Prediction of Γ-L and L-X crossovers in Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>Sb alloys by empirical pseudopotential method

Using empirical pseudopotential method Γ-L crossover is found for the Ga_0.74Al_0.26Sb. The conduction band minimum is observed to switch at the (0.87, 0, 0) point for Ga_0.51Al_0.49Sb which shifts to the X point for Ga_0.21Al_0.79Sb and remains at X leading finally to indirect band gap in AlSb. Band structure calculations for a large number of alloys are performed and bowing parameters b_X and b_L are proposed for the E_X and E_L respectively. Our findings may serve as directive to select the materials in a range of composition to examine the bowing parameters and thereby effective mass experimentally for the Ga_xAl_1-xSb alloys.     

Infrared crystal spectra of C2H4, C2D4, and as-C2H2D2 and the general harmonic force field of ethylene

Carbon-13 frequency shifts for C₂H₄, C₂D₄, and as-C₂H₂D₂ have been measured in isotopic solid solutions in crystalline films at 60 K. All but two of the shifts (for as-C₂H₂D₂) are compatible with recently determined ζ data for C₂H₄, with ¹³C frequency shifts for C₂H₄ and C₂D₄ in the gas phase and with conventional frequency data. Together, these data completely determine with precision all 18 parameters of the GHFF for ethylene, the previous ambiguity in choice between two sets of A_g species force constants being removed. The force field reproduces closely the observed centrifugal distortion constants for C₂H₄, a ζ constant observed for trans-C₂H₂D₂, and the inertia defects for C₂H₄, C₂D₄, and as-C₂H₂D₂. Vibration and rotation constants for all isotopically deuterated ethylenes are calculated.Possible explanations for the two anomalous crystal shifts in as-C₂H₂D₂ involve the effects of the crystal field, and failure of the use of Dennison's rule for making anharmonic corrections to the shifts. The former explanation is preferred as a result of thorough analysis of the anharmonicity constants for as-C₂H₂D₂ determined from many overtone and combination bands in the gas and crystal spectra.     

Crystal field analysis of the magnetization curves of R2Fe17 and R2Fe17H3 (R=Tb,Ho,Er)

In this paper the values of the crystalline-electric-field parameters A_nm for R₂Fe₁₇ and R₂Fe₁₇H₃ (R=Tb,Ho,Er) are evaluated by fitting calculations to the magnetization curves measured on the single crystal at several temperatures. The fitted A_nm for R₂Fe₁₇ are strikingly different from those for the corresponding R₂Fe₁₇H₃. The energy gaps between the lowest four energy levels for Ho ions in Ho₂Fe₁₇₂ can be reproduced by using the fitted A_nm and exchange field 2μ_BH_ex, which estimated from the fit of the temperature dependence of the spontaneous magnetization combined with inelastic neutron scattering experiment.     

Description of ferroelectric phase transitions in solid solutions of relaxors in the framework of the random-field theory

A model based on the random-field theory is proposed for calculating the properties of solid solutions of ferroelectric relaxors. The electric dipoles randomly distributed in the system are treated as sources of random fields. The random field distribution function is calculated taking into account the contribution of nonlinear and correlation effects and the differences in the dipole orientations for different solid solution components. The dependence of the phase transition temperature T _c on the concentration of solid solution components is analyzed. Numerical calculations are performed for the lead scandoniobate and lead scandotantalate solid solutions (PbSc_1/2Nb_1/2O₃)_1?x (PbSc_1/2Ta_1/2O₃)_x with different degrees of ordering and the lead magnoniobate and lead titanate solid solution (PbMg_1/3Nb_2/3O₃)_1?x (PbTiO₃)_x. It is shown that the higher transition temperature for more disordered solid solutions of the composition (PbSc_1/2Nb_1/2O₃)_1?x (PbSc_1/2Ta_1/2O₃)_x in the range 0≤x<0.5 is associated with the larger nonlinearity coefficient for PbSc_1/2Nb_1/2O₃ as compared to that for PbSc_1/2Ta_1/2O₃. The theory provides a means for calculating the region of the coexistence of the phases with different symmetry groups in the temperature-composition phase diagram of the (PbMg_1/3Nb_2/3O₃)_1?x (PbTiO₃) solid solution. Numerical calculations with the use of the fitting parameters obtained from the known transition temperatures T _c for the solid solution components adequately describe the experimental phase diagrams for the aforementioned solid solutions of ferroelectric relaxors.     

A theoretical study on tuning band gaps of monolayer and bilayer SnS2 and SnSe2 under external stimuli

Based on density functional theory, we systematically study the mechanical and electronic properties of monolayer and bilayer SnS₂ and SnSe₂. The electronic properties of these layers can be significantly tuned by applying in-plane strains and electric fields perpendicular to the sheets. The band gaps of monolayer SnS₂ and SnSe₂ slightly increase with the in-plane tensile strains, and they start to decrease after critical strains (5% for monolayer SnS₂ and 7% for monolayer SnSe₂). The band gaps of bilayer SnS₂ and SnSe₂ have a similar tendency to the monolayers with smaller critical strains (1% for bilayer SnS₂ and 2% for bilayer SnSe₂), which enables a semiconductor-to-metal transition at 10% strain for bilayer SnSe₂. We also find that an external electric field perpendicular to bilayer SnS₂ and SnSe₂ modulates their electronic band gaps. Semiconductor-to-metal transitions are achieved at the electric fields of 0.27 V/Å for bilayer SnS₂ and 0.13 V/Å for bilayer SnSe₂.     

Mean amplitudes of vibration of perhalogenated ethylenes

Mean amplitudes of vibration have been recalculated from spectroscopic data for C₂F₄, C₂Cl₄ and C₂Br₄. For C₂I₄ the calculated mean amplitudes are reported for the first time. The results for C₂Cl₄, C₂Br₄ and C₂I₄ agree perfectly satisfactorily with observed values from recent electron diffraction investigations of these molecules. Calculated values of generalized meansquare amplitudes for all the four molecules are also given.     

Probing the isomer,fluorination and bond effects in C<Subscript>3</Subscript>H<Subscript>6</Subscript>, cyclo-C<Subscript>3</Subscript>H<Subscript>6</Subscript> and C<Subscript>3</Subscript>F<Subscript>6</Subscript> molecules using electron impact

We have carried out experimental and theoretical studies on electron scattering from the C₃H₆ isomers and C₃F₆ molecules and we report on total, differential as well as theoretical integral elastic cross-sections for these molecules. Vibrational excitation functions are also presented for the typical vibrational peaks in C₃H₆ and cyclo-C₃H₆ for the angle of 90^○, impact energy range of 1–16 eV and loss energies of 0.12 eV and 0.13 eV, respectively. In the cross-sections, clear differences in peak positions and magnitudes between the C₃H₆ isomers can be viewed as the isomer effect. The same is observed between C₃H₆ and C₃F₆ in a clear manifestation of the fluorination effect. The resemblance of the π^* shape resonance in the cross-sections, observed at about 2.2 eV for C₃H₆ and 3.5 eV for C₃F₆, to those in C₂H₄ and C₂F₄ clearly points to the effect of the double bond in the molecular structures for these molecules. Theoretical analysis is performed to provide rationales for the scattering dynamics.     

Fundamental frequency and sound pressure level of phonation in pathological states

The fundamental frequency (F₀) for the habitual pitch (F_0HAB), F₀ for the lowest physiological tone (F_0L), F₀ for the highest physiological tone (F_0H), F₀ range of phonation (F₀ Rg), sound pressure level (SPL) for habitual loudness (SPL_HAB), SPL for the softest tone (SPL_S), SPL for the loudest tone (SPL_L), and SPL range of phonation (SPL Rg) were measured in 40 normal adult subjects and 1,563 voice patients with varying diseases. F_0H, F₀ Rg, SPL_L, and SPL Rg were decreased in many disease groups. F_0HAB and F_0L varied. SPL_HAB and SPL_S were increased in some disease groups. F₀-related parameters reflected effects of treatments in the cases with polyp, Reinke's edema, epithelial hyperplasia, carcinoma, and paralysis. Effects of treatments were manifested in SPL-related parameters in the cases with nodule, polyp, carcinoma, and paralysis.     

Shape memory behaviour of annealed Ti48.5Ni(51.5− x )Cu x (x = 6.2–33.5) thin films

The shape memory behaviour of (Ni,Cu)-rich Ti–Ni–Cu thin films (Ti_48.9Ni_44.9Cu_6.2, Ti_48.5Ni₄₀Cu_11.5, Ti_48.6Ni_35.9Cu_15.5, Ti_48.3Ni_28.4Cu_23.3, Ti_48.3Ni_23.9Cu_27.8 and Ti_48.5Ni₁₈Cu_33.5) annealed at 773, 873 and 973 K for 1 h was investigated. The films with 6.2, 11.5–15.5 and 23.3–33.5 at% Cu showed a single-stage deformation due to a B2 ? B19′ transformation, a two-stage deformation due to the B2 ? B19 ? B19′ transformation and a single-stage deformation due to the B2 ? B19′ transformation, respectively. The martensitic transformation start temperature (M _s) increased with increasing Cu content and then levelled off for more than 15 at% Cu, indicating a high Ms temperature of 345 K. Temperature hystereses were almost 15 K for all films with more than 10 at% Cu. The critical stress for slip increased with increasing Cu content and increased significantly for the Ti_48.5Ni₁₈Cu_33.5 film, whereas the maximum recoverable strain significantly decreased for the Ti_48.5Ni₁₈Cu_33.5 film. With decreasing annealing temperature, the critical stress for slip increased, but the M _s temperature decreased. It was found that films with 11.5 at% Cu or more, annealed at 873 K, showed a high martensitic transformation temperature and a high critical stress for slip.     

Interaction of different types of nanocages (Al12N12, Al12P12, B12N12, Be12O12, Mg12O12, Si12C12 and C24) with HCN and ClCN: DFT,TD-DFT,QTAIM, and NBO calculations

In this work, the ability of different types of nanocages including Al₁₂N₁₂, Al₁₂P₁₂, Be₁₂O₁₂, B₁₂N₁₂, Si₁₂C₁₂, Mg₁₂O₁₂ and C₂₄ for the adsorption and detection of poisonous gases HCN and ClCN has been investigated, theoretically using the D3 dispersion corrected density functional theory (DFT-D3). The absorption spectra of HCN–nanocage and ClCN–nanocage complexes were calculated by the time-dependent density functional theory (TD-DFT) and compared with the calculated absorption spectrum of isolated nanocage to investigate the ability of nanocage for sensing of HCN and ClCN gases. It was found that the strongest interaction between HCN (ClCN) molecule and nanocage takes place when the molecule is adsorbed via its N atom on the surface of nanocage except for C₂₄. Also, it was shown that the Al₁₂N₁₂ is the best adsorbent for HCN and ClCN gases among the selected nanocages and Si₁₂C₁₂ is the best sensor for the detection of these gases using the electroconductivity and absorption spectroscopy techniques.     

Analytische Fern- und Laborkontrolle bei der Wideraufarbeitung von Brennelementen aus KKW

The activity of Ni–Cr₂O₃ catalyst for the deuterium exchange reactions between hydrogen and ammonia, as well as for hydrogen and water vapor has been measured in dependence on the reaction temperature and on partial pressure of ammonia and water vapor, respectively. In both cases the activity in dependence on the partial pressure shows a maximum; the maximum of activity for H₂–NH₃ exchange is situated between partial pressures of 0.05 and 0.25, and for H₂–H₂O reaction between 0.25 and 0.5. The Ni–Cr₂O₃ catalyst is about 2.4 more active for the exchange reaction H₂–H₂O than for H₂–NH₃. For both reactions, chromia has a strong promoting effect, enhancing the activity per gram of catalyst of about three orders of magnitude in comparison with that of the Nickel black.     

Band structure of new layered arsenides SrRu<Subscript>2</Subscript>As<Subscript>2</Subscript> and BaRu<Subscript>2</Subscript>As<Subscript>2</Subscript>

This paper reports on the results of the ab initio FLAPW-GGA band structure calculations for two new layered phases SrRu₂As₂ and BaRu₂As₂, which are isostructural and isoelectronic to the known tetragonal (Ca,Sr,Ba)Fe₂As₂ basis phases of the FeAs superconductor family. The energy bands, densities of states, topology of the Fermi surface, low-temperature electron specific heats, and molar Pauli paramagnetic susceptibilities of SrRu₂As₂ and BaRu₂As₂ are determined for the first time and discussed in comparison with those for BaFe₂As₂ and BaRh₂As₂.     

Superparamagnetic-like behavior and spin-orbit coupling in (Co,Zn)RE4W3O16 tungstates (RE=Nd, Sm, Eu, Gd, Dy and Ho)

Magnetic susceptibility measurements carried out on (Co,Zn)RE₄W₃O₁₆ compounds revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to magnetic field and temperature for ZnSm₄W₃O₁₆ and ZnEu₄W₃O₁₆ samples. The temperature independent component of magnetic susceptibility has a negative value for ZnGd₄W₃O₁₆ and a positive one for the rest of the tungstates, indicating a domination of van Vleck contribution. The magnetization isotherms of majority of the tungstates under study revealed both spontaneous magnetic moments and hysteresis characteristic for the superparamagnetic-like behavior with blocking temperature T_B∼30 K, except for ZnEu₄W₃O₁₆. Fitting procedure of the Landé factor revealed that the stronger the orbital contribution, the weaker the superparamagnetic effect, namely for ZnRE₄W₃O₁₆. In case of CoRE₄W₃O₁₆ a significant participation of the Co²⁺ moment in the spontaneous magnetization was observed.     

Inelastic magnetic X-ray scattering from highly correlated electron systems: La1.2Sr1.8Mn2O7, La0.7Sr0.3MnO3 and Fe3O4

Magnetic Compton profiles have been measured for the colossal magnetoresistance manganites La_1.2Sr_1.8Mn₂O₇ and La_0.7Sr_0.3MnO₃, and for magnetite Fe₃O₄, along various crystallographic directions, over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations for the double layer manganite, La_1.2Sr_1.8Mn₂O₇, and by using a simple model involving atomic d-orbitals and free electrons for the other two compounds. For all three materials a preference for the occupation of e_g orbitals is found, particularly, for orbitals of dx²−y² symmetry. An itinerant electron contribution is adduced at all temperatures in magnetite; such a contribution also appears in La_1.2Sr_1.8Mn₂O₇, but it is present only at low temperatures in La_0.7Sr_0.3MnO₃.     

A statistical model for the product energy distribution in reactions leading to prompt dissociation

Direct dynamics calculations have been performed for three reactions: C₃H₈ + H → i-C₃H₇ + H₂, C₃H₈ + H → n-C₃H₇ + H₂, and C₂H₃ + O₂ → HCO + CH₂O. The fraction of the population for the radical products that promptly dissociates is computed. The results for C₃H₈ + H are qualitatively similar to previous results for C₃H₈ + OH, but the new results exhibit a slightly higher branching fraction for prompt dissociation products, owing to the fact that a greater fraction of the internal energy in the transition state ends up in the radical. For C₂H₃ + O₂ → HCO + CH₂O, the fraction of HCO that promptly dissociates is in excess of 99%. Consequently, the main product for C₂H₃ + O₂ at lower temperatures should be written as H + CO + CH₂O and not HCO + CH₂O. These results are then compared with four previous systems: CH₂O + H → HCO + H₂, CH₂O + OH → HCO + H₂O, C₃H₈ + OH → i-C₃H₇ + H₂O, and C₃H₈ + OH → n-C₃H₇ + H₂O. Based upon these seven system, several statistical models are presented. The goal of these statistical models is to predict the fraction of the transition state energy that ends up in the rovibrationally excited radical. On average, these statistical models provide an excellent prediction of product energy distribution. Consequently, these models can be used instead of costly trajectory simulations for predicting prompt radical dissociation for larger species.     

The elastic constants of isomorphous sodium bromate and sodium chlorate from 77 to 350 K

The elastic constants of NaBrO₃ and NaClO₃ are evaluated from ultrasonic velocity measurements using pulse superposition techniques. The values of C₁₁, C₁₂ and C₄₄ for NaBrO₃ at 298°K are 5.57₈, 1.70₅, 1.51₀ (x 10¹⁰ N/m²) and for NaClO₃ the values are 4.89₇, 1.38₉, 1.17₄. The values at 77°K are respectively 6.35, 1.98 and 1.65 for NaBrO₃ and 6.15, 2.16 and 1.32 for NaClO₃.     

Photocatalytic properties of lanthanide tungstates Ln2W2O9 (Ln=La,Pr, Nd,Sm, and Gd)

Lanthanide tungstates, Ln₂W₂O₉ (Ln=La, Pr, Nd, Sm, and Gd), were prepared via the polymerized complex method at 1273 K for 2 h, and their photocatalytic activities for hydrogen and oxygen evolution were investigated. Pt-loaded Gd₂W₂O₉ exhibited activity for H₂ evolution from an aqueous methanol solution under light irradiation (λ>300 nm). The remaining Ln₂W₂O₉ were inactive for H₂ evolution due to the influence of the Ln elements and their crystal structures. All Ln₂W₂O₉ were inactive for O₂ evolution from an aqueous AgNO₃ solution due to the lack of O₂ evolution sites on the surface.     

Site–site potential function and second virial coefficients for linear molecules

The second virial coefficients for several linear molecules were calculated using the 2CLJ potential including the electrostatic and induction effects with modified mixing rules for unlike pairs. Least squares fits of experimental values for B(T) were used to calculate the energy parameters σ and ε in the LJ core potential for N₂, O₂, Cl₂, F₂, CO, CO₂, NO, N₂O, C₂H₆, C₂F₆ and the strongly polar molecules CH₃Cl, CH₃F, CH₃CF₃, CH₃CHF₂, and CF₃CH₂F. The analysis takes into account rotation of the dipole out of the molecular axis. The calculated results for the second virial coefficient agree well with experimental data. In addition, the effect of the induction terms on the potential for calculating the second virial coefficient is shown to be important only for the molecules with strong dipole or quadrupole moments.     

A revisit of photoluminescence property for vanadate oxides AVO3 (A:K, Rb and Cs) and M3V2O8 (M:Mg and Zn)

We revisited the vanadium oxide phosphors, AVO₃ (A:K, Rb, and Cs) and M₃V₂O₈ (A:Mg and Zn) for a revaluation of possibility of these compounds for lighting applications, and the internal quantum efficiency (η) and luminescent colour properties for AVO₃ (A:K, Rb, and Cs) and M₃V₂O₈ (A:Mg, and Zn) have been presented. The AVO₃ showed the broadband emission from 380 to 800 nm, and the η for the KVO₃, RbVO₃ and CsVO₃ were 4%, 79% and 87%, respectively. The CIE colour coordinates are located at white region on the chromaticity diagram. The M₃V₂O₈ (A:Mg and Zn) also exhibited a quite broadband emission between 410 and 900 nm, indicating yellow luminescent colour. The Zn₃V₂O₈ showed high η value, 52%, compared to that of the Mg₃V₂O₈ (η=6%). This enhancement of η in the Zn₃V₂O₈ could be due to the increasing exciton diffusion assisted by the hybridizations of Zn 3d and O 2p orbitals for the valence band, and Zn 4s and Ti 3d orbitals for the conduction band.     

Pressure dependence of magnetic ordering temperatures of rare earth - Sn3 compounds

Measurements of the hydrostatic pressure dependence of the Neél temperatures, T_N, are reported for PrSn₃, GdSn₃, GdSn₃ and CeIn₃. T_N is found to increase with applied pressure for PrSn₃ and NdSn₃, whereas T_N is pressure independent within experimental error for GdSn₃ and CeIn₃. Slightly Sn-deficient RESn₃ (RE=rare earth) compounds are found consistently to be weakly ferromagnetic. The physical properties of the RESn₃ compounds exhibit analogies with the corresponding properties of dilute superconducting ($\text{La}$RE)Sn₃ alloys. The high pressure data for PrSn₃ and CeIn₃ are qualitatively consistent with a “Kondo necklace” model for magnetically ordered RE compounds with unstable 4f shells.