearly unchanged. The saturation magnetization of the compounds decreases with x and shows a linear dependence on the degree of cation ordering. The resistivity of the parent compound shows a semiconducting behavior below room temperature, but those of the doped samples exhibit a metal–semiconductor transition. A correlation between the resistivity and metal-semiconducting transition temperature (T_M−S) is observed. The resistivity and T_M−S of the compounds decrease with x for x0.2 and increase for x0.2. Magnetoresistance of the compounds is reduced by the La/Ba doping. All these observations can be understood based on the interplay of the electron doping, change in bandwidth and the anti-site defect concentration.     

Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

A first-principles calculation on the electronic properties of Si/N-codoped TiO2

To deeply understand the effects of Si/N-codoping on the electronic structures of TiO₂ and confirm their photocatalytic performance, a comparison theoretical study of their energetic and electronic properties was carried out involving single N-doping, single Si-doping and three models of Si/N-codoping based on first-principles. As for N-doped TiO₂, an isolated N 2p state locates above the top of valence band and mixes with O 2p states, resulting in band gap narrowing. However, the unoccupied N 2p state acts as electrons traps to promote the electron-hole recombination. Using Si-doping, the band gap has a decrease of 0.24 eV and the valence band broadens about 0.30 eV. These two factors cause a better performance of photocatalyst. The special Si/N-codoped TiO₂ model with one O atom replaced by a N atom and its adjacent Ti atom replaced by a Si atom, has the smallest defect formation energy in three codoping models, suggesting the model is the most energetic favorable. The calculated energy results also indicate that the Si incorporation increases the N concentration in Si/N-codoped TiO₂. This model obtains the most narrowed band gap of 1.63 eV in comparison with the other two models. The dopant states hybridize with O 2p states, leading to the valence band broadening and then improving the mobility of photo-generated hole; the N 2p states are occupied simultaneously. The significantly narrowed band gap and the absence of recombination center can give a reasonable explanation for the high photocatalytic activity under visible light.     

Rietveld refinement and electronic structure studies for the Sm2FeMnO6 new complex perovskite

We report synthesis and crystalline structure study of the Sm₂FeMnO₆ new complex perovskite, by X-ray diffraction experiments and through the application of Rietveld refinement. Results revealed the crystallization of system in a structure given by Pmn21 (#31) space group and lattice parameters a=7.621(1) Å, b=5.675(3) Å and c=5.378(3) Å. Ab initio calculations of density of states (DOS) and electronic structure were carried out for this perovskite-like system by the density functional theory (DFT) and using the full-potential linearized augmented plane waves (FP-LAPW) method. All calculations were carried out using spin polarization. Material evidences a conductor-like character, predominantly due to d–xy Fe orbital of the spin down channel. Magnetic response of system has contributions of Fe and Mn spin up orientation. The calculated magnetic moment in cell was 34.48 μ_B and the magnetic moment in interstitial was 1.54 μ_B.     

过渡金属Fe或Ag掺杂K2Ti6O13的电子结构和光学性质

本文利用基于密度泛函理论（DFT）的第一性原理计算研究了它们的电子结构和光学性质.光学性质的计算结果和实验相一致.结果表明,Fe或Ag掺杂后,K₂Ti₆O₁₃的带隙中出现了杂质带且其带隙值变小,因而使掺杂后的K₂Ti₆O₁₃的吸收边发生红移并实现了其对可见光吸收.其中杂质带主要由Fe 3d态或Ag 4d态与Ti 3d态和O 2p态杂化而成.对于Fe掺杂的K₂Ti₆O₁₃,杂质带位于带隙中间,因此可以作为电子从价带跃迁到导带的桥梁.对于Ag掺杂的K₂Ti₆O₁₃,杂质带位于价带顶附近为受主能级,可以降低光生载流子的复合概率.实验和计算研究表明,通过Fe或Ag的掺杂可以实现了K₂Ti₆O₁₃对可见光的吸收,这对进一步研究K₂Ti₆O₁₃的光学性质具有重要意义.     

First-principles study of the structural,electronic, and magnetic properties of double perovskite Sr_2FeReO_6 containing various imperfections

The structural, electronic, and magnetic properties of double perovskite Sr_2FeReO_6 containing eight different imperfections of FeReor ReFeantisites, Fe1–Re1 or Fe1–Re4 interchanges, V_(Fe), V_(Re), VOor V_(Sr) vacancies have been studied by using the first-principles projector augmented wave(PAW) within generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction(GGA+U). No obvious structural changes are observed for the imperfect Sr_2FeReO_6 containing FeReor ReFeantisites, Fe1–Re1 or Fe1–Re4 interchanges, or VSrvacancy defects. However, the six(eight) nearest oxygen neighbors of the vacancy move away from(close to) VFeor V_(Re)(VO) vacancies. The half-metallic(HM) character is maintained for the imperfect Sr_2FeReO_6 containing FeReor ReFeantisites, Fe1–Re4 interchange, V_(Fe),VO or V_(Sr) vacancies, while it vanishes when the Fe1–Re1 interchange or VRevacancy is presented. So the Fe1–Re1 interchange and the VRevacancy defects should be avoided to preserve the HM character of Sr_2FeReO_6 and thus usage in spintronic devices. In the FeReor ReFeantisites, Fe1–Re1 or Fe1–Re4 interchanges cases, the spin moments of the Fe(Re)cations situated on Re(Fe) antisites are in an antiferromagnetic coupling with those of the Fe(Re) cations on the regular sites. In the V_(Fe), V_(Re), VO, or V_(Sr) vacancies cases, a ferromagnetic coupling is obtained within each cation sublattice,while the two cation sublattices are coupled antiferromagnetically. The total magnetic moments μtot(μB/f.u.) of the imperfect Sr_2FeReO_6 containing eight different defects decrease in the sequence of VSrvacancy(3.50), VRevacancy(3.43),FeReantisite(2.74), VOvacancy(2.64), VFevacancy(2.51), ReFeantisite(2.29), Fe1–Re4 interchange(1.96), Fe1–Re1interchange(1.87), and the mechanisms of the saturation magnetization reduction have been analyzed.     

Dangling bond modulating the electronic and magnetic properties of zigzag SiGe nanoribbon

First-principles nonmagnetic calculations reveal a metallic character in zigzag SiGe nanoribbons (ZSiGeNRs) regardless of their width. The partial DOS projected onto the Si and Ge atoms of ZSiGeNR shows that a sharp peak at the Fermi level is derived from the edge Si and Ge atoms. The charge density contours show the Si–Ge bond is covalent bond, while for the Si–H bond and Ge–H bond, the valence charges are strongly accumulated around H atoms due to their stronger 1 s potential and the higher electronegativity of 2.20 than that of 1.90 for Si atom and 2.01 for Ge atom, so that a significant charge transformation from Si or Ge atoms to H atoms and thus an ionic binding feature. Spin–polarization calculations show that the band structures of ZSiGeNR are modified by the dangling bonds. Compared with perfect ZSiGeNR which is a ferrimagnetic semiconductor, the bands of the ZSiGeNRs with bare Si edge, bare Ge edge, and bare Si and Ge edges shift up and nearly flat extra bands appear at the Fermi level. The ZSiGeNR with bare Si edge or bare Ge edge is a ferrimagnetic metal, while ZSiGeNR with bare Si and Ge edges is a nonmagnetic metal.     

Structural, magnetic and electrical transport properties of Co doping in LaKFeMoO6 double perovskite

The structural, magnetic and transport properties of La_1+xK_1−xFe_1−yCo_yMoO₆ (0.0≤x≤0.1 and 0.1≤y≤0.2) series are studied. At room temperature, the crystal structure is a monoclinic system with space group P2₁/n. The antisite defect lowers with Co doping in LaKFe_1−yCo_yMoO₆ series. However, it increases with the substitution of K by La. Magnetizations increase with the increase in Co content (x=0) and with the La substitution for K, respectively. All compounds demonstrate semiconducting behavior. Their electrical resistivities increase with Co content for LaKFe_1−yCo_yMoO₆ and also increase with La for La_1+xK_1−xFe_1−yCo_yMoO₆. For the LaKFe_1−yCo_yMoO₆ the electrical transport behavior can be described by Mott variable range hopping model in the studied temperature range, whereas for the La_1+xK_1−xFe_1−yCo_yMoO₆ (x≠0 and y≠0) the electrical transport behavior follows the Mott and ES variable range hopping model in high and low temperature ranges, respectively. Each sample exhibits a large magnetoresistance effect.     

First-principles study on electronic structure and elastic properties of Fe16N2

We have performed first-principles study on structural stability, elastic properties and electronic structure of Fe₁₆N₂ by applying LSDA+U method. The calculated values of formation energy and reaction enthalpy for decomposition reaction indicate that Fe₁₆N₂ is a thermodynamically stable phase at the ground state. The six independent elastic constants are derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio are determined as 180 GPa, 199 GPa, 76 GPa and 0.32, respectively. The elastic constants meet all the mechanical stability criteria. The ductility of Fe₁₆N₂ is predicted by Pugh's criterion. The strong bonding between Fe and N atoms results in high values of elastic constants C₁₁ and C₃₃, and contributes to the strengthening of the Fe₁₆N₂ structural stability. The total and partial densities of states (DOS) suggest the existence of hybridization between N-p and Fe-d bands. The position of the Fermi level in DOS curve implies that Fe₁₆N₂ is a metastable phase.     

Structural and electronic behavior of Sr2GdRuO6 complex perovskite

We report experimental and theoretical study of crystallographic lattice and electronic structure of Sr₂GdRuO₆ complex perovskite, which is used as precursor in the fabrication process of superconducting ruthenocuprate RuSr₂GdCu₂O₈. Samples were produced by the standard solid state reaction. Rietveld refinement of experimental X-ray diffraction patterns shows that material crystallizes in a monoclinic structure, which belongs to the P2₁/n (#14) space group, with lattice parameters , , , and tilt angle β=90.258. Calculations of electronic structure were performed by the density functional theory. The exchange and correlation potentials were included through the LDA+U approximation. Density of states (DOS) study was carried out considering the two spin polarizations. Results show Gd are majority responsible for the magnetic character in this material, but Ru contribution is also relevant because d-orbital is closer to Fermi level. Theoretical results evidence that Sr₂GdRuO₆ material behaves as a magnetic semiconductor, with 20μ_B effective magnetic moment.     

High-pressure synthesis,characterization, and equation of state of double perovskite Sr2CoFeO6

Double perovskite oxide Sr2Co Fe O6(SCFO)has been obtained using a high-pressure and high-temperature(HPHT)synthesis method.Valence states of Fe and Co and their distributions in SCFO were examined with X-ray photoelectron spectroscopy.The electric transport behavior of SCFO showed a semiconductor behavior that can be well described by Mott’s law for variable-range hopping conduction.The structural stability of SCFO was investigated at pressures up to 31GPa with no pressure-induced phase transition found.Bulk modulus B0was determined to be 163(2)GPa by fitting the pressure–volume data to the Birch–Murnaghan equation of state.