Variation of electronic density in the superconducting phase transition in Nb<Subscript>3</Subscript>Al

The variation of electronic density in the superconducting phase transition in the classical superconductor Nb₃Al with critical temperature T_c=18.6 K was studied using ⁷³Ge emission Mössbauer spectroscopy. A comparison of the results obtained and the data available for the ⁶⁷Zn isotope in the lattices of high-temperature superconductors revealed a correlation between the electronic density variation at the Mössbauer probe nuclei sites and the value of T_c. This correlation is assumed to be related to the dependence of the electronic density variation on the standard correlation length.     

Acoustic emission and thermal expansion of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> and Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> crystals

The temperature dependence of the elongation per unit length for Pb(Mg_1/3Nb_2/3)O₃ crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1?x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.     

Structural and electrical properties of KCa<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> ceramics

A polycrystalline sample of KCa₂Nb₅O₁₅ with tungsten bronze structure was prepared by a mixed oxide method at high temperature. A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound shows a uniform grain distribution throughout the surface of the sample. Studies of temperature variation on dielectric response at various frequencies show that the compound has a transition temperature well above the room temperature (i.e., 105°C), which was confirmed by the polarization measurement. Electrical properties of the material have been studied using a complex impedance spectroscopy (CIS) technique in a wide temperature (31–500°C) and frequency (10²–10⁶ Hz) range that showed only bulk contribution and non-Debye type relaxation processes in the material. The activation energy of the compound (calculated from both the loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers. A possible ‘hopping’ mechanism for electrical transport processes in the system is evident from the modulus analysis. A plot of dc conductivity (bulk) with temperature variation demonstrates that the compound exhibits Arrhenius type of electrical conductivity.      

Ionic displacements and structure of disordered PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript>

A sequence of structural models of unit cells of complex oxides with perovskite-type structure has been constructed to refine the average structure of PbFe_1/2Nb_1/2O₃ (PFN) at temperatures above the ferroelectric Curie point (T _C ～ 110°C). Owing to the analysis of the probability theory considerations, each model is characterized by only two positional fitting parameters. With the use of the intensities of 95 symmetrically independent X-ray reflections from a PFN single crystal at 160°C, a model with the R factor below 3% (K = 2.44%) was chosen. The ravine method has been used to verify the existence of a single minimum for the R factor with respect to the fitting parameter.     

Electronic structure of A15-type compounds: V<Subscript>3</Subscript>Co,V<Subscript>3</Subscript>Rh,V<Subscript>3</Subscript>Ir and V<Subscript>3</Subscript>Os

Band structure and Fermi surfaces of the A₃B compounds V₃Co, V₃Rh, V₃Ir and V₃Os are calculated in FP-LAPW calculations. From V₃Co to the V₃Os compound one observes a decrease of the overlap for d-states from both V and B atoms; the center of gravity of the d-band for V moves upwards, while for the B-atom it moves toward lower energies. Hence, despite the band widening, a weakening of interactions takes place throughout this series, which leads to a lattice expansion as experimentally observed. The bonding mechanism in these compounds is found to be dominated by the lower energy d-states. Comparatively, the DOS at E_F does not change appreciably between these compounds, except for V₃Os, where a sharp peak is observed at E_F. This feature leads to the highest electronic heat-capacity coefficient γ (2.31 ) in this compound, which otherwise possesses the smallest Bulk modulus (209.05 GPa). In V₃Os, for whom no report has been found, a stronger admixture between p-states from both metals near E_F is observed, and the Os d-states form a common d-band with the V d-states between 0.4–0.6 Ry. A stronger s–s hybridization is observed for V₃Co, which is in the source of the fact that this compound has an anomalously large quadrupole interaction and exhibits a positive Knight shift.     

Nanostructured multiferroic PbFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> and its physical properties

Polycrystalline multiferroic PbFe_0.5Nb_0.5O3 (PFN) fabricated by a solid-phase method is studied. Before sintering, a synthesized PFN powder is processed in Bridgman anvils via a force action in combination with shear deformation (FASD) at room temperature. The electrophysical properties and structural parameters of processed samples and a reference sample are compared. Point defects are shown to play a key role in the formation of the physical properties beginning from an FASD of 200 MPa.     

Dielectric properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)(Nb<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript>)O<Subscript>3</Subscript> ferroelectric ceramics modified with BaTiO<Subscript>3</Subscript>

Processes of the polarization and repolarization of ferroelectric ceramics based on potassium sodium niobate in the region of infralow frequencies are discussed. The effect aging and subsequent annealing in a strong alternating electric field have on the nonlinearity of the dielectric response of a sample at different temperatures is determined.     

Heterophase states in 0.10PbTiO<Subscript>3</Subscript>-0.90Pb(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> crystals

A new model of the elastic matching of phases is proposed, and heterophase structures near the morphotropic phase boundary in 0.10PbTiO₃-0.90Pb(Zn_1/3Nb_2/3)O₃ crystals are studied. Unique behavior of the unit cell parameters is found to favor the elastic matching of the ferroelectric tetragonal and orthorhombic phases under the conditions of complete or partial relaxation of internal mechanical stresses at a volume concentration ratio of these phases of about 20/80% and temperatures of T=20–300 K. Interrelations between the volume concentrations of different domain (twin) types and of the coexisting phases are analyzed.     

Electro-optical polarization mode dispersion in PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> single crystals

The polarization mode dispersion induced by an external ac electric field in PbMg_1/3Nb_2/3O₃ single crystals is studied experimentally at frequencies of 10⁴–10⁷ Hz. It is established that the discovered electro-optical polarization mode dispersion (EPMD) displays anomalous properties related to the orientation of the induced dipole moments in microregions of the crystal. Analytical expressions describing the peculiarities of the physical properties of relaxors giving rise to the EPMD are obtained.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Analysis of the exchange magnetic structure in Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The indirect-coupling model is used to analyze the exchange magnetic structure of Pb₃Mn₇O₁₅ in the hexagonal setting. The ratios of manganese ions Mn⁴⁺/Mn³⁺ in each nonequivalent position are determined. Pb₃(Mn_0.95Ge_0.05)₇O₁₅ and Pb₃(Mn_0.95Ga_0.05)₇O₁₅ single crystals are grown by the solution–melt method in order to test the validity of the proposed model. The structural and magnetic properties of the single crystals are studied. The magnetic properties of the grown single crystals are compared with those of nominally pure Pb₃Mn₇O₁₅.     

Band structure of superconducting dodecaborides YB<Subscript>12</Subscript> and ZrB<Subscript>12</Subscript>

The band structure of superconducting UB₁₂-like cubic dodecaborides, namely, YB₁₂ and ZrB₁₂, is calculated in the framework of the self-consistent full-potential linearized muffin-tin orbital (FLMTO) method. The calculated parameters of the electronic subsystems of YB₁₂ and ZrB₁₂ dodecaborides are analyzed and compared with the relevant parameters of the hypothetical dodecaborides □B₁₂ (□ is a metal vacancy) and BB₁₂; nonsuperconducting AlB₂-like layered diborides, namely, YB₂ and ZrB₂; and a new superconductor, MgB₂.     

Heat capacity study of relaxor PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> in a wide temperature range

Heat capacity of the PbMg_1/3Nb_2/3O₃ compound is measured using the methods of adiabatic and differential scanning calorimetry in the temperature range 80–750 K. Two blurred anomalies on the C _p (T) dependence are observed in wide temperature intervals of 200–400 K and 500–700 K. The results of studies are discussed together with data on the structure and phonon spectrum in the framework of spherical random bond-random field model.     

On the possible separation of the phase enriched with Nb in superconducting intermetallic Nb<Subscript>3</Subscript>Sn irradiated with fast protons

The results of the study of magnetization and dynamic magnetic susceptibility are correlated with changes in the microstructure of superconducting intermetallic Nb₃Sn plates irradiated at the Kurchatov Institute cyclotron with fast protons with an energy of 12.8 MeV to a fluence of 1 · 10¹⁸ cm^?2. The dependence of the superconducting transition temperature on the total irradiation dose is determined. For one of the samples, the dynamic magnetic susceptibility exhibits several steps corresponding to superconducting transitions at different temperatures. It is assumed that a Nb-enriched phase is separated in the region of maximum radiation damages. A microstructural analysis shows the appearance of randomly oriented Nb-enriched regions from 0.1 to 0.5 μm in size.     

Magnetic properties and the magnetoimpedance effect of nanostructured Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons with induced magnetic anisotropy

The structure and magnetic properties and the magnetoimpedance effect of nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons, obtained from the amorphous state by annealing under different conditions, were comparatively analyzed. Despite the similarity of the samples’ structural states and the processes of their quasi-static magnetization reversal, the features of the magnetoimpedance effect are indicative of significant differences in the processes of their dynamic magnetization.     

A comparative Raman study of 0.65(PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>) -0.35(PbTiO<Subscript>3</Subscript>) single crystal and thin film

We report a comparative Raman study of 0.65(PbMg_1/3Nb_2/3O₃)-0.35(PbTiO₃) (PMN-0.35PT) single crystal and thin film. Raman spectra investigation indicates a change in bulk from the high temperature cubic to the tetragonal phase and then to the low temperature Mc monoclinic phase. The transition temperatures are in good agreement with the ones previously observed by dielectric measurements on the same sample. In contrast, we observe no phase transition to the monoclinic phase in the PMN-0.35PT 4000 Å thick film and only a cubic to tetragonal diffuse transition has been determined at high temperature. The enhanced stability of the tetragonal phase and the absence of low temperature monoclinic phase have been attributed to the in plane strain.     

X-ray diffraction studies of the structure of nanocrystals in Fe<Subscript>73.5</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> soft magnetic alloys before and after thermomechanical treatment

The structure of the Fe_73.5Si_13.5B₉Nb₃Cu₁ soft magnetic alloy has been investigated using X-ray diffraction in transmission geometry. The initial alloy prepared by rapid quenching from the melt has a short-range order (∼2 nm) in the atomic arrangement, which is characteristic of the Fe-Si structure with a body-centered cubic lattice. The alloy subjected to annealing contains Fe-Si nanocrystals with sizes as large as 10–12 nm. The annealing under a tensile load leads to an extension of the nanocrystal lattice so that, after cooling, a significant residual deformation is retained. This can be judged from the relative shifts of the (hkl) peaks in the X-ray diffraction patterns measured for two orientations of the scattering vector, namely, parallel and perpendicular to the direction of the load applied. The deformation is anisotropic: within the accuracy of the experiment, no distortions in the [111] direction are observed and the distortions in the [100] direction are maximum. It is known that crystals with a composition close to Fe₃Si exhibit a negative magnetostriction; i.e., their magnetization induced under a load (Villari effect) applied along the [100] direction is perpendicular to this direction along one of the easy magnetization ([010] or [001]) axes. In the alloy, the orientation of the nanocrystal axes is isotropic and the majority of the nanocrystals have a composition close to Fe₃Si. The direction of magnetization of these nanocrystals is determined by the residual deformation of their lattice and lies near the plane perpendicular to the direction of the tensile load applied during heat treatment. This is responsible for the appearance of transverse magnetic anisotropy of the easy-plane type in the Fe_73.5Si_13.5B9Nb₃Cu₁ alloy.     

Controlling the physical properties of PbIn<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> ferroelectric ceramics by intense mechanical action with shear

PbIn_0.5Nb_0.5O₃ (PIN) ferroelectric relaxer ceramics obtained by the solid-phase method is studied. Prior to sintering, the synthesized charge is subjected to mechanical action in Bridgman anvils at room temperature. The electrophysical properties of the reference sample and test samples are compared. It is concluded that mechanical treatment of the material at the stage of preparation allows one to control the physical properties of ferroelectric ceramics. Starting from a certain amount of the action, the electrophysical properties of PIN ceramics vary nonmonotonically.