The electronic structures,Born effective charge tensors,and phonon properties of cubic,tetragonal,orthorhombic,and rhombohedral K0.5Na0.5NbO3： A first-principles comparative study

The electronic structures, Born effective charges（BECs）, and full phonon dispersions of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5Nb O3 are investigated by the first principles method based on density functional theory.The hybridized states of Nb 4d and O 2p states are observed in the valence band, showing the formation of a strong Nb–O covalent bond which should be responsible for the displacement of Nb and O atoms. The abnormally large BECs of Nb and O indicate the possibility of phase instability induced by the off-center displacement of Nb and O atoms. The phonon dispersions reveal that the ferroelectric instability of K0.5Na0.5Nb O3 is dominated by Nb and O displacements with significant Na characteristics. In addition to the ferroelectric instability, there is also rotational instability coming from the oxygen octahedra rotation around one axis. Moreover, the Γ phonon properties of orthorhombic KNb O3, Na Nb O3, and K0.5Na0.5Nb O3 are also studied in detail.     

Room-Temperature Ferromagnetism in Semiconducting TiO2-δ Nanoparticles

TiO2-δ nanoparticles are synthesized by the sol-gel method and annealed under different reducing atmosphere. The x-ray diffraction patterns show that anatase is the dominant phase with small amounts of the futile phase of TiO2-δ for all the samples. Magnetic measurements indicate that the samples annealed in reducing atmosphere exhibit unprecedented room-temperature ferromagnetism, in particular, the saturation magnetization Ms is up to about 8.6 × 10^-3 emu/g for the sample annealed in H2/Ar mixture. Analysis of the x-ray photoelectron spectroscopy spectra for the samples processed under different conditions indicates that the amounts of Ti^3＋ or Ti^2＋ cations, namely, the concentration of oxygen vacancies, increase with intensifying reducing atmosphere during processing, which shows that ferromagnetism in this material strongly depends on the concentration of oxygen vacancies. The relationships between the ferromagnetism and the crystal structure as well as the grain size in this material are also discussed.     

Mechanical and thermodynamic properties of the monoclinic and orthorhombic phases of SiC_2N_4 under high pressure from first principles

First principles calculations are preformed to systematically investigate the electronic structures, elastic and thermodynamic properties of the monoclinic and orthorhombic phases of Si C2N4 under pressure. The calculated structural parameters and elastic moduli are in good agreement with the available theoretical values at zero pressure. The elastic constants of the two phases under pressure are calculated by stress–strain method. It is found that both phases satisfy the mechanical stability criteria within 60 GPa. With the increase of pressure, the degree of the anisotropy decreases rapidly in the monoclinic phase, whereas it remains almost constant in the orthorhombic phase. Furthermore, using the hybrid density-functional theory, the monoclinic and orthorhombic phases are found to be wide band-gap semiconductors with band gaps of about 2.85 e V and 3.21 e V, respectively. The elastic moduli, ductile or brittle behaviors, compressional and shear wave velocities as well as Debye temperatures as a function of pressure in both phases are also investigated in detail.     

Phase transition and thermal expansion property of Cr_(2-x)Zr_(0.5x)Mg_(0.5x)Mo_3O_(12) solid solution

Compounds with the formula Cr2-xZr0.5xMg0.5xMo3O12（x = 0.0, 0.3, 0.5, 0.9, 1.3, 1.5, 1.7, 1.9） are synthesized, and the effects of Zr4＋ and Mg2＋ co-incorporation on the phase transition, thermal expansion, and Raman mode are investigated. It is found that Cr2-xZr0.5xMg0.5xMo3O12 crystallize into monoclinic structures for x 〈 1.3 and orthorhombic structures for x _〉 1.5 at room temperature. The phase transition temperature from a monoclinic to an orthorhombic structure of Cr2Mo3O12 can be reduced by the partial substitution of （ZrMg）6＋ for Cr3＋. The overall linear thermal expansion coefficient decreases with the increase of the （ZrMg）6＋ content in an orthorhombic structure sample. The co-incorporation of Zr4＋ and Mg2＋ in the lattice results in the occurrence of new Raman modes and the hardening of the symmetric vibrational modes, which are attributed to the MoO4 tetrahedra sharing comers with ZrO6/MgO6 octahedra and to the strengthening of Mo-O bonds due to less electronegativities of Zr4＋ and Mg2＋ than Cr3＋, respectively.     

Piezoelectricity in K_(1-x)Na_xNbO_3: First-principles calculation

The piezoelectric properties of K1-xNaxNbO3 are studied by using first-principles calculations within virtual crystal approximation. To understand the critical factors for the high piezoelectric response in K1-x Na x Nb O3, the total energy,piezoelectric coefficient, elastic property, density of state, Born effective charge, and energy barrier on polarization rotation paths are systematically investigated. The morphotropic phase boundary in K1-x Na x Nb O3 is predicted to occur at x = 0.521, which is in good agreement with the available experimental data. At the morphotropic phase boundary, the longitudinal piezoelectric coefficient d33 of orthorhombic K0.5Na0.5NbO3 reaches a maximum value. The rotated maximum of d*33is found to be along the 50° direction away from the spontaneous polarization(close to the [001] direction). The moderate bulk and shear modulus are conducive to improving the piezoelectric response. By analyzing the energy barrier on polarization rotation paths, it is found that the polarization rotation of orthorhombic K0.5Na0.5NbO3 becomes easier compared with orthorhombic KNbO3, which proves that the high piezoelectric response is attributed to the flattening of the free energy at compositions close to the morphotropic phase boundary.     

Multi-layer structures including zigzag sculptured thin films for corrosion protection of AISI 304 stainless steel

To increase corrosion resistance of the sample,its electrical impedance must be increased.Due to the fact that electrical impedance depends on elements such as electrical resistance,capacitance,and inductance,by increasing the electrical resistance,reducing the capacitance and inductance,electrical impedance and corrosion resistance can be increased.Based on the fact that these elements depend on the type of material and the geometry of the material,multilayer structures with different geometries are proposed.For this purpose,conventional multilayer thin films,multilayer thin film including zigzag structure(zigzag 1)and multilayer thin film including double zigzag structure(zigzag 2)of manganese nitride are considered to protect AISI 304 stainless steel against corrosion in salt solution.These multilayer coatings including zigzag structures are prepared by alternately using the conventional deposition of thin film and glancing angle deposition method.After deposition,the samples are placed in a furnace under nitrogen flux for nitriding.The cross sections of the structures are observed by field emission scanning electron microscopy(FESEM).Atomic force microscope(AFM)is used to make surface analyses of the samples.The results show that the multilayer thin films including zigzag structures have smaller grains than conventional multilayer thin films,and the zigzag 2 structure has the smaller grain than the other two samples,which is attributed to the effect of shadowing and porosity on the oblique angle deposition method.Crystallography structures of the samples are studied by using x-ray diffraction(XRD)pattern and the results show that nitride phase formation in zigzag 2 structure is better than that in zigzag 1 structure and conventional multilayer thin film.To investigate the corrosion resistances of the structures,electrochemical impedance spectroscopy(EIS)and potentiodynamic polarization tests are performed.The results reveal that the multilayer thin films with zigzag structures have better corrosion protection than the conventional multilayer thin films,and the zigzag structure 2 has the smallest corrosion current and the highest corrosion resistance.The electrical impedances of the samples are investigated by simulating equivalent circuits.The high corrosion resistance of zigzag 2 structure as compared with conventional multilayer structure and zigzag 1 structure,is attributed to the high electrical impedance of the structure due to its small capacitance and high electrical resistance.Finally,the surfaces of corroded samples are observed by scanning electron microscope(SEM).     

Structure, dielectric and magnetodielectric properties of Bi1-xGdxFeO3 Ceramics

Different magnetodielectric effects were observed in Bi1-xGdxFeO3 ceramics depending on gadolinium content. A positive one was observed in the samples with x ≤0.10 at 295K and 16K, and a negative one in the sample with x = 0.4 at 16 K. Structure analysis by x-ray diffraction (XRD) reveals that the samples crystallize in the R3c structure (ferroelectrics) for x<0.08 and in the Pbnm structure (paraelectrics) for x≥0.3 at room temperature. Temperature-dependent dielectric response and x-ray diffraction confirm the occurrence of a structural transition in the Pbnm phase at low temperature for the samples with x ≤0.4. While the positive magnetodielectric effects can be attributed to a coupling of magnetic and crystallographic structures of the R3c phase, the observed negative magnetodielectric effect in the Pbnm phase can be associated with a low-temperature modification of the Pbnm structure. The observed dualsigned magnetodielectric effects suggest that the Bi1-xGdxFeO3 oxides are a good prototype for understanding the magnetodielectric coupling mechanism in this kind of materials.     

Intensity Distribution and Phase Vortices of Speckle Fields Generated by Multi-Aperture Random Scattering Screens

The intensity distribution and phase vortices of the speckle fields generated by multi-aperture random scattering screens are simulated, and it is found that the vortices exhibit layer-like structures and the dislocation phenomena occur in the local phase patterns produced by the two-pinhole aperture, whose phase distributions appear as striped structures. For three- or four-pinhole aperture, there are many circular bright spots appearing in the speckle grains, and there is one vortex between the neighboring circular bright spots. The positive and negative phase vortex lattices appear in the phase distributions, and the regions circled by the isothetic phase lines form irregular quadrilaterals or hexagons. Moreover, the relative positions of the vortices or bright spots can be adjusted by changing those of the pinhole apertures.     

First-principles study on the geometric and electronic structures and phase transition of PbZr1-xTixO3 solid solutions

With first-principles virtual-crystal approximation calculations, we systematically investigate the geometric and elec- tronic structures as well as the phase transition of lead zirconate titanate (PbZr1-xTixO3 or PZT) as a function of Ti content for the whole range of 0 ≤ x Ti ≤ 1. It can be found that, with the increase of the Ti content, the PbZr1-xTixO3 solid solu- tions undergo a rhombohedral-to-tetragonal phase transition, which is consistent with the experimental results. In addition, we also show the evolution in geometric and electronic structures of rhombohedral and tetragonal PbZr1-x TixO3 with the increasing content of Ti.     

Elastic and thermodynamic properties of vanadium nitride under pressure and the effect of metallic bonding on its hardness

By the particle-swarm optimization method, it is predicted that tetragonal P42mc, 141md, and orthorhombic Amm2 phases of vanadium nitride （VN） are energetically more stable than NaCl-type structure at 0 K. The enthalpies of the predicted three new VN phases, along with WC, NaC1, AsNi, CsCl type structures, are calculated each as a function of pressure. It is found that VN exhibits the WC-to-CsCl type phase transition at 256 GPa. For the considered seven crystal- lographic VN phases, the structures, elastic constants, bulk moduli, shear moduli, and Debye temperatures are investigated. Our calculated equilibrium structural parameters are in very good agreement with the available experimental results and the previous theoretical results for the NaC1 phase. The Debye temperatures of VN predicted three novel phases, which are all higher than those of the remaining structures. The elastic constants, thermodynamic properties, and elastic anisotropies of VN under pressure are obtained and the mechanical stabilities are analyzed in detail based on the mechanical stability criteria. Moreover, the effect of metallic bonding on the hardness of VN is also investigated, which shows that VNs in P42mc, 141md, and Amm2 phases are potential superhard phases. Further investigation on the experimental level is highly recommended to confirm our calculations presented in this paper.     

A Comparative Study on the Self Diffusion of N-Octadecane with Crystal and Amorphous Structure by Molecular Dynamics Simulation

The straight chain n-Mkanes and their mixture, which can be used as phase change materials （PCM） for ther- mal energy storage, have attracted much attention in recent years. We employ the molecular dynamics （MD） simulation to investigate their thermophysical properties, including self diffusion and melting of n-octadecane with crystal and amorphous structures. Our results show that, although the initial and melted structures of n-octadecane with crystal and amorphous are different, the melting behaviors of n-octadeeane judged by the self diffusion behavior are consistent. The MD simulation indicates that both the crystal and amorphous structures are effective for the property investigation of n-octadecane and the simulated conclusion can be used as reference for modeling the alkanes-based PCM system.     

Predicted novel insulating electride compound between alkali metals lithium and sodium under high pressure

The application of high pressure can fundamentally modify the crystalline and electronic structures of elements as well as their chemical reactivity, which could lead to the formation of novel materials. Here, we explore the reactivity of lithium with sodium under high pressure, using a swarm structure searching techniques combined with first-principles calculations, which identify a thermodynamically stable Li–Na compound adopting an orthorhombic oP8 phase at pressure above 355 GPa. The formation of Li–Na may be a consequence of strong concentration of electrons transfering from the lithium and the sodium atoms into the interstitial sites, which also leads to open a relatively wide band gap for Li NaoP8. This is substantially different from atoms sharing or exchanging electrons in common compounds and alloys. In addition, lattice-dynamic calculations indicate that Li Na-oP8 remains dynamically stable when pressure decompresses down to 70 GPa.     

Femtosecond laser microstructuring of nickel foil

We observe the morphological change and grain structure of Ni foil when it is ablated with femtosecond laser pulses. Scanning electron microscopy and field emission transmission electron microscopy are used to study the nature of the morphology and grain structure of nickel foil and determine the essential features. The results indicate that there are many random uanostructures in the center of the ablated region composed of nanocrystalline grains as well as some core-shell structures phase explosion and extremely high cooling rate are the for the formation of surface nanostructures. The observed morphologies seem to suggest that most probable physical mechanisms responsible     

Energy band structure in optical of spin-1 condensates lattices

The energy band structure of spin-1 condensates with repulsive spimindependent and either ferromagnetic or antiferromagnetic spin-dependent interactions in one-dimensional （1D） periodic optical lattices is discussed. Within the two-mode approximation, Bloch bands of spin-1 condensates are presented. The results show that the Bloch bands exhibit a complex structure as the atom density of mF = 0 hyperfine state increases： bands splitting, reversion, intersection and loop structure are excited subsequently. The complex band structure should be related to the tunneling and spin-mixing dynamics.     

Pressure-induced structural transition and thermodynamic properties of RhN2 and the effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method. Three structures are chosen to investigate for RhN2, namely, simple hexagonal P6/mmm （denoted as SH）, orthorhombic Pnnm （marcasite）, and simple tetragonal P4/mbm （denoted as ST）. Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch Murnaghan equation of states, we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young＇s modulus, and Debye temperature of RhN2 are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN2 is investigated. This is a quantitative investigation on the structural properties of RhN2, and it still awaits experimental confirmation.     

Pressure-induced structural transition and thermodynamic properties of RhN₂ and the effect of metallic bonding on its hardness

The elastic constant,structural phase transition,and effect of metallic bonding on the hardness of RhN 2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method.Three structures are chosen to investigate for RhN 2,namely,simple hexagonal P6/mmm(denoted as SH),orthorhombic Pnnm(marcasite),and simple tetragonal P4/mbm(denoted as ST).Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure.On the basis of the third-order Birch-Murnaghan equation of states,we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa,respectively.Elastic constants,formation enthalpies,shear modulus,Young’s modulus,and Debye temperature of RhN 2 are derived.The calculated values are,generally speaking,in good agreement with the previous theoretical results.Meanwhile,it is found that the pressure has an important influence on physical properties.Moreover,the effect of metallic bonding on the hardness of RhN 2 is investigated.This is a quantitative investigation on the structural properties of RhN 2,and it still awaits experimental confirmation.     

铝、铜、镍三元合金系中τ相的晶体结构变迁

A thorough investigation by means of X-rays has been carried out with the purpose to determine the nature of the ternary phase τ in Al-Cu-Ni alloys. In contrast with the conventional concept of alloy phase which is characterized by a definite type of crystal structure, systematic structure changes are found in the single phase field of τ which occupies quite an extensive area in the isothermal section of the phase diagram at room temperature. There are eight types of structures altogether, all derived from a basic rhombohedron with corners occupied by Al atoms and centres either occupied by the heavy atoms or remaining vacant. The basic rhombohedron is the building stone in the crystal architecture. By transforming the basic rhombohedron into a hexagonal prism in the usual way, all structures may be considered to be built up by stacking together a number of these hexagonal prisms along the triad. The transformation of one structure into another is quite systematic in the way that the number of the stacking stories in the unit cell increases according to the order 10, 11, 12, 13, 14, 15, 16, 17. The atomic arrangements in the different structures are closely related too, in the respect that they are all superstructures due to the presence of ordered vacancies in the rhombohedral centres.The principal factor controlling the formation of these structures has been fully considered. In view of the fact that the change of structure types follows closely with the content of Ni or Cu for alloys of constant Al content, the atomic size factor appears to be unimportant in the formation of these alloys. It has been shown that for alloy phases of the defect lattice type as the r-phase, the most fundamental factor is the average number of valency electrons per structural unit which is the basic rhombohedron in the present case. By assuming Hume-Rothery's valencies, the average number of valency electrons remains remarkably constant throughout the entire phase field, while the electron concentration varies with compositions. It has also been pointed out that for alloy phases where there is no unit cell change, the average number of electrons per structural unit is equivalent to the number of electrons per unit cell, and for alloy phase where there is no defect, this is in effect equivalent to the electron concentration.     

Thermal expansion properties of Lu2-xFexMo3O12

The structures and thermal expansion properties of Lu2-xFexMo3O12 have been investigated by X-ray diffraction(XRD).XRD patterns at room temperature indicate that compounds Lu2-xFexMo3O12 with x≤1.3 exhibit an orthorhombic structure with space group Pnca;compounds with x=1.5 and 1.7 have a monoclinic structure with space group P21/a.Studies on thermal expansion properties show that the linear thermal expansion coefficients of orthorhombic phase vary from negative to positive with increasing Fe content.Attempts to make zero thermal expansion materials indicate that zero thermal expansion can be observed in Lu1.3Fe0.7Mo3O12 in the temperature range of 200-400℃.     

Generation and motion control of optical multi-vortex

The generation and propagation dynamics of multiple optical vortices hosted in a Gaussian beam are experimentally demonstrated by use of the computer-generated holography. Fluid-like motions of the multi-vortex beam are observed owing to the helical phase structure. The multi-vortex beam with identical topological charge presents rotation, which can be suppressed by changing the sign of the topological charge alternately. In addition, the transverse motion control of the multi-vortex is proved by inserting an additional vortex. Finally, rotary and stationary vortex lattices with different periodic arrays are experimentally constructed. The results exhibit potential applications in inducing twisted or stable waveguide arrays and new types of optical traps.     

Fabrication and Magnetic Properties of Co-Doped TiO2 Powders Studied by Nuclear Magnetic Resonance

Co0.04 Ti0.96O2 powders are fabricated by sol-gel method. The structure and magnetic properties are investigated under different annealing conditions systematically with emphasis on the influence of oxygen pressure. Pure anatase structure was acquired for all the samples annealed at 450℃ for one hour. The samples annealed in air exhibit evident room-temperature ferromagnetism (RTFM) with a small magnetic moment of 0.029μB per Co atom and coercivity Hc of 26 Oe, while the samples annealed in vacuum have strong RTFM with a larger magnetic moment of 1.18μB per Co atom and Hc of 430 Oe. The zero-field spin echo nuclear magnetic resonance spectrum of ^59Co is obtained to prove the existence of Co dusters in the latter samples, implying that the Co dusters are responsible for the strong RTFM in the samples annealed in vacuum. No Co cluster could be observed using both XPS and NMR techniques in the samples annealed in air, implying that the RTFM found in these sample sis intrinsic.