Dynamics of photo-induced changes in the elastic characteristics of lithium niobate crystals doped with Jahn–Teller Fe<Superscript>2+</Superscript> ions

The effect of admixture Jahn–Teller Fe²⁺ ions on the elastic characteristics of lithium niobate was studied. The appearance of photostrains and a change in elastic moduli under the influence of laser radiation was established, thus enabling the creation of device elements with optically controlled elastic characteristics.     

Relaxation Contribution of a System of Jahn–Teller Complexes to the Elastic Moduli of Doped Fluorites

Journal of Experimental and Theoretical Physics - The results of the study of the temperature dependence of attenuation and velocity of ultrasonic waves are presented for CaF2:Cr and CaF2:Ni...     

Contribution to the study of structural and elastic properties of wüstite under pressure up to 140 GPa by pseudopotential calculations

Using first-principles calculations based on the density functional theory and the generalized gradient approximations, we have studied the effect of high pressures up to 140 GPa on the structural and elastic properties of wüstite. Our results indicate that FeO undergoes a structural phase transition from NaCl-type (B1) to NiAs-type (B8) almost at the pressure of 77 GPa. The density increases across this transition by about 5%, which is a higher value than that obtained in other researches. We can clearly present the wüstite elastic properties and isotropic wave velocities which are not already studied in this range of pressure, and we could compare these results with the available experiment data, especially with that of PREM model.     

First-principles study of the structural and elastic properties of AuxV1–x and AuxNb1–x alloys

Ab initio total energy calculations, based on the Exact Muffin-Tin Orbitals (EMTO) method in combination with the coherent potential approximation (CPA), are used to calculate the total energy of Au_xV_1–x and Au_xNb_1–x random alloys along the Bain path that connects the body-centred cubic (bcc) and face-centred cubic (fcc) structures as a function of composition x (0 ≤ x ≤ 1). The equilibrium Wigner–Seitz radius and the elastic properties of both systems are determined as a function of composition. Our theoretical prediction in case of pure elements (x = 0 or x = 1) are in good agreement with the available experimental data. For the Au–V system, the equilibrium Wigner–Seitz radius increase as x increases, while for the Au–Nb system, the equilibrium Wigner–Seitz radius is almost constant. The bulk modulus B and C₄₄ for both alloys exhibit nearly parabolic trend. On the other hand, the tetragonal shear elastic constant C′ decreases as x increases and correlates reasonably well with the structural energy difference between fcc and bcc structures. Our results offer a consistent starting point for further theoretical and experimental studies of the elastic and micromechanical properties of Au–V and Au–Nb systems.     

Jahn–Teller and coupled Jahn–Teller/Renner–Teller effects in the calculation of adiabatic-to-diabatic transformation angle for the lowest three 2A′ states of NH2 (NHH)

We report here on the first study of topological effects for the NHH system, as carried out by treating simultaneously the two dominant effects of this system, namely, the Jahn–Teller (JT) effect and the Renner–Teller (RT) effect. Both the effects were treated rigorously as demanded by the Born–Oppenheimer approach. No approximations were made and in those cases where convergence was required, it was achieved by including the required number of states. The study concentrates on calculating the privileged adiabatic-to-diabatic transformation (ADT) angle γ_12, along closed contours, which is the only needed angle to carry out the ADT in the case of relatively low energies. For this purpose, three coupled A′-states are usually considered and only in the last two extreme cases, where the area in configuration space becomes relatively large, namely 15–35 Ų, we had also to include an A^″ state (the second Δ-state), a situation that enforces the more elaborate (JT/RT) effect. In this paper, we also report on potential energies as calculated along the above-mentioned contours. Among them are considered the energies associated with the two adiabatic Δ-states, 1A′ and 1A^″, of different symmetry and therefore are responsible for the RT effect. These states are expected to be degenerate along the collinear axis. It was revealed that these states, in contrast to the Renner theory, are not degenerate along a finite interval of the collinear axis at the vicinity of the JT conical intersection (JT-CI). In other words, the JT-CI annihilates the RT degeneracy along this interval.     

Jahn–Teller effect and the formation of anticorrosive protective layers of nitrilo-tris-methylene-phosphonates on surfaces of steel

Nitrilo-tris-methylene-phosphonato-three-aqua-complexes of chromium(II) and iron(II), the main components of protective layers for inhibiting steel corrosion with nitrilo-tris-methylene-phosphonates, are isolated and studied. Their ability to form protective layers is explained by their insolubility and resistance to water and air. These properties of Cr and Fe complexes are explained by Jahn–Teller distortion of the coordination environment of Cr and Fe atoms.     

Ultrasonic Determination of the Jahn–Teller Effect Parameters in Impurity-Containing Crystals

Journal of Experimental and Theoretical Physics - A method is developed to determine the symmetry properties of strains and the type of Jahn–Teller effect in crystals with impurity ions in a...     

The role of the second order Jahn–Teller effect in the thermal reactions of ethylene

The UV absorption spectrum of ethylene during the pulse heating of an ethylene–argon mixture to prepyrolysis temperatures in the range 950 < Т < 1285 K was studied by kinetic spectroscopy in a free-piston adiabatic compression unit. New intense light absorption bands were found at 210 < λ < 260 and 440 < λ < 490 nm. Ab initio quantum-chemical calculations of the mechanism of the thermal cis-trans isomerization of ethylene were performed. When this mixture is heated, the point symmetry group of the ethylene molecule in the ground state S₀ reduces to С ₁, which is characteristic for ethylene at the minimum of the S ₁ state, due to the second order Jahn–Teller effect.     

The structural,elastic, and electronic properties of ZrxNbl-xC alloys from first principle calculations＊

The structural, elastic, electronic, and thermodynamic properties of ZrxNbl xC alloys are investigated using the first principles method based on the density functional theory. The results show that the structural properties of Zr~.Nb1 xC alloys vary continuously with the increase of Zr composition. The alloy possesses both the highest shear modulus （215 GPa） and a higher bulk modulus （294 GPa）, with a Zr composition of 0.21. Meanwhile, the Zr0.2! Nb0.79C alloy shows metallic conductivity based on the analysis of the density of states. In addition, the thermodynamic stability of the designed alloys is estimated using the calculated enthalpy of mixing.     

Determining the Parameters of the Jahn–Teller Effect in Impurity Centers from Ultrasonic Experiments: Application to the ZnSe : Ni2+ Crystal

Physics of the Solid State - The earlier developed methodology of ultrasonic investigation for determining the parameters of the Jahn–Teller effect (JTE) in impurity centers in crystals is...     

Investigation of structural,half-metallic and elastic properties of a new full-Heusler compound–Ir2MnSi

The electronic structure, magnetic and elastic properties of Ir₂MnSi full-Heusler compound is studied within the framework of Density Functional Theory (DFT). The ferromagnetic (FM) and non-magnetic (NM) states are compared in Cu₂MnAl and Hg₂CuTi prototype structures. The ferromagnetic state in Cu₂MnAl structure has been found energetically more stable than non-magnetic state in these two types of structures. Due to this stability, all calculations are carried out for FM-state. The spin-polarized calculations show that the spin-up electrons of Ir₂MnSi compound have metallic nature, but the spin-down electrons have semiconducting behavior with 0.55 eV energy gap around the Fermi level. The calculated Cauchy pressure and Poisson's ratio indicated that Ir₂MnSi compound is a ductile material. Ir₂MnSi compound is a half-metallic ferromagnet (HMF) and it has 5µ_B magnetic moment. This study will theoretically lead to experimental works in the spintronic field and its applications.     

Ferroelectric,pyroelectric and electrical properties of new tungsten–bronze tantalate

The polycrystalline sample of Li₂Pb₂Y₂W₂Ti₄Ta₄O₃₀ was prepared by a high-temperature solid-state reaction technique. Room temperature X-ray structural analysis confirms the formation of a single-phase compound. The surface morphology of the sintered pellet sample recorded by SEM (scanning electron microscope) exhibits a uniform grain distribution with few voids. Detailed studies of dielectric constant, tangent loss and remanent and spontaneous polarization with temperature and frequency exhibit the existence of ferroelectricity in the material. The temperature and frequency dependence of impedance parameters (impedance, modulus, etc) of the material exhibits a strong correlation between these electrical parameters with its micro-structure (i.e., bulk, grain boundary, etc). The nature of variation of pyroelectric-coefficient and current with temperature suggests that material has good pyroelectric properties useful for fabrication of pyroelectric detector.     

On the structural properties of B–C–N nanotubes

We apply a first-principles method, based on the density functional theory, to calculate the structural stability of B–C–N armchair nanotubes, comparing such results with the ones obtained for zigzag configuration. Analysis of the corresponding strain energies confirm that the stability of BC₂N nanotubes is independent of their chirality and demonstrate that such nanostructures have lower strain than BCN and carbon nanotubes. The results show that the formation energy decreases with the tube diameter and indicate that the most stable nanotubes have the maximum number of B–N and C–C bonds. Therefore, from the experimental point of view, larger diameter BC₂N model-I nanotubes should be more probable to be synthesized.     

First-principles calculation of the structural,electronic, elastic,and optical properties of sulfur-doping ε-GaSe crystal

The structural,electronic,mechanical properties,and frequency-dependent refractive indexes of GaSe_(1-x)S_x(x=0,0.25,and 1) are studied by using the first-principles pseudopotential method within density functional theory.The calculated results demonstrate the relationships between intralayer structure and elastic modulus in GaSe_(1-x)S_x(x=0,0.25,and 1).Doping of ε-GaSe with S strengthens the Ga-X bonds and increases its elastic moduli of C_(11) and C_(66).Born effective charge analysis provides an explanation for the modification of cleavage properties about the doping of e-GaSe with S.The calculated results of band gaps suggest that the distance between intralayer atom and substitution of S_(Se),rather than interlayer force,is a key factor influencing the electronic exciton energy of the layer semiconductor.The calculated refractive indexes indicate that the doping of ε-GaSe with S reduces its refractive index and increases its birefringence.     

Relativistic Jahn–Teller Effect for Triplet States of Tetrahedral Molecular Complexes

A relativistic multimode Jahn?Teller effect for tetrahedral molecular complexes in a triplet electronic state is considered. The analysis is based on the symmetry properties of the electronic Hamiltonian and its generalized symmetry operators, acting on both the coordinates (spatial operations) and spins (matrix operations) of the electrons. As a result, a 9 × 9 vibronic matrix that includes the vibronic coupling constants of orbital and spin-orbital nature and depends on the five normal modes of t₂ and e symmetry has been obtained.     

Magnetic properties of the Ashkin–Teller model on a hexagonal nanotube

A new theoretical model for nanomagnets represented by the Ashkin–Teller model on a core-shell hexagonal nanotube is proposed. The Mean Field Theory from the Bogoliubov inequality is applied to study the magnetizations, phase boundaries and tricritical points. For a positive couplings system (ferromagnetic), first order and continuous phase transitions between the stable, metastable and unstable states are observed. For a negative core-shell coupling system, only continuous phase transitions between the stable and unstable states are observed. The phase diagram is presented to illustrate the different phases and transitions exhibited by the model.     

First-principles investigation of the elastic and electronic properties of the binary intermetallics in the Al–La alloy system

The structural, elastic and electronic properties of Al₂La, AlLa₃ and Al₃La binary intermetallics in the Al–La alloy system were investigated using the first-principles method. The calculated lattice constants were consistent with the experimental values. Formation enthalpy and cohesive energy showed that the studied Al₂La, AlLa₃ and Al₃La all have a higher structural stability, and the alloying ability of Al₂La and Al₃La is stronger than that of AlLa₃. The single-crystal elastic constants (C_ij) as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio υ and anisotropy value A) were calculated by the Voigt–Reuss–Hill (V–R–H) approximations, and the relationship of these elastic parameters between Al₂La, AlLa₃ and Al₃La phases were discussed in detail. The results showed that Al₂La and Al₃La which are anisotropic materials are absolutely brittle, while the isotropic AlLa₃ is slightly ductile. Finally, the electronic density of states (DOS) was also calculated to reveal the underlying mechanism of structural stability.