Second-order ferroic properties of a Pb5Ge3O11 uniaxial ferroelectric

The ferrobielastic properties (ferroic properties of the second order) earlier theoretically predicted for lead germanate uniaxial ferroelectric crystals are justified experimentally. It is demonstrated that single-domain samples are formed upon cooling to temperatures below the Curie point under uniaxial mechanical stresses corresponding to a combination of mechanical stresses σ₁₁σ₁₃ or σ₂₂σ₂₃. The macroscopic mechanism of this phenomenon is considered.     

On isotropic states in α-LiNH4SO4 crystals

The spectral and baric dependences of the birefringence of α-LiNH₄SO₄ crystals are studied. It is found that the birefringence is rather sensitive to uniaxial compressions. A baric shift of the isotropic point to longer or shorter wavelengths depending on the uniaxial compression direction is observed and a generalized temperature-spectral-baric diagram of the isotropic state is plotted. A possibility of the appearance of a pseudoisotropic state under the action of uniaxial pressure σ _z or simultaneous pressures σ _x = σ _z is shown.     

Optical properties of mechanically compressed K<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of a uniaxial mechanical compression (σ_m ≤ 100 bar) on the spectral dependences (300–800 nm) of the birefringence Δn _i and refractive indices n _i of K₂SO₄ crystals is studied. The electronic polarizabilities, refractions, and parameters (λ_0i , B _1i ) of ultraviolet oscillators of mechanically compressed crystals are calculated. It is shown that the dispersions of Δn _i(λ) and n _i(λ) are normal and sharply increase near the absorption edge. It is found that the uniaxial compression changes the value of the dispersions dΔn _i/dλ and dn _i/dλ rather than their character. It is ascertained that the simultaneous action of the compressions σ_x and σ_z, as well as of σ_y and σ_z, leads to the appearance of new isotropic states in the K₂SO₄ crystal, which manifests itself in the equality of corresponding birefringences. It is shown that the baric dependences n _i(σ) are determined by the change in the density of oscillators (～30%), by the shift of the absorption edge and effective band maximum and by the change in the oscillator strength (～70%).     

Refractometry of uniaxially compressed triglycine sulphate crystals doped with L-valine

The temperature and spectral dependences of the refractive indices n _i of triglycine sulphate (TGS) crystals doped with L-valine have been investigated. Doping is found to weaken the temperature dependence of n _i of TGS crystals. The electronic polarizabilities ?? _i , refractions R _i , and parameters of UV oscillators (??_0i , B _1i ) of mechanically distorted doped TGS crystals have been calculated. The temperature coefficients of the shift of the phase-transition point, ?T _c /??? _m , are found to be somewhat smaller than those for pure TGS crystals, which is confirmed by the increase in the hardness of TGS crystals after doping.     

Magnetoelastic contribution in domain wall dynamics of amorphous microwires

We studied the domain wall (DW) propagation of magnetically-bistable Fe–Co-rich microwires paying attention to the effect of applied and internal stresses. Magnetic field, H, dependences of DW velocity, v, were measured in Co_41.7Fe_36.4Si_10.1B_11.8 microwires with metallic nucleus diameters (from 13 μm to 18 μm) and with different ρ-ratio between the metallic nucleus diameter, d, and total microwire diameter, D. DW velocity decreased under the application of stresses. From measured dependences we evaluated DW mobility, S, dependence on the applied stresses. The results obtained for Co_41.7Fe_36.4Si_10.1B_11.8 sample show that S decreases with the increasing of applied stresses, σ_a. The observed dependences manifest that increasing of magnetoelastic anisotropy results in the decreasing of DW mobility and DW velocity     

Structure of triglycine sulfate embedded in porous aluminum oxide

The X-ray diffraction investigations have been performed for nanocomposite materials based on porous aluminum oxide with inclusions of TGS and TGS, which is doped with L,α-alanine (ATGS). The presence of the TGS and ATGS textures in pores of Al₂O₃ films has been found. It has been established that, under conditions of confined geometry, the broadening of diffraction maxima of the reflection is caused by the size effect. The temperature dependences of the order parameter for porous aluminum oxide with TGS inclusions have been constructed.     

Analysis of the relationship between the electron-capture and electron-loss cross sections for carbon ions

The electron-loss cross sections σ _{i, i + 1} and the electron-capture cross sections σ _{i, i ? 1} for carbon ions with energies of 35–330 keV/nucleon in hydrogen and neon are determined from experimental data. It is demonstrated that, for particle energies which satisfy the condition σ _{i, i + 1} = σ _{i, i ? 1} or σ _{i, i ? 1} = σ _{i ? 1, i} , the average equilibrium ion charge can be evaluated without solving the system of differential equations for charge exchange. The dependence of the average equilibrium ion charge on the ion energy is investigated for carbon ions.     

Refractive indices of K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals in an incommensurate phase under uniaxial stresses

The influence of uniaxial mechanical stresses directed along the principal crystallophysical axes on refractiveindex temperature dependences in K₂ZnCl₄ crystals was studied. It is established that the refractive indices ni are quite sensitive to uniaxial stresses. Significant baric shifts of the paraphase–incommensurate–commensurate phase transition points to different temperature regions were observed, which is due to the effect of the uniaxial stress on the K₂ZnCl₄ crystal structure. It is found that applying uniaxial pressure increases the value of the temperature hysteresis of the commensurate–incommensurate phase transition.     

Influence of microporosity on the strength properties of SiC ceramic materials

A series of silicon carbide ceramic samples with variable characteristics of the microporosity and strength, such as the ballistic strength σ _B and the static strength σ _S , are investigated. The dependences of the strength on the integral porosity for ceramic materials are determined. It is established that the strength (both σ _B and σ _S ) is directly proportional to the average length of the bridges between micropores. The mechanism of the influence of microporosity on the strength of the ceramic materials is elucidated. According to this mechanism, interpore bridges are concentrators of stresses and, hence, are broken when a load is applied to the ceramic material. Numerous breakings of bridges bring about the failure of the ceramic body. The average stress concentration coefficient is estimated as a function of the integral porosity of the ceramic material. It is demonstrated that the static strength of the ceramic material is determined by the presence of large micropores (50–100 μm).     

Electrostriction in a uniaxial ferroelectric with second order phase transition

The electrostriction constant γ(m²V^?2) of a uniaxial ferroelectric with a second order phase transition has been calculated as a function of the dielectric constant (?P/?E)₀. The results have been experimentally verified on triglycine sulphate (TGS) by measuring γ and (?P/?E)₀. A sign reversal of γ above the Curie temperature is presented.     

Stability and stabilization of 2H martensite in Cu–Zn–Al single crystals

The stabilization of the 2H martensitic phase in Cu–Zn–Al single crystals with an electron concentration e/a?=?1.53 was investigated. This orthorhombic 2H martensite was first induced from the cubic β phase by the direct β?→?2H or the indirect β?→?18R?→?2H transformations. On loading the 2H martensite, a transition without hysteresis is observed at a stress which was denoted σ_T1. It was found that this stress is associated with a change in the behaviour of the 2H martensite. A high stabilization of the 2H martensite, around 300?K, is only obtained if an ageing is performed at a stress above σ_T1. Additionally, the stresses of the transformation to another martensitic phase, called 18R₂, were found to be constant when the value of σ_T1 is below the retransformation stress. The 2H martensite and its behaviour on ageing were studied by dilatometry, calorimetry, mechanical testing, optical microscopy and transmission electron microscopy (TEM). Models accounting for the stabilization of the 2H martensite on ageing are proposed.     

Study of cross sections for the loss of outer 1s, 2s, and 2p electrons by fast ions and atoms of light elements

In the case of light-element ions propagating with velocities V = 1.83 and 3.65 au in H₂, He, N₂, Ne, and Ar, the loss cross sections σ_{i, i+m} for m electrons (m = 1, 2, 3) are considered. The partial loss cross sections σ_i(nl) for one of the outer 1s, 2s, or 2p electrons are determined using the obtained data. It is shown that the experimental cross sections for the loss of the 1s and 2s electrons by positive ions qualitatively agree with the theoretical values calculated in the Born approximation. In the case of the ion velocity V = 1.83 au, the cross sections σ_i for 2p electrons are greater than the cross sections σ_i (1s) and σ_i (2s) by a factor of 1.2–3 for the same binding energies of electrons in the ion (I _nl > 20 eV). It is found experimentally that, at V = 1.83 au, the cross sections σ_i (2p) for I _nl ～ 10–20 eV are less than the cross sections σ_i (1s) by a factor of 2–3, which is probably caused by a decrease in the screening parameter (θ_2p < 1) of the outer shell of atoms.     

Pressure effect on magnetization for (CoTm)90Zr10 (Tm = Cr,Mo) amorphous alloys

Temperature dependences of the forced volume magnetostriction dω/dH and the saturation magnetization σ_s for (CoTm)₉₀Zr₁₀ (Tm = Cr, Mo) amorphous alloys have recently been measured by the 3-terminal capacitance method and a vibrating sample magnetometer and magnetic balance at temperatures from 77 K to the Curie temperature T_c or the crystallization temperature. The pressure coefficient of σ_s0 at 0 K, d ln σ_s0/dp, is estimated from (dω/dH)₀ extrapolated to 0 K using the thermodynamical relation. The values of d ln σ_s0/dp increase in negative value with increasing Tm concentration. The relation between d ln σ_s0/dp and the pressure coefficient of T_c, d ln T_c/dp, estimated indirectly from dω/dH is discussed.     

Electronic structure and related properties of the ferroelectric crystal triglycine sulfate

The electronic band structure and related physical properties of the crystal triglycine sulfate (TGS), (NH₂CH₂COOH)₃·H₂SO₄, in the ferroelectric phase P2₁ have been calculated using the first principles Linear Combination of Atomic Orbitals (LCAO) code CRYSTAL’06 at the B3LYP level of theory. The interactions between the quasi-separate three glycine groups and the sulfate complex have been studied by the analysis of the density of states of the crystal. The glycine 2 group was found to be the zwitterion in the optimized structure. Ten coefficients of the elastic stiffness tensor c_kl, four coefficients of the tensor of the elastoelectrical effect e_ki, and the spontaneous polarization P_s of TGS have been calculated for the first time and have been found to agree well with experimental data. Extrema are found in the elastoelectric coefficient e₂₂ as a function of the strain ε₂, and in the spontaneous polarization P_s as a function of the unit cell volume.     

Magnetic anisotropy and saturation of LaFe12O19 and some related compounds

The saturation magnetization σ_s and uniaxial anisotropy constants K₁ and K₂ are measured on a polycrystalline, crystal-oriented sample of LaFe₁₂O₁₉ contaminated with known amounts of Fe₃O₄ and LaFeO₃. K₁ and K₂ increase strongly with decreasing temperature and the value of K₁ = (19–24) × 10⁵ erg/g at T = 0 shows that the substance is considerably more anisotropic than BaFe₁₂O₁₉ (K₁ ? 8·5 × 10⁵ erg/g) at low temperatures. The σ_s-T curve is more convex than that of BaFe₁₂O₁₉, so that σ_s is 11 per cent higher at room temperature but lower at T = 0. The value σ_s(T = 0) = 96·2 G cm³/g (19·2 μ_B/molecule) and the anisotropic behaviour are attributed to the presence of 1 Fe²⁺/molecule occupying the octahedral 2a sites in the magnetoplumbite lattice and having a uniaxial anisotropy of 10–15 cm^?1/ion.From measurements on polycrystalline, crystal-oriented samples of BaFe_10·8Fe²⁺_0·6Ti⁴⁺_0·6O₁₉ and BaFe_10·5Fe²⁺_1·0Sb⁵⁺_0·5O₁₉ it was found that, in comparison with LaFe₁₁Fe²⁺O₁₉, σ_s (T = 0) is smaller and K₁ is much smaller and much less temperature-dependent. The difference in anisotropic behaviour is attributed to a different distribution of the Fe²⁺ ions among the lattice sites due to the effective positive charge of the Ti⁴⁺ and Sb⁵⁺ ions.     

Acoustic symmetry and the third order elastic constants of tetragonal TII (4/m) crystals

The mode equations which relate the measured hydrostatic and uniaxial pressure derivatives of the “natural” ultrasonic velocity to the second and third order elastic constants for the tetragonal TII (4/m) Laue group have been transformed from the crystallographic (XYZ) axial frame to a reference frame (k, k + π/2, Z) comprised of two acoustic symmetry axes and the fourfold axis. It is found that, if the acoustic symmetry axes do not shift appreciably with pressure, the mode equations for TII crystals in this reference frame are much simpler than those in the crystallographie axial frame (XYZ), having the same form as those for the higher symmetry TI Laue group. Hence 12 transformed third order elastic constants can be obtained experimentally for TII crystals by making measurements of the hydrostatic and uniaxial pressure derivatives of ultrasonic wave velocity of modes propagated in the (k, k + π/2, Z) axial set with the uniaxial stresses also applied in this reference frame. The approach has been used to determine the 12 transformed third order elastic constants of scheelite (CaWO₄).     

Possible influence of the quasiparticle-pair interference term on the current-voltage characteristics of oxide barrier Josephson junctions

The coefficients σ₀, σ₁ and j₁ of the current-phase relation of a planar oxide barrier Josephson junction are evaluated in the BCS theory as a function of voltage and gap energy. Since σ₁/σ₀ is ? - 1 for small voltages, and ? 1 above the gap voltage, σ₁ may have some observable consequence on the current-voltage characteristics.     

Ionic conductivity of polycrystalline samples of KBrKI solid solutions

Ionic conductivity of KBr_xI_1?x(0?x?1) mixed crystals, σ_x, has been measured as a function of temperature in the range of 370°C to close to their melting points. The variation in conductivity, σ_x, with composition in the intrinsic region was found to be non-linear, having a maximum value at x=0.3. The maximum conductivity of KBr_xI_1?x mixed crystals was never far outside the range of conductivity of the component crystals. Several expressions of the relative conductivity, σ_x/σ₁ (σ₁ refers to KBr) have been suggested.     

Features of electrical conductivity in an incommensurate phase of graphite intercalation compound C10HNO3

Electrical conductivity and conduction-electron spin resonance (CESR) have been studied in stage-2 acceptor α-graphite-nitric acid intercalation compound C₁₀HNO₃. It is found that the electrical conductivity σ_c along the c axis in the structurally incommensurate phase of this compound is temperature independent, whereas the electrical conductivity σ_a along carbon layers exhibits “metallic” temperature behavior. Analysis of the temperature dependences of σ_c, σ_a, and the CESR linewidth demonstrates that, in the incommensurate phase of the graphite intercalation compound, the electrical conductivity along the c axis is realized through a nonband mechanism—the transfer of free charge carriers along thin high-conductivity channels shunting the carbon layers adjacent to the intercalate.