Ab initio study of La-doped BaSnO<Subscript>3</Subscript> proton conductor

The hydration properties of Lanthanum-doped barium stannate have been studied using density functional calculations. The interaction energy between elementary defects (proton-dopant and oxygen vacancy-dopant) have been calculated in charged states corresponding to p-type conditions. Surprisingly, the most attractive energies are not found for first-neighbor relative positions. The geometric distortions and the electronic densities of state in the different configurations involved are presented. With these data, two simple models are used to estimate the energy of the hydration reaction. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Ab initio study of 2DEG at the surface of topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

By means of ab initio DFT calculation, we analyze the mechanism that drives the formation and evolution of the 2D electron gas (2DEG) states at the surface of Bi₂Te₃ topological insulator (TI). As it has been proved earlier it is due to an expansion of the van der Waals (vdW) spacing produced by intercalation of adsorbates. We will show that the effect of this expansion, in this particular surface, leads to several intriguing phenomena. On one hand we observe a different dispersion of the Dirac cone with respect to the ideal surface and the formation of Parabolic Bands (PB) below the conduction band and M-shaped bands in the valence band, the latters have been observed recently in photoemission experiments. On the other hand the expansion of the vdW gaps changes the symmetry of the orbitals forming the Dirac cone and therefore producing modifications in the local spin texture. The localization of these new 2DEG-states and the relocalization of the Dirac cone will be studied as well.     

Reactions C<Subscript>2</Subscript>H<Subscript>2</Subscript> + OH and C<Subscript>2</Subscript> + H<Subscript>2</Subscript>O: Ab initio study of the potential energy surfaces

The stationary points of the potential energy surfaces for the reactions C₂H₂ + OH and C₂ + H₂O are calculated using density functional theory and the coupled cluster method. The relative energies and geometric parameters of the stable intermediates and transition states are in good agreement with the results of independent studies. In most cases, the relative energies differ from the earlier published values by no more than 3 kcal/mol, whereas the rotational constants, by 1–2%. The mechanism of the reaction CCOH₂ → C₂ + H₂O is studied in detail. The possible sources of errors in the calculation methods are examined.     

Structure and dynamics of the Lu<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> lattice: Ab initio calculation

The ab initio calculations have been carried out for the crystal structure and Raman spectrum of a single crystal of lutetium pyrosilicate Lu₂Si₂O₇. The types of fundamental vibrations and their frequencies and intensities in the Raman spectrum for two polarizations of the crystal have been determined. The calculations have been performed within the framework of the density functional theory (DFT) using the hybrid functionals. The ions involved in the vibrations have been identified using the method of isotopic substitution. The results of the calculations are in good agreement with the experiment.     

Phonon spectrum of lead oxychloride Pb<Subscript>3</Subscript>O<Subscript>2</Subscript>Cl<Subscript>2</Subscript>: Ab initio calculation and experiment

IR and Raman spectra of Pb₃O₂Cl₂ in the range of 50–600 cm^–1 have been detected for the first time. Ab initio calculations of the crystal structure and the phonon spectrum of Pb₃O₂Cl₂ in the framework of LCAO approach have been performed by the Hartree–Fock method and in the framework of the density functional theory with the use of hybrid functionals. The results of calculations have made it possible to interpret the experimental vibration spectra and reveal silent modes, which do not manifest themselves in these spectra but influence the optical properties of the crystal.     

Synthesis and magnetic properties of Laves phase Fe<Subscript>2</Subscript>Nb amorphous alloy

The formation process and magnetic properties of the Laves phase compound of nominal composition Fe₂Nb in its amorphous phase prepared by mechanical alloying have been investigated. The effect of milling time on the formation of amorphous phase has been studied using an X-ray diffraction technique. Further characterizations were carried out by particle size measurement, dc magnetization, ac susceptibility and ferromagnetic resonance studies. Magnetization and susceptibility studies show soft ferromagnetic behaviour, whereas ferromagnetic resonance studies show some sort of disorder/strain introduced during the mechanical alloying process. PACS 74.70.Ad; 61.82.Bg; 74.25.Ha; 75.60.Ej; 76.50.+g     

Ion-pair states of I<Subscript>2</Subscript>, Br<Subscript>2</Subscript>, IBr,and ICl

The experimentally determined energies and rotational constants of the vibrational levels v = 0–20 of the Ion-Pair states Ω = 0⁺, Ω = 1 of the I₂, Br₂, IBr, and ICl molecules are modeled. The model used includes three diabatic states, which correlate to X⁺(3P, 1D) + Y^～(1S₀). These states are coupled by the spin-orbit interaction, which is assumed to be independent of the internuclear distance. For IBr and ICl, as well as for the ungerade states of I₂ and Br₂, satisfactory results are obtained. The model is less applicable to the gerade states of I₂ and Br₂, which is possibly results from the retainment of the asymptotic J _A J _B coupling of the angular momenta at equilibrium internuclear distances.     

Structure and lattice dynamics of PrFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>: Ab initio calculation

The crystal structure and phonon spectrum of PrFe₃(BO₃)₄ are ab initio calculated in the context of the density functional theory. The ion coordinates in the unit cell of a crystal and the lattice parameters are evaluated from the calculations. The types and frequencies of the fundamental vibrations, as well as the line intensities of the IR spectrum, are determined. The elastic constants of the crystal are calculated. A “seed” frequency of the vibration strongly interacting with the electron excitation on the praseodymium ion is obtained for low-frequency A₂ mode. The calculated results are in agreement with the known experimental data.     

Ab initio study on elastic and thermodynamical properties of Ti<Subscript>1-x</Subscript>Zr<Subscript>x</Subscript>C

Elastic and Thermodynamical properties of Ti_1-xZr_xC have been investigated using LAPW + lo within the density-functional theory with the generalized gradient approximation. We have studied the stability of the alloy Ti_1-xZr_xC as a function of Zr composition in rocksalt (B1) structure by calculating the elastic constants C₁₁, C₁₂ and C₄₄ using the tetragonal and trigonal distortions. Mechanical properties such as Poisson ratio, bulk, shear and Young’s modulii of Ti_1-xZr_xC are calculated. The Debye temperature and hardness are also computed for the first time to our knowledge for Ti_1-xZr_xC in various compositions.     

Control of the Coulomb explosion of I<Subscript>2</Subscript>

We present calculated results for the optimization highly-charged fragment ion formation in the Coulomb explosion of I₂ in an intense laser field. Calculations are performed using a simple genetic algorithm and a classical model for the Coulomb explosion process. We find that at low intensity the production of highly-charged fragment ions is optimized by a Fourier-limited pulse, whereas at higher intensity the Coulomb explosion is optimized by a sequence of pulses, with a time-separation determined by enhanced ionization at the critical internuclear distance. Our calculations provide insight into the sensitivity of adaptive pulse shaping experiments to the parameters and evolutionary approaches used.Received: 6 January 2003, Published online: 15 July 2003PACS: 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 82.53.Eb Pump probe studies of photodissociation - 02.60.Pn Numerical optimization     

Ab initio study of thermoelectric properties of Cu<Subscript>3</Subscript>PSe<Subscript>4</Subscript> and Cu<Subscript>3</Subscript>PS<Subscript>4</Subscript>: alternative materials for thermoelectric applications

This letter discusses the thermoelectric properties of Cu₃PSe₄ and Cu₃PS₄ compounds, using the Ab initio calculations. These compounds are predicted to be good thermoelectric materials thanks to the nature of their band edge states. Seebeck coefficient of Cu₃PSe₄ exhibits a maximum value of 1256 µV/K at roopm temperature, whereas it is 2389 µV/K for Cu₃PS₄. Furthermore, the electrical conductivity is significantly enhanced with doping level while the electronic thermal conductivity is weakly increased. Besides, the factor of merit of these compounds shows a value around the unity only at low doping levels. Hence, this predicts that these compounds may present excellent thermoelectric properties, therefore they could be considered as alternatives for thermoelectric applications.     

The magnetocaloric effect in hydrogen-doped Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B and Er<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B intermetallic compounds

The nature of the magnetocaloric effect (MCE) in compounds R₂Fe₁₄B (R = Nd, Er) and their hydrides in a wide temperature range is investigated. The investigation is carried out on initial samples of a special purity including single-crystalline ones. The highest value of the magnetocaloric effect is established in the Curie-temperature region. The hydrogenation of samples affects the value of the MCE. The model explaining the dependence of the value and sign of the magnetocaloric effect on the hydrogen contents in R₂Fe₁₄B compounds with participation of erbium and neodymium is proposed.     

Anti-stokes luminescence of solid AgCl<Subscript>0.95</Subscript>I<Subscript>0.05</Subscript> solutions

An anti-Stokes luminescence band with λ_max = 515 nm of microcrystals of solid AgCl_0.95I_0.05 solutions excited by a radiation flux of density 10¹³–10¹⁵ quanta/cm²·sec in the range 600–800 nm at 77 K was detected. It is shown that the intensity of this luminescence and the frequency of its excitation depend on the prior UV-irradiation of samples. Analysis of the stimulated-photoluminescence spectra and the anti-Stokes luminescence excitation spectra of the indicated microcrystals has shown that to the centers of anti-Stokes luminescence excitation correspond local levels in the forbidden band of the crystals. These states are apparently due to the atomic and molecular disperse silver particles that can be inherent in character or formed as a result of a low-temperature photochemical process. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 738–742, November–December, 2005.     

Phonon linewidths in YNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN₂B₂C was chosen as a test example because it is a conventional superconductor with a fairly high T _c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron-phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.      

Electric-discharge He/Xe/I<Subscript>2</Subscript> excimer-halogen UV lamp

The characteristics of a low-size low-pressure UV lamp operating on the atomic iodine resonant line (λ=206 nm) and xenon iodide (λ=253 nm) and dimer iodine (λ=342 nm) bands are investigated. A lamp with an interelectrode distance of 19 cm was pumped by a longitudinal dc glow discharge. The working gas mixtures were He/I₂ and He/Xe/I₂ mixtures with a total pressure of 50–1500 Pa. The output parameters of the electric-discharge excimer-halogen lamp were optimized in terms of the gas mixture pressure and composition and the power deposited in the discharge. It is shown that the total UV power emitted from the entire aperture of the lamp in the spectral range 200–350 nm attains 5–7 W with an efficiency of ≤5%.     

Structure,lattice dynamics,and exchange interaction in Lu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>, Y<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>: an ab initio approach

Ab initio calculations of the crystal structure and fundamental vibrations of vanadium pyrochlores Lu₂V₂O₇ and Y₂V₂O₇ are performed. The calculations are performed in the framework of the density functional theory (DFT) with the use of hybrid functionals. The ions involved in the vibrations are determined by the isotope substitution method. Values of the isotropic exchange interaction constant were calculated. Theoretical results for the crystal structure parameters, the vibrational frequencies, and the isotropic exchange interaction parameter are compared with the experimental data.     

Metamagnetism near <Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> in Mn-substituted chalcopyrite Cd<Subscript>0.90</Subscript>Mn<Subscript>0.10</Subscript>GeAs<Subscript>2</Subscript>

The magnetic and electric properties of a Cd_0.90Mn_0.10GeAs₂ solid solution with the chalcopyrite structure have been investigated in wide temperature and field ranges. It has been found that a metamagnetic transition from a low-magnetization state to a high-magnetization one is initiated in Cd_0.90Mn_0.10GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of magnetic properties. An external magnetic field at temperatures above T _C also induces the metamagnetic transition. When the temperature increases above T _C, the magnetization jump decreases, whereas fields inducing the metamagnetic transition increase. The band character of magnetism and metamagnetism in the effective magnetic field is assumed on the basis of the behavior of magnetization in the metamagnetic transition and analysis of the band structure of the solid solution of cadmium-germanium diarsenide with manganese.     

Magnetic dynamics of phase separation domains in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> and Gd<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.     

Electronic and optical properties of Fe<Subscript>2</Subscript>SiO<Subscript>4</Subscript> under pressure effect: ab initio study

We report first-principles studies the structural, electronic, and optical properties of the Fe₂SiO₄ fayalite in orthorhombic structure, including pressure dependence of structural parameters, band structures, density of states, and optical constants up to 30 GPa. The calculated results indicate that the linear compressibility along b axis is significantly higher than a and c axes, which is in agreement with earlier work. Meanwhile, the pressure dependence of the electronic band structure, density of states and partial density of states of Fe₂SiO₄ fayalite up to 30 GPa were presented. Moreover, the evolution of the dielectric function, absorption coefficient (α(ω)), reflectivity (R(ω)), and the real part of the refractive index (n(ω)) at high pressure are also presented.