Die elastische Wechselwirkung zwischen O 2 − -Zentren in Alkalihalogeniden-Zentren in Alkalihalogeniden

The interaction between two elastic dipoles has been studied directly by means of electron spin resonance. The investigated system consists of pairs of O ₂ ^? centers on nearest neighbor anion sites in KCl, KBr and KI. Since the O ₂ ^? centers reorient at temperatures as low as 1°K a pair assumes the configuration of lowest energy at these temperatures. The investigation of the EPR spectrum of the pairs and its response to externally applied uniaxial stresses has yielded the orientations of the axes of the pair partners in the configuration of lowest energy as well as some information on configurations of higher energy. The experiments are in agreement with the predictions based on continuum elasticity theory. Electric quadrupole interaction and covalency effects are less important. In particular the magnetic interaction is small compared to the elastic interaction.     

Elastic model of protein-protein interaction

Summary The elastic distortions of a model membrane are discussed, by taking into account those due to incorporated proteins. Two types of distortions exist: i) membrane compression and ii) displacement at constant thickness. The first mode decays on a very-short-length scale (≈nm). The second one is important since the characteristic lengh is of the order of the μm. This last distortion is characterized by a multipole expansion, in a way similar to electrostatics. The interaction of different elastic distortions is described in the context of their multipole expansion: for example, i) the monopole-monopole interaction is attractive (repulsive) in the case of opposite (equal) signs of the monopoles. The interaction energy behaves asW _AB≈Inr _AB (r _AB distance between the monopoles A and B) and may be even at large distance much higher thankT. ii) The dipole-dipole interaction is always attractive if the elastic dipole can rotate (fluid membrane). iii) The interaction energy varies asW _AB≈r _AB ⁻² (r _AB distance between the dipoles) and so on. This multipole concept may be used for the understanding of the recognition of different proteins. In addition, the particles which creaste the distortion may be polarizable. This means that shape changes of the particle could be induced by an external elastic field. We expect for a large particle a large polarizability. As a consequence, two static monopoles which reped each other may attract if they become polarizable. This feature is reminiscent of the Van der Waals interaction in electrostatics. Paper presented at the “Meeting on Lyotropics and Related Fields?, held in Rende, Cosenza, September 13–18, 1982.     

Observation of defect clustering in Gd-doped calcium fluoride

Abstract

Solid solutions Ca_1-xGd_xF_2+x for 3 × 10^?7≤ x ≤10^?1 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). The EPR experiments show the presence of two single-ion sites a cubic and a tetragonal Gd³⁺ center which co-exist with comparable abundances for intermediate impurity concentrations. The cubic center predominates at very low and high concentrations. Seven different relaxation processes have been identified from the ITC spectra and the variation of their intensity vs. x was measured. The absolute concentrations of the cubic and nn Gd³⁺ dipoles were calculated. The scavenging of interstitial fluorines by the neutral clusters explains both the abundance of cubic sites at high concentration and the variety of orientable clusters detected by ITC.     

The anharmonic force fields of PH3, PHF2, PF3, PH5, and H3PO

The cubic and quartic force fields of the title compounds are determined from ab initio SCF calculations using 6-31G^** and TZP/TZ2P basis sets. The computed geometries, vibration-rotation interaction constants, l-doubling constants, anharmonicity constants, and vibrational wavenumbers are compared with the available experimental data, especially for PH₃ and PF₃. Many experimentally unknown spectroscopic constants are predicted. A scaling procedure based on calculated harmonic and anharmonic force fields is proposed for predicting the vibrational wavenumbers of unknown molecules such as PH₅.     

Magnetic dipole energies in tetragonally-distorted cubic crystals

Dipole energies in tetragonally-distorted cubic crystals have been calculated by the Ewald-Kornfeld method for the case in which the dipoles are parallel to the c-axis. The results are accurate to at least 1 part in 10⁵, sufficient to provide accurate values of the strain-dependence and hence the dipolar magnetostriction constant h₁ within the range 0.98 ?c/a?1.02. Calculations are presented for bcc, fcc and diamond lattices.     

Effect of activator content on the TL of pretreated NaCl:Sr

Investigation of the thermoluminescence of Sr⁺⁺ doped NaCl phosphors indicate that a glow peak at 410 K has marked dependence on the thermal and mechanical teratments as well as on the concentration of the dopant. The results obtained are explained on the basis of the elastic interaction of the dipoles with the generated dislocation.     

Generation of disclination dipoles and nanoscopic cracks in deformed nanoceramic materials

The processes of generation of disclination dipoles and nanoscopic cracks (nanocracks) in deformed nanoceramic materials are investigated theoretically. It is demonstrated that disclination dipoles are formed at grain boundaries in the course of grain-boundary sliding. The geometric features of the generation of disclination dipoles are analyzed. The conditions under which the nucleation of nanocracks in the vicinity of the disclination dipoles is energetically favorable are calculated for the nanoceramic materials α-Al₂O₃ (corundum) and 3C-SiC (the cubic phase of silicon carbide). The equilibrium lengths of these nanocracks are also calculated. It is shown that the equilibrium lengths of nanocracks can be comparable to the grain size. As a consequence, these nanocracks can coalesce, thus eventually resulting in the brittle fracture of nanoceramic materials.     

The Beam-Wave Interaction in a Ka-Band Relativistic Coupled-Cavity TWT

By the method of the eigen mode expansion, a full three-dimensional (3-D) model has been developed that can be used to investigate the beam-wave interaction in a high-power, Ka-band relativistic coupled-cavity traveling-wave tube (TWT). In the tube studied by us, a sever in the interaction section is used to restrain the oscillation of RF electromagnetic fields between the input and output end. In this case, the asymmetric hybrid HTM₁₁ mode has little impact on the main interaction process ^[1,2], so the RF electromagnetic fields mainly interacting with the modulated electron beam belong to the symmetric mode TM₀₁. The presented model includes three-dimensional RF fields, three-dimensional electron motions, and improved three-dimensional space-charge fields including dc and ac space-charge fields. Moreover, this model can also calculate backward radiation excited by modulated electron beam and the direct effect of the transverse electron motion on the energy exchange. Our calculation results show that the space-charge field has evident effect on the interaction process, the transverse electron motion has some, and the backward radiation has little.     

Low temperature thermal expansion and magnetostriction of TmCd

Thermal expansion measurements in various external magnetic fields confirm the recently found Jahn-Teller transition in TmCd at 3.16 K. The tetragonal strain ?₃ at T = 0 K is found to be 0.62 × 10^?3. From magnetostriction data we determine a coupling constant g₀² = 0.4 × 10^?3 K which is in good agreement with the value found from ultrasonic experiments. In order to explain our magnetostriction results in the cubic phase, g₀ has to be taken negative.     

Negative pressure effects in SrTiO3 nanoparticles investigated by Raman spectroscopy

The size effects on SrTiO₃ nanoparticles have been investigated by means of Raman spectroscopy with changing the grain size in the range 10–80 nm. The intensities of the first-order polar TO₂ and TO₄ modes increase as the grain size reduces, suggesting the enhanced interaction of the surface-defect dipoles on the grain boundary. By contrast, the intensities for the first-order nonpolar TO₃ mode decrease with reducing the grain size. Further we have found that the Raman frequencies of the vibration modes are very sensitive to the variation of the grain size. The softening of the TO₂ and TO₃ modes with decreasing the grain size indicates the increase of the Ti–O bond length, which is consistent with the lattice expansion investigated by XRD. We have ascribed the size effects to the negative pressure effects due to the enhanced interaction of the surface-defect dipoles.     

On the phase transition in CsCN

X-ray diffraction and dielectric measurements have been performed on the molecular crystal CsCN as a function of temperature. The order parameter of the cubic to rhombohedral phase transition (T _c =186 K) was determined and interpreted by a coupling of the (CN)^? orientations to the shear strain. At lower temperatures the dielectric response is dominated by thermally activated relaxations processes rather than by electrical ordering of the CN dipoles.     

Neutron diffraction study of the plastic phases of polycrystalline SF6 and CBr4

An investigation of the plastic phases of polycrystalline specimens of sulphur hexafluoride (SF₆) and carbon tetrabromide (CBr₄) by neutron elastic scattering and neutron diffraction experiments is reported. A theory of neutron diffraction in plastic crystals, which treats the Bragg scattering and the diffuse scattering from a unified point of view, is developed and applied in the interpretation of the neutron results. The Debye-Scherrer peaks are analysed, both by a cumulant expansion technique and a cubic harmonic analysis, to determine the crystal structures of the plastic phase which are found to be body-centred cubic (space group Im3_m) for SF₆ and face-centred cubic (space group Fm3m) for CBr₄. The bond-orientation distribution function, f(Ω), has maxima in the <100> directions for SF₆ and in the <110> directions for CBr₄. Since, in both cases, f(Ω) is appreciably different from zero for all orientations, it is apparent that significant thermal reorientation takes place in both these crystals. The translational and librational displacements in CBr₄ are exceptionally large and give rise to extensive diffuse scattering which is analysed on the basis of a simple Einstein model. The model predicts that the centre-of-mass thermal vibration and the orientational disorder give approximately equal contributions to the total diffuse scattering. The calculated scattering is in good agreement with experiment for all wave vector transfers outside the range 2 to 3 Å^-1. Inside this range discrepancies occur which are interpreted as evidence for the existence of orientational short-range order in CBr₄.     

The cooperative spin transition in [FexZn1 − x(ptz)6](BF4)2: II. Structural properties and calculation of the elastic interaction

The cooperativity of the thermal spin transition in the Fe(II) spincrossover compound [Fe(ptz)₆](BF₄)₂ (ptz = 1-propyltetrazole) and in isomorphous mixed crystals with the isostructural zinc complex is investigated. From powder X-ray measurements the lattice deformation (tensor ε) accompanying the spin transition is determined. For diluted mixed crystals with x < 0.44 the lattice deformation is directly related to the spin transition, whereas in concentrated mixed crystals with x > 0.44 a first order crystallographic phase transition (R3i → P1i) is observed on cooling, which is triggered by the spin transition and can be suppressed by cooling rapidly. The thermal spin transition is measured with UV/VIS optical absorption spectroscopy on mixed single crystals in the R3i structure. From this metal dilution experiment an interaction constant of 169 cm ^{? 1} for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is calculated on the grounds of elasticity theory: The lattice deformation due to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fields and indirectly via the surface of the elastic crystal by an image pressure. The elastic properties of the crystalline matrix are taken in the isotropic approximation. They are derived from the complete sets of anisotropic elastic constants of the pure iron and zinc compounds, which have been measured previously by Brillouin spectroscopy. The contribution of elastic energy calculated this way is ? 80% of the experimental value of the interaction constant, i.e. the cooperativity in crystalline spincrossover compounds is quantitatively of elastic nature.     

Defect clustering in double doped fluorite crystals with Lu and Gd

Abstract

Solid solutions Ca¹ x-^yLu^xGd^y F_2+x+y for 10^?4 ≤ x ≤ 2 × 10^?2 and y=0.0001 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). It has been found that the ITC spectrum from 77 to 420 K is very weak and the main peak is attributed to the relaxation of both Lu³⁺-F^? _x and Gd³⁺F^? _i nn dipoles. No polarizable clusters are present in the temperature range explored here. The EPR spectra show the presence of Gd³⁺ tetragonal and cubic centers due to the local and non local compensation, respectively. The continuous decrease in the molar fraction of Gd³⁺ tetragonal centers together with the low concentration of Lu nn dipoles is an evidence of the existence at these low and intermediate concentrations of large clusters such as the cubo-octahedral hexamer which has been proposed for CaF₂ crystals very highly doped with small trivalent cations.     

Multicaloric effect in a solid: New aspects

The multicaloric effect that is the result of interaction between various caloric effects has been studied theoretically. The effects attributable to the pairwise interactions of fields (piezomagnetocaloric, piezoelectrocaloric, and magnetoelectrocaloric effects) have been added to the previously known electrocaloric, magnetocaloric, and elastocaloric effects that exist when the electric, magnetic, and elastic fields change. These new effects are shown to be determined by the temperature dependence of the piezomagnetic (magnetostrictive), piezoelectric (electrostrictive), and magnetoelectric coefficients. According to the estimates obtained, the change in entropy in an isothermal process under the magnetoelectrocaloric effect for Cr₂O₃ is 2–5 mJ kg^?1 K^?1. The caloric effects caused by the influence of the gradient in one of the fields on other fields are shown to contribute to the multicaloric effect. One of these gradient effects, the flexocaloric one, which consists in a change in temperature and/or entropy when a strain gradient is applied or removed, has been studied in detail as an example. It follows from the derived formulas that the greatest values of this effect should be expected for materials with strong temperature dependences of the flexocaloric coefficient, permittivity, or permeability. The change in temperature calculated from experimental data for a PMN ferroelectric is estimated as 2–6 mK at a strain gradient of 1 m^?1. The interaction between fields of a different nature is known to lead to the synergetic effect, and the multicaloric effect can reach values that are commonly called giant ones, expanding considerably the possible domains of its application.     

Resonanzaufspaltung im Schwingungsspektrum des CaSO4·2H2O

The resonance- or Davydov- or correlationfield-splittings of the vibrational levels of CaSO₄·2H₂O (gypsum) are calculated using the exciton theory ofDavydov. This theory is extended to include the induced polarization of crystalline matter by the electric fields of the transition dipoles. The numerical calculations are based on an experimental determination of the transition dipole moments and on a calculation of the dipole lattice sums by the Ewald method. The results obtained show, that in most cases the assumption of Coulomb interaction between transition dipoles alone does not completely explain the observed splittings. However, the shift of transition frequency with an increasing longitudinally polarized component of a polar crystal vibration can be predicted by this theory within the limits of accuracy of the experimental determination of transition moments.     

Terahertz studies of carrier localization in spontaneously forming polar lateral heterostructures

We employ near‐bandgap terahertz emission spectroscopy to study lateral heterostructures resulting from basal plane stacking faults in m ‐plane GaN. The predominant stacking faults have I₁ character and behave as an array of spontaneously forming layers comprised of a single cubic stacking sequence within the wurtzite matrix that terminate the spontaneous polarization along the in‐plane c‐axis, leading to strong lateral electric fields. Spectral tuning of femtosecond excitation pulses enables observation of the transition from carrier transport in the continuum to formation of instantaneous dipoles and nonlinear susceptibility associated with both the quantum‐well‐like regions of the cubic layers and polarization‐induced triangular‐like potentials. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The elastic behaviour of tetrahedral materials with vacant sites

As part of a study of defect tetrahedral structure compounds, the elastic constants of single crystal specimens of Hg₃Ga₂□Te₆ and HgIn₂□Te₄ have been measured between 77 K and room temperature using the pulse superposition technique. These compounds are included in the series HgTe, Hg₅In₂□Te₈, Hg₃In₂□Te₆ and HgIn₂□Te₄ and HgTe, Hg₅Ga₂□Te₈, Hg₃Ga₂□Te₆ where there is a progressive increase in the concentration of vacant sites. While the other compounds studied are cubic, HgIn₂□Te₄ has a tetragonal structure with a $\text{c}\text{a}$ ratio of 2.0. The components of the elastic stiffness tensor of this material at 77 K are (in units of 10¹¹ dyne cm^?2) C₁₁ = 4.31 C₁₂ = 2.54 C₄₄ = 2.14 C₃₃ = 4.47 C₁₃ = 2.18 C₆₆ = 2.41. In a cubic material C₁₁ = C₃₃, C₄₄ = C₆₆ and C₁₂ = C₁₃: the elastic behaviour of this tetragonal compound closely resembles that of a cubic material, as might be anticipated from its structure. This similarity is further illustrated by reference to the symmetry of phase velocity and Young's modulus surfaces. Examination of the elastic constants and reduced elastic constants of these compounds shows a regular trend, the elastic stiffness decreases as the number of vacant sites increases. There is an approximately linear relationship between the reduced bulk modulus and the number of sited vacancies.     

Low temperature heat capacity and magnetic excitation of HoAl2 in a magnetic field

The specific heat of single crystalline HoAl₂ in magnetic fields up to 7.5 T has been measured for the temperature range 1.5–16 K. In addition the energy of a magnetic excitation in a magnetic field of 5 T at 4.2 K has been determined by inelastic neutron scattering. The results have been interpreted with a cubic crystalline electric field and an exchange interaction using the same parameter set B₄=-0.85×10^-4 meV, B₆=+0.71× 10^-6 meV and T_C=31.5 K previously obtained by magnetization measurements.     

Possible mechanisms of dielectric relaxation of liquid water and calculation of the temperature dependence of the complex permittivity of water

The spectra of the complex dielectric permittivity and absorption of water (H₂O) in the frequency range 0–1000 cm^?1 are calculated for a wide temperature interval. Using the method of autocorrelation functions, the dielectric response of dipoles rotating in potential wells of three types is found. The majority of dipoles (about 90%) rotate in a deep and comparatively narrow potential well, whose profile resembles an upside-down hat. Such a potential models a molecular structure with strongly bent and/or broken hydrogen bonds. The hat model describes the complex permittivity in the low-frequency (Debye) range and in the range 300–1000 cm^?1. The remaining dipoles (～10%) execute harmonic vibrations of two types: rotational vibrations about the equilibrium direction of a hydrogen bond and translational vibrations along this direction. These types of motion yield the dielectric response in the frequency range 10–300 cm^?1. This response is described by the Lorentz lines in terms of the harmonic oscillator model and the truncated parabola model. The hat–harmonic oscillator–truncated parabola composite model provides good agreement with experimental spectra. The lifetimes of the three types of motion considered are about 10, 0.2, and 0.05 ps, respectively. They characterize (i) tetrahedral translations of molecules accompanied by their rotations, (ii) librations of dipoles in the hatlike potential well, and (iii) elastic interactions of hydrogen-bonded molecules. Based on data of independent methods of investigation, it is concluded that the temperature 300 K is a singular point with respect to the properties of liquid water.