First-principles study of zinc-blende to rocksalt phase transition in BP and BAs

The structural and electronic properties of BP and BAs are investigated by first-principles pseudopotential method. The calculations show the structural phase transition from zinc-blende (ZB) structure to rocksalt (RS) structure at the transition pressure of 142 GPa for BP and 134 GPa for BAs. The ZB phase of BP and BAs remains indirect gaps upon applying hydrostatic pressure, while RS phase of BP and BAs is semimetal at the transition pressure.     

Unusual phase transitions in two-dimensional telluride heterostructures

Phase-change heterostructure (PCH) holds great promise for overcoming the low-precision bottleneck that limits multibit storage and parallel computing in conventional phase-change random-access memory. However, the origin of high-accuracy control of electrical resistance achieved in programming PCH memory devices has yet to be established. Via in situ transmission electron microscopy, here we unveil the unusual microscopic processes during the order–disorder phase transitions driven by electrical pulse in a Sb₂Te₃/TiTe₂ PCH architecture. The template-modulated phase transition is confirmed to be two-dimensional (2D) in nature. The structural transformation path and dynamics in the confined Sb₂Te₃ sublayers are found to be profoundly changed with respect to those in bulk monolithic Sb₂Te₃, leading to markedly suppressed amorphous relaxation and substantially reduced crystallization stochasticity, both highly desirable for swift and accurate device operations. Our atomic-scale observations provide direct evidence of, and much-needed insight into, the working mechanisms that may enable superior 2D phase-change electronic devices.     

First-principles study of phase transition and structural properties of AlAs

We have investigated the phase transition and structural properties of AlAs in three crystallographic structures, i.e., B3 (zinc blende), B1 (rocksalt), and B8 (nickel arsenide), at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory (DFT). For B8 structure, it is found that the c/a ratios kept nearly constant (0.2% fluctuation) corresponding to V/V₀  0.7–1.05 (V is the primitive cell volume and V₀ is the experimental equilibrium volume of B3 structure), which is in full agreement with experiment, but the c/a ratios increase linearly with the values of V/V₀ decreasing corresponding to V/V₀  0.4–0.7. This indicates under low pressure the compression along c-axis and a-axis is the same, but the compression along c-axis is more difficult than along a-axis under higher pressure. Based on the condition of equal enthalpies AlAs is found to undergo a structural phase transition from B3 to B8 at 5.34 GPa, in agreement with the experimental value of 7 ± 5 GPa, and is speculated to undergo the B3–B1 transition at 6.24 GPa.     

First-principles study of structural and elastic properties of MgSe under hydrostatic pressure

The structural and elastic properties were calculated using ab initio plane wave pseudopotential method within the generalized gradient approximation (GGA). Our results indicated that MgSe undergoes a structural phase transition from NaCl-type (B1) to FeSi-type (B28) at a pressure near to about 111 GPa. The calculated elastic stiffness coefficients presented a linear behaviour versus pressure. The structural parameters and elastic constants of the fundamental ground are generally in good agreement with the available theoretical and experimental data.     

Quantum effects on phase transitions in high-pressure ice

The H₂O ice phases VIII, VII, and X as well as their phase transformations are studied theoretically at 100 K as a function of pressure up to about 100 GPa. A combination of ab initio electronic structure calculations within the framework of density functional theory and the path integral representation of the nuclei is used. This allows the effects of thermal and quantum mechanical fluctuations on the properties of ice at high compression to be assessed separately and also in conjunction. Pronounced quantum effects are uncovered and different mechanisms are found to be at work at the antiferroelectric to paraelectric transition and the symmetrization transition.     

Facile synthesis and thermoelectric studies of n-type bismuth telluride nanorods with cathodic stripping Te electrode

Bismuth telluride (Bi₂Te₃) nanorods (NRs) of n-type thermoelectric materials were prepared using an electrogenerated precursor of tellurium electrode in the presence of Bi³⁺ and mercapto protecting agent in aqueous solution under atmosphere condition. The optimal preparation conditions were obtained with ratio of Bi³⁺ to mercapto group and Te coulomb by photoluminescence spectra. The mechanism for generation of Bi₂Te₃ precursor was investigated via the cyclic voltammetry. The highly crystalline rhombohedral structure of as-prepared Bi₂Te₃ NRs with the shell of Bi₂S₃ was evaluated with high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) spectroscopy. The near-infrared absorption of synthetic Bi₂Te₃ NRs was characterized with spectrophotometer to obtain information of electron at interband transition. The thermoelectric performance of Bi₂Te₃ NRs was assessed with the result of electrical resistivity, Seebeck coefficient, thermal conductivity, and the figure of merit ZT parameters, indicating that thermoelectric performance of as-prepared Bi₂Te₃ nanocrystals was improved by reducing thermal conductivity while maintaining the power factor.     

Effect of heating rate on the hysteresis in the phase transition in silver telluride thin films

The electrical resistance of vacuum deposited silver telluride thin films was measured in the temperature range from 300 to 430 K at different heating rates. It is found that silver telluride films undergo a structural phase transition, with a hysteresis. The phase transition occurs over a wide temperature range of about 30 K and the transition temperature as well as the hysteresis width are found to be influenced by the heating rate. The effect of heating rate on the phase transition temperature and the hysteresis are discussed.     

First-principles elastic constants and electronic structure of beryllium chalcogenides BeS, BeSe and BeTe

A theoretical study of structural, elastic and electronic properties of BeS, BeSe and BeTe is presented using the full-potential augmented plane-waves plus local orbitals (APW + lo) within density-functional theory (DFT). Results are obtained using both the local-density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation potentials. The ground-state properties, like lattice constant, bulk modulus and its first derivative obtained from our calculations agree very well with experimental and other theoretical calculations. Band structures, and total valence charge densities including spin–orbit interaction are analyzed in great detail. The calculated values of the energy gaps, bandwidths, and spin–orbit splittings and the correct band degeneracies are compared to experimental and/or ab initio results. The calculated energy gap for the series of beryllium chalcogenides BeS, BeSe and BeTe is found to be indirect (Γ–X) and underestimated by about 40% for both LDA and PBE-GGA compared to experiment. We have also reported the elastic constants of these materials; the elastic constants have been derived by the stress–strain relation.     

First-principles calculations on the electronic structure of FeS

Spin-polarized first-principles band structure calculations have been performed for antiferromagnetic FeS. The experimental band gap of 0.04 eV is not opened in the local spin density approximation (LSDA) density of states but a marked dip appears at the Fermi energy located within the t_2g minority subband. The result is a consequence of the octahedral surroundings which split the minority t_2g states into one low-lying state and two degenerate states higher in energy. The dip can be enlarged to a small gap for very low U values when the Hubbard correction is employed. The calculated magnetic moment of 3.5μ_B is close to the value of 4μ_B deduced from the ionic model. The density of states (DOSs) compares satisfactorily with the photoemission and bremsstrahlung isochromat spectroscopy (BIS) spectra.     

First-principles calculations of elastic constants for epsilon-carbide and the consequences

ABSTRACT

The elastic constants of ?-carbide at 0?K and zero pressure have been calculated using first-principle methods. The iron to carbon ratio for the carbide is not established and may, in fact, vary between 3 and 2. As a consequence, the calculations have been conducted as a function of the chemical composition using the special quasi-random structures method. In all cases, the elastic constants obtained are consistent with ?-carbide being mechanically stable. The analysis indicates that in comparison with cementite, ?-carbide should be more brittle; that when present as a precipitate in steel, it would be less effective in participating in the overall plastic deformation, and that its coherency strain field in the surrounding matrix should be less extensive than cementite.     

First-principles calculations of mechanical and thermodynamic properties of YAlO3

First-principles calculations are performed to investigate the crystal structure, electronic properties, the elastic properties, hardness and thermodynamic properties of YAlO₃. The calculated ground-state quantities such as lattice parameter, bulk modulus and its pressure derivative, the band structure and densities of states were in favorable agreement with previous works and the existing experimental data. The elastic constants C_ij, the aggregate elastic moduli (B, G, E), the Poisson’s ratio, and the elastic anisotropy have been investigated. YAlO₃ exhibits a slight elastic anisotropy according to the universal elastic anisotropy index A^U = 0.24. The estimated hardness for YAlO₃ is consistent with the experimental value, and Al–O bond in AlO₆ octahedra plays an important role in the high hardness. The Y–O bonds in YO₁₂ polyhedra exhibit different characteristic. Using the quasi-harmonic Debye model considering the phonon effects, the temperature and pressure dependencies of bulk modulus, heat capacity and thermal expansion coefficient are investigated systematically in the ranges of 0–20 GPa and 0–1300 K.     

Growth and electrical properties of mercury indium telluride single crystals

A novel photoelectronic single crystal, mercury indium telluride (MIT), has been successfully grown by using vertical Bridgman method (VB). The crystallinity, thermal and electrical properties of the MIT crystal were investigated. The results of X-ray rocking curve show that the as-grown MIT crystal has good crystal quality with the FWHM on (3 1 1) face of about 173 in. DSC measurement reveals that the Hg element is easy to solely evaporate from the compound when the temperature is higher than 387.9 °C in the open system. Hall measurements at room temperature show that the resistivity, carrier density and mobility of the MIT crystal were 4.79 × 10² Ω cm, 2.83 × 10¹³ cm⁻³ and 4.60 × 10² cm² V⁻¹ s⁻¹, respectively. The reduction of carrier mobility and the increase of the resistivity are related to the adding of In₂Te₃ into HgTe, which changes the energy band structure of the crystal.     

First-principles study of phase transition and elastic properties of XBi (X = Ce, Pr) compounds

The pressure-induced phase transitions of CeBi and PrBi compounds were investigated by using full-potential linearized augmented plane-wave (FP-LAPW) method. The calculations indicate that the transition pressure for CeBi compound from the NaCl-type (B1) structure to the body centered tetragonal (BCT) structure are 11.53 GPa from total energy (E)-volume (V) data and 6.48 GPa from equal Gibbs free energy (G). For PrBi compound, the same phase transition sequence occurred at 10.94 GPa obtained from the slope of the common tangent of E-V curves and 6.04 GPa from the equal G. The detailed structural changes during the phase transition were analyzed. From the elastic constants at zero pressure, we can conclude that B1 phase of XBi (X = Ce, Pr) compounds are mechanical stable, consistent with the experimental observations.     

Preparations of porous AlN particles from an aluminum–magnesium alloy melt solution

Porous AlN particles were prepared from an aluminum–magnesium melt solution. These samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and N₂-adsorption isotherms. The results show that there are many pore formations in every AlN particle, and the wall thickness of the pores is about 10 nm, the diameter of the pores is hundreds of nanometers and the Brunauer–Emmett–Teller (BET) surface area of the samples is 65.183 m²/g.     

First-principles calculations on the crystal, electronic structures and elastic properties of Ag-rich γ′ phase approximates in Al-Ag alloys

Due to the controversy on the structure and lack of experimental data, it is difficult to understand the effects of γ′ phase on the mechanical properties of Al-Ag alloys. In this work, the bulk γ′ phase in Ag-rich solid-solution has been investigated by means of first-principles calculations within the generalized gradient approximation. The lattice parameters are determined theoretically by structural optimization of full relaxation. It is found that the Neumann phase is energetically stable from the calculated formation enthalpy and cohesive energy. The electronic structure is analyzed to understand the underlying mechanism of the structural stability and mechanical properties of the γ′ phase. The six calculated independent elastic constants indicate that the Neumann phase, a proposed γ′ structure in literature, is mechanically stable. The polycrystalline bulk modulus B, Young’s modulus E, shear modulus G, Poisson ratio ν of the γ′ phase are obtained by the Voigt-Reuss-Hill (VRH) approximation. Since there is currently no experimental measurement of elastic constants, we also calculated the elastic constants of a competing hcp structure (N.A. Zarkevich, D.D. Johnson, Phys. Rev. B 67 (2003) 064104.) at 50 at.% Ag for comparison. The results indicate that the prototype hcp phases could have large effect on the mechanical properties of the Al-Ag alloys. The results on elastic anisotropy show that the Young’s modulus E along the c axis is larger than that of the other directions.     

TiN振动和热力学性质的第一性原理研究

基于平面波赝势的第一性原理方法,对TiN的B1(NaCl)和B2(CsCl)以及B3(闪锌矿)复合结构的力学性能以及焓与压力之间的关系进行了研究,并讨论了它们的相对稳定性.通过研究分析可知,TiN在347 GPa左右时会发生从B1到B2的结构相变,在-17.5 GPa左右时会发生从B1到B3的结构相变.由声子频率对压强的依赖关系可知频率和带隙随着压力的增加而增加.另外高温区TiN的热膨胀系数受温度的影响较小.     

Theoretical studies of the pressure-induced zinc-blende to cinnabar phase transition in CdTe and thermodynamical properties of each phase

Luminescence of CdTe quantum dots embedded in ZnTe is quenched at pressure of about 4.5 GPa in the high-pressure experiments. This pressure-induced quenching is attributed to the “zinc-blende–cinnabar” phase transition in CdTe, which was confirmed by the first-principles calculations. Theoretical analysis of the pressure at which the phase transition occurs for CdTe was performed using the CASTEP module of Materials Studio package with both generalized gradient approximation (GGA) and local density approximation (LDA). The calculated phase transition pressures are equal to about 4.4 GPa and 2.6 GPa, according to the GGA and LDA calculations, respectively, which is in a good agreement with the experimental value. Theoretically estimated value of the pressure coefficient of the band-gap luminescence in zinc-blende structure is in very good agreement with that recently measured in the QDs structures. The calculated Debye temperature, elastic constants and specific heat capacity for the zinc-blend structure agree well with the experimental data; the data for the cinnabar phase are reported here for the first time to the best of the authors' knowledge.     

Computer calculations of phase diagrams

The thermodynamic route of establishing phase diagrams is a relatively recent activity, considering that till about the fifties most phase diagrams were determined by the measurement of certain physical property or quantitative microscopy using light optics or x-ray diffraction. The thermodynamic formalism used by Kaufman and Bernstein is explained and illustrated with examples of the development of hypothetical binary phase diagrams. The calculation of ternary phase diagrams can begin with the binary phase diagram data as a first approximation. However, to calculate a reasonably accurate ternary phase diagram a certain amount of ternary solution data is necessary. Various empirical equations have been proposed in the literature to express ternary thermodynamic data. Calculation of simple ternary isothermal sections is illustrated with the examples of Mo-V-W and Cd-Sn-Pb systems. The numerical techniques which involve the differentiation of thermodynamic parameters with respect to composition get more involved with the number of components becoming 3 or more. A simpler approach has been applied recently to find the minimum position on the Gibbs free energy surface.     

First-principles study of electronic structure and metal-insulator transition of plutonium dihydride and trihydride

The electronic structures of cubic PuH₂ and hexagonal PuH₃ have been calculated by combining the full potential linearized augmented plane wave method (FLAPW) with the local spin density and generalized gradient approximation plus a Hubbard parameter U (LSDA + U and GGA + U) for considering the strong Coulomb correlation between localized Pu 5f electrons. Our study indicates that PuH₂ is metallic, while PuH₃ is a semiconductor with a small band gap about 0.26 eV. The bonds in PuH₂ system have some covalent character. For PuH₃, besides the covalent bonds, particularly, the bonding between Pu and octahedral H atoms is of prominent ionicity. In addition, the conductivity and resistivity data is also worked out at 300 K and low temperature of 4 K. The conductivity decreases from a metallic behavior of PuH₂ to the semiconducting region of PuH₃. The experimentally undetermined metal-insulator transition has been firstly discovered in Pu-H system theoretically.