First-principles Calculations of Structural,Magnetic Electronic and Optical Properties of Rare-earth Metals TbX (X=N,O, S,Se)

The structural, magnetic, electronic and optical properties of Terbium-based binaries (TbX) (X = N, O, S and Se) in two cubic structures NaCl and CsCl have been investigated. This study is carried out by Full-Potential linearized Muffin-Tin orbitals (FP-LMTO) method in the framework of the functional theory of density (DFT) implemented in the lmtart code. The presence of f-state electrons in these induced high-correlation materials led us to study these systems using local density approximation (LDA) for the paramagnetic state and spin local density approximation (LSDA) for the ferromagnetic state within two cubic structures. We have demonstrated that these binaries are stable in the ferromagnetic state in the cubic phase of NaCl, which allows us to deduce the magnetic moments of these components. From the electronic band structures and density states, we have concluded that TbX (X = N, O, S and Se) are metallic in the NaCl phase. The results obtained in this work show that the theoretical parameters of the ground state, structure of the bands, density of the states (DOS) and optical properties agree well with other available theoretical and experimental data.     

Thermal expansion of TlInX2 (X = S, Se, Te)

The thermal expansion coefficients of THnS₂, TlInSe₂, and TlInTe₂ were measured in the range 77– 350 K. The data for both single-crystal and polycrystalline TlInS₂ were found to exhibit features attributable to ferroelectric and commensurate-incommensurate phase transitions. The measured thermal expansion coefficients of THnS₂, TlInSe₂, and TlInTe₂ were used to calculate the Debye temperatures and root-mean-square dynamic atomic displacements. In going from TlInS₂ to TlInSe₂ and to TlInTe₂, the Debye temperature increases and the root-mean-square atomic displacement decreases, which can be attributed to the increase in bond ionicity.     

PbX(X=S,Se)纳米结构材料的制备方法

PbX(X=S,Se)纳米结构材料因其良好的光电性能,在太阳能电池等方面有着较好的应用前景,目前已成为半导体领域的研究热点.概括和总结了几种制备PbX纳米材料的经典和新型方法,其中包括水热法、溶剂热法、化学气相沉积法、中孔材料模板法、熔盐籽晶法、纳米晶的取向附属物法和微波法等,并分析和讨论了各方法的特点及对应产物的特征.     

Half-metallic Ferromagnetism in the Rocksalt and Zincblende NaX (X=O, S, Se, Te, and Po)

The first-principles full potential linearized augmented plane wave method within spin-polarized density functional theory was used to investigate the structural, electronic, and magnetic properties of NaX compounds in the NiAs, rocksalt (RS), and zincblende (ZB) phases. The results showed that in all compounds, the stable state phase is a nonmagnetic (NM) NiAs structure except for NaO in which the stable state phase is a ferromagnetic (FM) RS structure. The ZB NaX (X=O, S, Se, Te, and Po) and RS NaX (X=O, S, and Se) structures are half-metallic (HM) ferromagnets with an integer magnetic moment of 1?? _B per formula unit. The half-metallicity originates from hybridization between X p and Na s and p states. The ZB NaO compound is a very robust half-metal which has the HM band gap of 1.25?eV and maintains HM characteristic up to above 40% of the lattice constant contraction. The half-metallicity is also found up to the lattice constant contraction of about 26.3%, 19.6%, 10.4%, and 7.5% for ZB NaS, NaSe, NaTe, and NaPo and 27.1%, 6.4%, and 1.7% for RS NaO, NaS, and NaSe, respectively. The robustness of half-metallicity with respect to the lattice contraction in the HM NaX compounds and their compatibility with the binary semiconductors make them suitable candidates in spintronic applications.     

Etude des phases semiconductrices de formule AgTlX (X = S,Se, Te)

The results of preparation and crystal growth studies of AgTlS, AgTlSe, AgTlTe are reported. Their temperature dependance of electric conductivities were determined. They are p type semiconductors at room temperature. A continuous serie of solid solutions between AgTlSe and AgTlTe was formed and studied in the same way.     

Pressure dependence of elastic properties of ZnX (X = Se,S and Te): Role of charge transfer

An effective interaction potential (EIOP) is developed to invoke the pressure induced phase transition from zinc blende (B3) to rocksalt (B1) structure and anharmonic properties in ZnX (X = Se, S, Te) semiconductors. The effective interaction potential incorporates the long range Coulomb interaction, van der Waals interaction and short-range repulsive interaction up to second neighbour ions within the Hafemeister and Flygare approach as well as the charge transfer effects caused by the electron-shell deformation of the overlapping ions. The van der Waals coefficients are computed by the Slater Kirkwood variation method as a first step. Later on, we evaluate volume collapse, second order and third order elastic constants with pressure pointing to the systematic trends in all compounds of zinc blende structure and their thermal properties such as force constant, Gruneisen parameter, compressibility, Debye temperature etc. The vast volume discontinuity in pressure-volume (PV) phase diagram identifies the structural phase transition from zinc blende (B3) to rock salt (B1) structure and is consistent with those revealed from earlier reports.     

Structural,Electronic and Magnetic Properties of NaKZ (Z = N,P, As,and Sb) Half-Heusler Compounds: a First-Principles study

The electronic and magnetic properties of the half-Heusler compounds of NaKZ (Z = N, P, As, and Sb) are investigated on the basis of density functional theory. The spin-polarized calculations indicate that these materials are half-metallic ferromagnets with an integer magnetic moment of 1 μ _B at their equilibrium lattice constants. The mechanism that leads to half-metallicity in these materials is also investigated. It is found that these compounds are half-metallic ferromagnets on a wide range of lattice constants, and as a result, they could be used in the spintronic devices that contain heterojunctions of half-metal/semiconductors. The Curie temperatures of NaKN, NaKP, NaKAs, and NaKSb are estimated to be 526.3, 494.7, 475.9, and 358.1 K in the mean field approximation, respectively.     

Structural studies of the hydrated lithium intercalates of TiX2 (X=S,Se)

Lithium intercalated titanium disulphide and diselenide form a number of hydrated complexes. Two types of hydrate are found, one containing a monolayer and the other a bilayer of water molecules between the layers TiX₂ (X=S, Se). By powder X-ray diffraction the crystal structures have been determined of Li(H₂O)Ti(S,Se)₂, Li(H₂O)₂Ti(S,Se)₂ and refined parameters are given for the sulphur and selenium positions. The compounds Li_0.4(H₂O)Ti(S,Se)₂ and Li_0.4(H₂O)₂Ti(S,Se)₂ are isostructural with these materials, with statistical occupation of the lithium sites.     

Electronic band structure and optoelectronic properties of SrCu2X2 (X = As,Sb): DFT calculation

All-electron-full potential linear-augmented plane wave method with Engel Vosko approximation was used for calculating the electronic structure, Fermi surface, and optical properties of SrCu₂X₂ (X = As, Sb). The calculated band structure and Fermi surface show that the metallic behavior of SrCu₂X₂ increases as one move from As to Sb. The calculated partial density of states shows that As-s/p/d, Cu-s/p, and Sr-s/p/d states are forming the Fermi surface for SrCu₂As₂, whereas Sb-s/p/d, Cu-s/p, and Sr-s/p/d states are forming the Fermi surface for SrCu₂Sb₂. The calculated densities of states at Fermi level and electronic specific heat are 14.2 (42.57) states/Ryd-cell and 2.60 (7.37) mJ/mol K² for SrCu₂As₂ (SrCu₂Sb₂). The complex optical dielectric function’s dispersion and the related optical properties such as refractive indices, extension coefficient, absorption coefficient, reflectivity, energy loss function, and optical conductivity were calculated and discussed in detail. The optical properties show a considerable anisotropy between the two components.     

Preparation and properties of ALCON (Al28C6O21N6) ceramics

The synthesis of Al₂₈C₆O₂₁N₆ powder (ALCON), starting from the binary compounds is described. The powder is resistant to oxidation in air up to 760°C. From the prepared powder, fully dense ceramics have successfully been prepared using hot pressing. The as-prepared ceramics had a thermal conductivity of 20 W m^–1 K^–1. Experiments showed that it is also possible to prepare ALCON ceramics by reactive hot-pressing, starting from Al₂O₃, AlN and Al₄C₃. Further optimization is expected to raise the thermal conductivity significantly. The strength, about 300 MPa, is similar to that of AlN. The thermal expansion coefficient of 4.8 × 10^–6K^–1 closely matches that of silicon, making application of ALCON ceramics as heat sinks an interesting possibility.     

Large valley-polarized state in single-layer NbX2 (X =S,Se):Theoretical prediction

Exploring two-dimensional valleytronic crystals with large valley-polarized state is of considerable importance due to the promising applications in next-generation information related devices.Here,we show first-principles evidence that single-layer NbX₂(X=S,Se)is potentially the long-sought two-dimensional valleytronic crystal.Specifically,the valley-polarized state is found to occur spontaneously in single-layer NbX₂,without needing any external tuning,which arises from their intrinsic magnetic exchange interaction and inversion asymmetry.Moreover,the strong spin-orbit coupling strength within Nb-d orbitals renders their valley-polarized states being of remarkably large(NbS₂～156 meV/NbSe₂～219 meV),enabling practical utilization of their valley physics accessible.In additional,it is predicted that the valley physics(i.e.,anomalous valley Hall effect)in single-layer NbX₂ is switchable via applying moderate strain.These findings make single-layer NbX₂ tantalizing candidates for realizing high-performance and controllable valleytronic devices.     

Electronic structures of Mg3Pn2 (Pn=N, P, As, Sb and Bi) and Ca3N2 calculated by a first-principle pseudopotential method

Electronic structure calculations for Mg₃N₂, Mg₃P₂, Mg₃As₂ (low and high temperature modifications), Mg₃Sb₂, Mg₃Bi₂, and Ca₃N₂ have been performed. Mg₃Sb₂ is predicted to be an indirect semiconductor with the gap value of about 0.41 eV. Mg₃As₂ with a high temperature modification is also predicted to be a semiconductor with the gap value of about 1.1 eV, but the valence band maximum and the conduction band minimum of Mg₃Bi₂ contacts at Γ which would make it a semimetal. Mg₃N₂, Mg₃P₂, and Mg₃As₂ (low temperature phase) are semiconductors with the direct band gaps of 1.64 eV, 1.73 eV, and 1.57 eV, respectively. Ca₃N₂ is a semiconductor with a gap of about 1.2 eV.     

Energy investigations on the mechanical properties of magnesium alloyed by X = C,B, N,O and vacancy

The generalized stacking fault （GSF） energies and surface energies of magnesium and its alloys with alloying atoms X- C, B, N, O and vacancy have been investigated using the first-principles methods. It is found that the predominant reducing effects of the alloying atoms and vacancy on the stacking fault energy are resulted from the position of them in the 1st layer near the slip plane. The stacking fault energies are nearly the same as the pure magnesium while the alloying atoms and vacancy are placed in the 2nd, 3rd, 4th, 5th and 6th layers. It has been shown that O strongly reduces the GSF energy of Mg. The alloying atoms C, B and N increase the surface energy, but O and vacancy reduce the surface energy of Mg. The ductilities of Mg and Mg alloys have been discussed based on the Rice criterion by using the ratio between surface energy and unstable stacking fault energy.     

Self-surface passivation of CdX (X = Se, Te) quantum dots

A small portion of a reaction mixture including unpurified CdX (X = Se or Te) quantum dots (QDs), in which unreacted Cd and Se ions were left together with coordinating solvents, was dropped into an organic solvent. The CdX QDs in this organic solution showed enhancement of photoluminescence (PL) efficiency, growth of particles, and focusing of size distribution for more than 10 h at room temperature (RT, -23 degrees C). These effects were attributed to passivation of QDs' surface by Cd and X ions present in the solution. No external energy source was used for these achievements; therefore, the process is termed as self-surface passivation. The self-surface passivation was reproduced using purified CdX QDs with additional Cd and X ions in an organic solvent. The self-surface passivation method was applied to RT-synthesized CdSe QDs, which is characterized by a broad PL spectrum (fwhm - 150 nm) for monodispersed QDs, to modify their emission characteristics. On self-surface passivation, the broad PL spectrum was narrowed (fwhm - 35 nm) and the QDs were grown. The X-ray diffraction measurements of RT-synthesized CdSe QDs and that subsequently aged in 1-butanol showed that crystallinity of the samples was improved on aging.     

Thermal stability of sputtered Mo/X and W/X (X = BN:O, B(4)C:O, Si, and C) multilayer soft-x-ray mirrors

We prepared eight samples of Mo/X and W/X (X = BN:O, B(4)C:O, Si, and C) multilayers by magnetron sputtering. Analyses of x-ray photoelectron spectroscopy for the boron nitride and the B(4)C layers showed the concentration of O to be nonnegligible. We have evaluated the thermal stability by measuring soft-x-ray specular reflectances before and after thermal annealing occurs at temperatures as high as 700 °C. The results suggest that the thermal stability depends largely on the inclusion of low-density materials and not on the type of metal. Of the four low-density materials studied, BN:O is thermally the most stable, and the Mo/BN:O multilayer, the most stable among the eight samples, shows stability as high as 700 °C.     

Sol-Gel Processing of Zr(C,N,O) and Al(C,N,O) Powders

,-Diaminoparaxylene was reacted with Zr(OPr ⁿ )₄ at a molar ratio r = 2 or with Al(OBu ^s )₃ with a molar ratio r = 1.5 or 2. Characterization of the obtained hybrid organic–inorganic gels by FTIR, XRD, TGA, and ¹³C CP-MAS NMR showed that a substitution reaction occurred between the OPr ⁿ or OBu ^s groups of the alkoxides and the HN—CH₂—C₆H₄—CH₂—NH groups of the diamine. Powders derived from hybrid gels pyrolysis were also analyzed.