Pressure and doping dependent elastic and thermodynamical properties of Ga1−xInxP mixed valent compounds

Using a phenomenological lattice model incorporating the long-range Coulomb and charge transfer caused by the deformation of the electron shells of the overlapping ions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction, we present a comprehensive study to understand the effects of pressure on the elastic behavior as ductility (brittleness) and thermodynamical properties of Ga_1−xIn_xP. Estimated phase-transition pressure and the vast volume discontinuity in pressure-volume phase diagram confirm the structural phase transition from zinc blende (B3) to rock salt (B1) phase. From the elastic constants the Poisson's ratio ν, the ratio R_S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic wave velocity, average wave velocity and thermodynamical property Debye temperature is calculated. The Poisson's ratio ν and the ratio R_S/B allows one to conclude that Ga_1−xIn_xP is brittle in zinc blende (B3) and ductile nature is inferred in sodium chloride (B1) phase. To our knowledge this is the first quantitative theoretical prediction of the doping and pressure dependent elastic properties for mixed valent Ga_1−xIn_xP compounds and still awaits experimental confirmations.     

Pressure effects on the structural and electronic properties of ABX4 scintillating crystals

Studies at high pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as, metals, semiconductors, superconductors, or minerals. In particular, the phase behaviour of ABX₄ scintillating materials is a challenging problem with many implications for other fields including technological applications and Earth and planetary sciences. A great progress has been done in the last years in the study of the pressure-effects on the structural and electronic properties of these compounds. In particular, the high-pressure structural sequence followed by these compounds seems now to be better understood thanks to recent experimental and theoretical studies. Here, we will review studies on the phase behaviour of different ABX₄ scintillating materials. In particular, we will focus on discussing the results obtained by different groups for the scheelite-structured orthotungstates, which have been extensively studied up to 50 GPa. We will also describe different experimental techniques for obtaining reliable data at simultaneously high pressure and high temperature. Drawbacks and advantages of the different techniques are discussed along with recent developments involving synchrotron X-ray diffraction, Raman scattering, and ab initio calculations. Differences and similarities of the phase behaviour of these materials will be discussed, on the light of Fukunaga and Yamaoka’s and Bastide’s diagrams, aiming to improve the actual understanding of their high-pressure behaviour. Possible technological and geophysical implications of the reviewed results will be also commented.     

Principal band gaps and bond lengths of the alloy (AlxGa1−x)1−zInzPyAs1−y lattice matched to GaAs

The principal band gaps (E(Γ),E(L), and E(X)) and bond lengths (d(x,y,z))of the alloy (Al_xGa_1−x)_1−zIn_zP_yAs_1−y (where, 0 < x + z < 1, and 0 < y < 1) are calculated over the entire composition range based on the first order correlated function expansion (CFE) scheme. Defining the lattice strain parameter as , it is found that a good quality of alloy (defining ? < ∼ 0.5%) can be obtained in the composition region : 0 < x < ∼ 0.3, 0 < y < ∼ 0.2 and 0 < z < ∼ 0.1. The first order CFE lattice matching relations and corresponding band gaps for the alloy on the GaAs substrate are also determined. It is found that the principal band gaps of the alloy (Al_xGa_1−x)_1−zIn_zP_yAs_1−y lattice matched to GaAs covers band gap ranges: 1.45 eV E < (Γ) 2.69 eV, 1.80 eV < E(L) < 2.38 eV, and 1.97 eV < E(X) < 2.20 eV, while the direct band gap covers from 1.45 eV to 2.05 eV. Our theoretical prediction was compared with the existing experimental data.     

Elastic properties and hardness calculations of lanthanide nitrides in rocksalt structure

First principles calculations were performed to study the hardness, electronic and elastic properties of lanthanide nitrides in rocksalt structure. The calculated lattice parameters are in good agreement with experimental and other theoretical values. The calculations indicate that NdN, PmN, SmN, EuN, TbN are mechanically unstable. The lower B/G and Poisson's ratio of HoN and ErN show that they are brittle than other lanthanide nitrides. Results reveal that the ligand field stabilization energy and lanthanide contraction play important roles in determining the hardness of lanthanide nitrides in rocksalt structure.     

Effect of isoelectronic substitution of Bi on the photoelectrical properties in amorphous Sn-Sb-Se films

Amorphous thin films of Sn₁₀Sb_20 − xBi_xSe₇₀ (0 ≤ x ≤ 6) system have been prepared by thermal evaporation technique. The optical gap and dc activation energy first increases with the addition of Bi (x = 2) and then decreases sharply with further addition. The photocurrent initially increases with time and then saturates to a constant value for all the samples. The decay portion of photocurrent has two components, fast one followed by a slow decay. Photocurrent (I_ph) versus light intensity (F) follows the power law I_ph ∝ F^γ and the value of the exponent (γ) decreases from 0.76 to 0.53 as the Bi concentration varied from x = 0 to 6 in the present system. The photosensitivity of these samples varies from 1.27 to 1.13 as Bi content increases.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

On the ferroelastic nature of the scheelite-to-fergusonite phase transition in orthotungstates and orthomolybdates

New evidence supporting the ferroelastic nature of the pressure-induced scheelite-to-fergusonite phase transition in ABO₄ orthotungstates and orthomolybdates (A = Ca, Sr, Ba, Pb, Eu and B = W, Mo) has been obtained from the analysis of Raman spectroscopy data. In the studied scheelite-type compounds, one external translational mode of B_g symmetry in the scheelite phase softens up to the transition pressure and then becomes a hard A_g mode in the fergusonite high-pressure phase. However, other scheelite-type compounds not undergoing the scheelite-to-fergusonite transition do not show softening of the B_g mode. The reported results have allowed us to establish a relationship between the square of the soft-mode frequency at ambient pressure and the transition pressure in the series PbWO₄, BaWO₄, EuWO₄, SrWO₄, and CaWO₄ and also in the series PbMoO₄, BaMoO₄, SrMoO₄, and CaMoO₄. This fact is in accordance with the soft-mode theory of second-order phase transitions. The conclusions drawn confirm a previous characterization of the phase transition obtained from the Landau theory.     

Effect of pressure on structural and elastic properties of alkaline-earth oxide CaO

We have performed ab initio calculations using a plane wave pseudopotential method to investigate the phase transition of alkaline-earth oxide CaO from NaCl (B1) to CsCl (B2) type structure. The elastic constants for this material have been determined in the pressure range 0–140 GPa. Also, the effect of hydrostatic pressure on the propagation elastic waves has been studied. The specific elastic constants, bulk modulus and wave velocities that we calculated for both B1 and B2 type structures are in good agreement with the available experiment data.     

Effect of annealing on the structural and optical properties of non-coated and silica-coated ZnS:Mn nanoparticles

The Mn²⁺-doped ZnS nanoparticles stabilized by sodium citrate were synthesized through a simple chemical route. Using the ZnS:Mn nanoparticles as seeds, the silica-coated ZnS:Mn nanocomposites were formed in isopropanol by the controlled hydrolysis of tetraethyl orthosilicate. The photoluminescence spectra confirmed that the Mn²⁺ ions were incorporated into the ZnS nanoparticles. The annealing effect on the structural and optical properties of these particles was studied over a range of 100–400 °C. The results of X-ray diffraction and photoluminescence showed that the silica shell not only improved the thermal stability but also resisted the lattice-deformation and oxidation of the particles. The thermal analysis further confirmed that the non-coated ZnS:Mn nanoparticles were unstable beyond 200 °C.     

Effects of grafted carboxyl groups on structural stability and elastic properties of graphene

Molecular dynamics is employed to study the effect of grafted carboxyl groups on elasticity, high-temperature structural stability and compressive properties of armchair and zigzag graphene nanoribbons (AGNRs and ZGNRs). It is found that the Young's moduli decrease as the graft quantity increases from no graft to full grafts (–COOH). The fall for AGNR is larger than that for ZGNRs, which further increases the anisotropy of the elastic property. The structures after undergoing high temperature at 1500 K show that the AGNR exhibits less distortion than ZGNR and grafting carboxyl can obviously reduce structural deformation. Optimizing the GNRs with different in-plane compressive strains, it is evident that the buckling strain (∼11.1%) of ZGNR is larger than that (∼8.1%) of AGNRs, but the AGNR–COOH does not exhibit buckling until the strain is 9.7%, while ZGNR–COOH exhibits buckling at strain of 7.9%. The results indicate that grafting carboxyl reduces the elastic moduli of GNRs, but significantly improves structural stability at high temperature. Grafting carboxyl enhances resistance to compressive buckling of AGNR, but is bad for ZGNR.     

Effect of nanotube geometry on the elastic properties of nanocomposites

Many analytical models replace carbon nanotubes with “effective fibers” to bridge the gap between the nano and micro-scales and allow for the calculation of the elastic properties of nanocomposites using micromechanics. Although curvature of nanotubes can have a direct impact on these properties, it is typically ignored. In this work, the nanotube geometry in 3D is included in the calculation of the elastic properties of a modified effective fiber. The strain energy of the nanotube and the effective fiber are calculated using Castligiano’s theorem and constraints imposed by the matrix on the deformation are taken into consideration. Model results are compared to results from archived literature, and a reasonable agreement is observed. Results show that the effect of nanotube curvature on reducing the modulus of the effective fiber is not limited to in-plane curvature but also to curvature in 3D. The impact of the nanotube curvature on the elastic properties of nanocomposites is studied utilizing the modified fiber model and the approach developed by Mori–Tanaka. Analytical results show that for a low weight fraction of nanotubes the effect of curvature seems to be minor and as the weight fraction increases, the effect of nanotube curvature becomes critical.     

Numerical and experimental investigations of the influence of corner singularities on 3D fatigue crack propagation

In the case of surface breaking cracks, the typical square-root stress singularity is generally not sustainable and a 3D corner singularity in the vicinity of the intersection of crack front and free surface has to be considered. Only the crack front intersection under a special angle γ_r ensures a valid square-root stress singularity and the applicability of the classical SIF-concept. In this paper, the theory of the numerical determination of the intersection angle γ_r is briefly described and the influence of the 3D corner singularity on fatigue crack growth is experimentally investigated. Therefore, 3D fatigue crack propagation experiments under pure mode-I are performed. Transparent specimens of PMMA are used, in order to be able to observe and to document accurate sequences of real 3D crack front evolution profiles via in situ photographic measurement.     

On the definitions of effective stress and deformation gradient for use in MD: Hill’s macro-homogeneity and the virial theorem

The continuum notions of effective deformation gradient and effective stress for homogenization problems with large deformations are reviewed. The “local” problem to be homogenized can include inertia effects to allow for a link between continuum homogenization and the estimation of average properties for particle ensembles via molecular dynamics. The focus of this paper is on the role played by boundary conditions in: defining a meaningful space average of deformation, defining a meaningful space average of stress, and establishing a connection between the idea of effective stress from micro-mechanics and that based on the virial theorem.     

Effect of Pr substitution on structural and electrical properties of BiFeO3 ceramics

Investigation on structural, vibrational, dielectric and ferroelectric properties of Bi_1−xPr_xFeO₃ (x = 0.0, 0.15, 0.25) ceramic samples has been carried out. Room temperature Rietveld-refined X-ray diffraction pattern shows the crystal structure of Bi_1−xPr_xFeO₃ is rhombohedral for x = 0 and triclinic for x = 0.15, 0.25. The changes in Raman normal modes with increasing doping concentration infer the structural transformation is due to Pr substitution at A-site in BiFeO₃. Raman spectra also reveal suppression of ferroelectric behavior due to Pr doping. The dielectric parameters, namely, dielectric permittivity (ε′) and loss tangent (tan (δ)) were evaluated as a function of frequency at room temperature. The ferroelectric polarization reduces in Pr doped bulk BFO samples due to structural change.     

Effect of extrusion ratio on mechanical and corrosion properties of AZ31B alloys prepared by a solid recycling process

Some AZ31B magnesium alloy bars were prepared by a solid recycling process with different extrusion ratios. A reference specimen was processed by extruding an as-received AZ31 ingot. The microstructures, mechanical and corrosion properties of AZ31B magnesium recycled specimens were investigated. With increasing extrusion ratio, the yield strength, tensile strength and yield ratio increases. The reliability of the recycled alloy is poorer than the reference specimen. The corrosion rates of recycled AZ31B magnesium specimens increase immersed in both alkaline and neutral 4% NaCl solution with a decrease extrusion ratio. The corrosion resistance of recycled AZ31B magnesium specimens is improved with increasing pH of immersed solution. The recycled specimens show superior corrosion resistance than reference specimen.     

Si含量对纳米晶Fe_(87-x)Cu_1Nb_3Si_xB_9合金磁性能的影响

研究了Si含量对Fe87-xCu1Nb3SixB9合金经不同方式退火后磁性能的影响。结果表明:随Si含量的增加,Fe87-xCu1Nb3SixB9合金经普通退火后软磁性能逐渐得到优化;经磁退火后可感生出单轴磁各向异性,且磁退火特征随Si含量的增加而逐渐明显。根据横磁退火实验结果计算出的感生磁各向异性值Ku,则由26.7J/m3(Si=9.5at%)降低至14.1J/m3(Si=13.5at%)。由实验数据的分析认为Fe87-xCu1Nb3SixB9合金在高Si含量时经普通退火或纵磁退火后呈现优异的软磁特性,归因于析出的α-Fe(Si)相晶粒具有小的磁晶各向异性K1,从而导致合金具有更低的有效磁各向异性常数K所至。     

Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties

Composite films of P3HT/PCBM-(poly[3-2,5-diyl]/[6,6]-phenyl C₆₁ butyric acid methyl ester) are widely used as an active layer in plastic solar cells. We have studied the influence of thermal annealing on nano-structural and optical properties of thin spin-coated P3HT/PCBM-films. Their structural properties were studied by X-ray diffraction in grazing incidence geometry. It was found that the crystallinity of the investigated films is drastically increased upon annealing. Furthermore, the anisotropic dielectric function of such films was determined by spectroscopic ellipsometry. Significant changes were observed both in parallel and perpendicular components of the dielectric function after annealing. These changes were attributed to the formation of crystalline domains upon annealing.     

Effect of Mn substitution on structural and magnetic properties of ferromagnetic shape memory alloys

The structure and the magnetic transitions have been investigated as a function of Mn in stoichiometric Ni₂MnGa heusler alloys. Particular attention is paid to examine the linear increase of martensite transformation temperature on substituting Mn for Ga. It is observed that the martensite temperature increases and Curie temperature decreases with the effect of Mn content. Room-temperature magnetic measurements show the composition-dependent characteristics with decreasing magnetic saturation values and increasing coercivity values due to decrease in the magnetic exchange interaction strength with increasing Mn in place of Ga. The scanning electron microscopy image confirms that the Mn-rich alloys have the martensitic plates.     

Effect of thickness on the structural, electrical and optical properties of ZnO films

A series of ZnO films of different thickness have been deposited on glass substrates using sol-gel technique by varying the number of spin coatings and the effect of film thickness on the structural, electrical and optical properties have been investigated. The XRD results indicate that the full width at half maximum (FWHM) of the (0 0 2) diffraction peak and the strain along c-axis are decreased as the film is grown up to a thickness of 300 nm. Above 300 nm, the strain again becomes appreciable. The surface morphology shows that the grains become more uniform and bigger in size as the film thickness increases. Electrical result shows that although ZnO film with thickness of around 260 nm has the highest resistivity but is better for current conduction. The excitonic nature in the absorption spectrum becomes prominent for a film with thickness of around 260 nm. The band gap increases and then decreases as the film grows thicker.