First-principle calculation on nearly half-metallic antiferromagnetic behavior of double perovskites La2VReO6

While it was recently found that La₂VTcO₆ and La₂VCuO₆ are promising candidates for half-metallic antiferromagnets (HM-AFM), the search continues for other potential candidates of HM-AFM in the double perovskites structure La₂BB′O₆ (B, B′=transition metal). La₂VReO₆ is found to be a nearly HM-AFM. Furthermore, considering correlation and spin-orbital coupling (SOC) effects in transition metals, it is still nearly a HM-AFM after generalized gradient approximation with correction of on-site Coulomb interaction and SOC calculations, as reported herein.     

Comparison of the structural and magnetic properties of ground-state SrTcO3 and CaTcO3 from first principles

The generalized gradient approximation (GGA) plus on-site Coulomb interaction corrections (GGA+U) method is employed for the total energies and electronic structure calculations of SrTcO₃ and CaTcO₃. G-type antiferromagnetic (G-AFM) is found to be ground state for both compounds, in consistence with the previous experimental results. The mechanism of Neel temperature of SrTcO₃ being higher than that of CaTcO₃ is explored. The insulating band gaps of SrTcO₃ and CaTcO₃ are found to be 1.71 eV and 1.74 eV, respectively. The magnetic moment of Tc1 is found to be 2.237μ_B in SrTcO₃ unit cell and 2.266μ_B in CaTcO₃ unit cell. Structural parameters and electronic structure of the two compounds are examined to explore the origin of their different electrical and magnetic characters.     

Enhanced antiferromagnetic coupling in dual-synthetic antiferromagnet with Co2FeAl electrodes

We study dual-synthetic antiferromagnets (DSyAFs) using Co₂FeAl (CFA) Heusler electrodes with a stack structure of Ta/CFA/Ru/CFA/Ru/CFA/Ta. When the thicknesses of the two Ru layers are 0.45 nm, 0.65 nm or 0.45 nm, 1.00 nm, the CFA-based DSyAF has a strong antiferromagnetic coupling between adjacent CFA layers at room temperature with a saturation magnetic field of ∼11,000 Oe, a saturation magnetization of ∼710 emu/cm³ and a coercivity of ∼2.0 Oe. Moreover, the DSyAF has a good thermal stability up to 400 °C, at which CFA films show B2-ordered structure. Therefore, the CFA-based DSyAFs are favorable for applications in future spintronic devices.     

Electronic structures and mechanical properties of uranium monocarbide from first-principles and calculations

The electronic structure, elastic constants, Poisson's ratio, and phonon dispersion curves of UC have been systematically investigated from the first-principles calculations by the projector-augmented-wave (PAW) method. In order to describe precisely the strong on-site Coulomb repulsion among the localized U 5f electrons, we adopt the local density approximation (LDA)+U and generalized gradient approximation (GGA)+U formalisms for the exchange correlation term. We systematically study how the electronic properties and elastic constants of UC are affected by the different choice of U as well as the exchange-correlation potential. We show that by choosing an appropriate Hubbard U parameter within the GGA+U approach, most of our calculated results are in good agreement with the experimental data. Therefore, the results obtained by the GGA+U with effective Hubbard parameter U chosen around 3 eV for UC are considered to be reasonable.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

Stability of La2O3 and GeO2 passivated Ge surfaces during ALD of ZrO2 high-k dielectric

La₂O₃ grown by atomic layer deposition (ALD) and thermally grown GeO₂ are used to establish effective electrical surface passivations on n-type (1 0 0)-Ge substrates for high-k ZrO₂ dielectrics, grown by ALD at 250 °C substrate temperature. The electrical characterization of MOS capacitors indicates an impact of the Ge-surface passivation on the interfacial trap density and the frequency dependent capacitance in the inversion regime. Lower interface trap densities can be obtained for GeO₂ based passivation even though a chemical decomposition of the oxidation states occur during the ALD of ZrO₂. As a consequence the formation of a ZrGeO_x compound inside the ZrO₂ matrix and a decline of the interfacial GeO₂ are observed. The La₂O₃ passivation provides a stable amorphous lanthanum germanate phase at the Ge interface but also traces of Zr germanate are indicated by X-ray-Photoelectron-Spectroscopy and Transmission-Electron-Microscopy.     

Antiferromagnetism of perovskite EuZrO3 from a first-principles study

Electronic structure and magnetic properties of perovskite EuZrO₃ have been investigated using the ab initio density-functional calculations with local spin density approximation (LSDA) and LSDA+U methods. The results that are obtained reveal that the antiferromagnetic G-type arrangement is more stable than other possible configurations. The ground G-AFM state shows the insulator property with an energy gap of about 0.27 eV at U=0 eV. It is found that the energy gap strongly depends on the correction potential parameter of U due to the strong interaction of the f electrons of Eu in EuZrO₃. The spin magnetic moment of Eu ions is predited to be 6.82μ_B, which is in well agreement with the experimental result of 6.87μ_B.     

Electronic and magnetic properties of NdCrSb3: A first principles study

The electronic and magnetic properties of NdCrSb₃ are calculated by the first principles full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT). Density of states (DOS), magnetic moments and band structures of the system are presented. For the exchange and correlation energy, local spin density approximation (LSDA+U) with the inclusion of Hubbard potential U is used. Our calculation shows that the 3d state electron of Cr and 4f state electrons of Nd contribute to the total DOS and the band structures. The effective magnetic moment is found to be 5.77μ_B, which is comparable to the earlier experimental results of NdCrSb₃.     

The spin effect in zinc-blende CdEuS and CdEuSe: GGA and GGA+U studies

We have investigated the structural, electronic and magnetic properties of substitutional europium rare earth impurity in cubic CdS and CdSe by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the framework of spin-polarized density functional theory (DFT). The electronic exchange-correlation energy is described by generalized gradient approximation GGA and GGA+U (U is the Hubbard correction). The GGA+U method is applied to the rare-earth 4f states. We have calculated the lattice parameters, bulk modulii, the first pressure derivatives of the bulk modulii and the cohesive energies. The calculated densities of states presented in this study identify the metallic behavior of CdEuS and CdEuSe when we use the GGA scheme, whereas when we use the GGA+U, we see that these compounds are half-metallic.     

Stabilization of a very high-k crystalline ZrO2 phase by post deposition annealing of atomic layer deposited ZrO2/La2O3 dielectrics on germanium

The impact of the ZrO₂/La₂O₃ film thickness ratio and the post deposition annealing in the temperature range between 400 °C and 600 °C on the electrical properties of ultrathin ZrO₂/La₂O₃ high-k dielectrics grown by atomic layer deposition on (1 0 0) germanium is investigated. As-deposited stacks have a relative dielectric constant of 24 which is increased to a value of 35 after annealing at 500 °C due to the stabilization of tetragonal/cubic ZrO₂ phases. This effect depends on the absolute thickness of ZrO₂ within the dielectric stack and is limited due to possible interfacial reactions at the oxide/Ge interface. We show that adequate processing leads to very high-k dielectrics with EOT values below 1 nm, leakage current densities in the range of 0.01 A/cm², and interface trap densities in the range of 2-5 × 10¹² eV⁻¹ cm⁻².     

Magnetic and electronic properties of Cr- and Mn-doped SnO2: ab initio calculations

The ab initio calculations, based on the Korringa–Kohn–Rostoker (KKR) approximation method combined with the coherent potential approximation (CPA), indicated as KKR–CPA, have been used to study the stability of ferromagnetic and ferrimagnetic states, for systems that are SnO₂ doped and co-doped with two transition metals, that is, chromium and manganese. Our results indicate that the ferromagnetic state is more stable than the spin-glass state for the (Sn_1−xCr_xO₂; x = 0.07, 0.09, 0.12 and 0.15)-doped system, while the spin-glass state is more stable than the ferromagnetic state for the (Sn_1−xMn_xO₂; x = 0.02 and 0.05)-doped system. However, the ferromagnetic and/or the ferrimagnetic states are stable for the (Sn_0.98−xMn_0.02Cr_xO₂; x = 0.05, 0.09 and 0.13)-doped system depending on the Cr concentration. Moreover, we estimated the Curie temperature (T_c) for the Cr-doped tin dioxide (SnO₂), and we explained the origin of magnetic behaviour through the total density of states for different doped and co-doped SnO₂ systems.     

Co3+离子自旋态对六方相LaCoO3的低指数晶面稳定性影响的密度泛函理论研究

By the first-principles calculations,most studies indicated that the (11102)-CoO₂ termination of LaCoO₃ cannot be stabilized,which disagrees with the experimental observation.Besides the crystal structure,we found that the spin states of Co³⁺ ions could affect surface stability,which previously were not well considered.By examining the different states of Co³⁺ ions in hexagonal-phase LaCoO₃,including low spin,intermediate spin,and high spin states,the surface grand potentials of these facets are calculated and compared.The results show that the spin states of Co³⁺ ions have an important influence on stability of the LaCoO₃ facets.Different from the previous results,the stability diagrams demonstrate that the (11102)-CoO₂ termination can stably exist under O-rich condition,which can get an agreement with the experimental ones.Furthermore,the surface oxygen vacancy formation energies (E_Ov) of stable facets are computed in different spin states.The E_Ov of these possible exposed terminations strongly depend on the spin state of Co³⁺ ions:in particular,the E_Ov of the HS states is lower than that of other spin states.This indicates that one can tune the properties of LaCoO₃ by directly tuning the spin states of Co³⁺ ions.     

Oxygen vacancy in LiTiPO5 and LiTi2(PO4)3: A first-principles study

The structures of LiTiPO₅ and LiTi₂(PO₄)₃, as well as the possibility of oxygen vacancies formation in the systems are studied by first-principles calculations. It is found that oxygen vacancies can be formed in LiTiPO₅ and LiTi₂(PO₄)₃ under oxygen poor condition. The formation of oxygen vacancies introduce a defect band within their band gaps, which is expected to improve the electronic conductivity of LiTiPO₅ and LiTi₂(PO₄)₃ significantly. Meanwhile, a great concentration of oxygen vacancies may increase the discharge voltage of LiTiPO₅ and LiTi₂(PO₄)₃.     

First-principles linear muffin-tin orbital investigations in the electronic and magnetic structures of double perovskites Ba2TMoO6 (T=V,Cr, Mn,Fe and Co)

The electronic and magnetic structures of ordered double perovskites Ba₂TMoO₆ (T=V, Cr, Mn, Fe and Co) are systematically investigated by means of the first-principle linear muffin-tin orbitals with the atomic-sphere approximation (LMTO-ASA) method. The calculations are performed by using the both local spin density approximation (LSDA) and the LSDA+U Coulomb interaction schemes. The results show a half-metallic ferrimagnetic ground states for T=Cr, Fe and Co in LSDA+U treatment, whereas half-metallic ferromagnetic character is observed for T=V. For T=Mn, insulating ground state is obtained, stabilized in the antiferromagnetic state. The LSDA+U calculations yield better agreement with the theoretical and the experimental results than do the LSDA.     

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 .     

B-cation effect on the electronic and magnetic properties of CeBO3 (B=Ga, In) compounds from first principles study

The ab initio APW+lo method is used to study the cation effect on the electronic structure of CeBO₃ (B=Ga, In) compounds. High-pressure structural behavior, magnetic phase stabilities and electronic properties of both materials have been investigated. The observed most stable phases are the orthorhombic (Pnma) and hexagonal (P6₃cm) for CeGaO₃ and CeInO₃, respectively. It is shown that the ferromagnetic (FM) state in CeGaO₃ is energetically more favorable than the anti-ferromagnetic (AFM) one, unlike CeInO₃ where the AFM-III configuration is the lowest in energy. LSDA+U calculation shows that the valence band maximum is located at T point and the conduction band minimum is located at the center of the Brillouin zone, resulting in a wide indirect energy band gap of about 3.6 eV in the ferromagnetic ordering CeGaO₃ which is typical of semiconductor with large gap. CeInO₃ compound keeps the metallic character using DFT+U calculation.     

Double-ceramic-layer thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/La2Ce2O7 deposited by electron beam-physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and La₂Ce₂O₇ (LC) were deposited by electron beam-physical vapor deposition (EB-PVD). The composition, interdiffusion, surface and cross-sectional morphologies, cyclic oxidation behavior of DCL coating were studied. Energy dispersive spectroscopy and X-ray diffraction analyses indicate that both LZ7C3 and LC coatings are effectively fabricated by a single LZ7C3 ingot with properly controlling the deposition energy. The chemical compatibility of LC coating and thermally grown oxide (TGO) layer is unstable. LaAlO₃ is formed due to the chemical reaction between LC and Al₂O₃ which is the main composition of TGO layer. Additionally, the thermal cycling behavior of DCL coating is influenced by the interdiffusion of Zr and Ce between LZ7C3 and LC coatings. The failure of DCL coating is a result of the sintering of LZ7C3 coating surface, the chemical incompatibility of LC coating and TGO layer and the abnormal oxidation of bond coat. Since no single material that has been studied so far satisfies all the requirements for high temperature applications, DCL coating is an important development direction of TBCs.     

Investigation of crystallographic sites of Eu in La2BaZnO5 by site-selective laser-excitation spectroscopy

La₂BaZnO₅:Eu³⁺ (0.05 mol%) was prepared by a solid-state reaction at high temperature. X-ray powder diffraction analysis confirmed the formation of single phase La₂BaZnO₅. Luminescence properties of La₂BaZnO₅:Eu³⁺ are investigated by site-selective laser-excitation and emission spectroscopy at 18 K. Two different crystallographic sites for Eu³⁺ corresponding to the La³⁺ and Ba²⁺ sites are identified from the ⁷F₀→⁵D₀ excitation spectra obtained by monitoring the ⁵D₀→⁷F_J (J=1, 2, …, 6) emissions. It is found that Eu³⁺ substituted for the Ba²⁺ ion experiences stronger crystal-field strength than Eu³⁺ substituted for the La³⁺ ion. Energy transfer between the two crystallographic Eu³⁺ centers is investigated by luminescence decay curves at 18 K.     

Effects of humidified atmosphere on oxygen transport properties in La2Mo2O9

The effects of humidified atmosphere on oxygen surface exchange and diffusion in La₂Mo₂O₉ have been investigated. After annealing samples in D₂O vapour, the depth and line scan profiles of the OD^- species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La₂Mo₂O₉.     

Sol-gel growth of Gd2MoO6:Eu nanocrystalline layers on SiO2 spheres (SiO2@Gd2MoO6:Eu) and their luminescent properties

SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors. The XRD results demonstrate that the Gd₂MoO₆:Eu³⁺ layers on the SiO₂ spheres begin to crystallize after annealing at 600 °C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 600 nm), are not agglomerated, and have a smooth surface. The thickness of the Gd₂MoO₆:Eu³⁺ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀-⁷F₂ red emission at 613 nm) under the excitation of 307 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles.