Long-time relaxation phenomena in Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) single crystals

Long-time polarization relaxation in the temperature range where PBSN-6 single crystals reside in the relaxor state was studied. An analysis of the time dependence of the permittivity ε′(t) performed at measuring frequencies from 1 Hz to 1 kHz in weak electric fields E₀ showed that the relaxation (or freezing) times derived by extrapolating relations of the type ε′(t) ～ log(t/t₀) and ε′(t) ～ exp{?[ln(t/t₀)]_β} range from 10⁸ to 10¹¹ min and depend substantially on the bias voltage applied to the sample. A study of the pattern of the dielectric response in moderate and strong infralow-frequency fields revealed that, after a sample was maintained under a bias lower than the coercive force, it no longer exhibited the additional anomalies in the amplitude dependences of the effective loss tangent tan_eff(E₀) than were observed in a thermally recuperated sample.     

Optical studies of delayed relaxation in Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) solid solutions

The temperature dependences of the permittivity, birefringence, optical transmittance, and small-angle light scattering and their variations with time are studied for single crystals of the Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ relaxor (PBSN-6) in the heterophase region of coexistence of different phases. It is shown that an electric field induces a phase transition to the ferroelectric state, which manifests itself within some time (delay time τ) after application of the electric field to the crystal. The observed dependence of the temperature of this transition on the heating rate of the sample and the changes in the birefringence and small-angle light scattering intensity with time confirm the kinetic character of the induced transition. Temperature dependences of the delay time τ for different electric fields are constructed. It is revealed that, at low temperatures, the delay time τ decreases with increasing temperature. This agrees with the behavior of τ in classical relaxors. At the Vogel-Fulcher temperature, however, one observes that dτ/dT reverses sign and τ increasing as the temperature continues to increase. This anomalous behavior of τ in the heterophase region is accounted for by the coexistence of the cubic relaxor and rhombohedral macrodomain phases.     

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.     

Specific features in the ferroelectric properties of Sr<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystals

The dielectric properties of strontium-barium niobate crystals with a strontium concentration of 0.75, which corresponds to the most smeared ferroelectric phase transition, were measured. The dielectric hysteresis loops and the polarization relaxation are shown to exhibit clearly pronounced features characteristic of crystals with structural disorder and a broad distribution of a random internal electric field. The measurements were performed in dc, slowly varying (quasi-static), and pulsed electric fields. The experimental data obtained are used to construct the distribution function for the coercive field in the crystal bulk.     

Structure and Polarization Relaxation of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript>/(001)Si Films

The structure and dielectric characteristics of strontium barium niobate thin films deposited on single-crystalline silicon substrates without buffer layers are studied. It is found that the c axis in these heterostructures runs largely normally to the substrate surface and the a and b axes are randomly oriented in the plane of the substrate. The polarization relaxation in such heterostructures is investigated. It is shown that the film–substrate interface in the heterostructures grown by rf cathode sputtering may contain a low amount of long-lived charged defects.     

Structure,the lattice dynamic,and the dielectric characteristics of Sr<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films

Solid solution Sr_0.5Ba_0.5Nb₂O₆ films have been synthesized on a (111)Pt/(001)Si substrate by rf deposition in an oxygen atmosphere. The depolarized Raman spectra, the structure, and the dielectric characteristics of the films have been studied over a wide temperature range. It is found that the films were singlephase, had the tetragonal tungsten bronze structure, and had a pronounced axial texture with axis 001 directed perpendicular to the substrate surface. It is shown that the film material undergoes a diffuse phase transition to the state of a relaxor ferroelectric in the temperature range 300–425 K. Possible reasons of the regularities observed are discussed.     

Formation of nanodomain ensembles during polarization reversal in Sr<Subscript>0.61</Subscript>Ba<Subscript>0.39</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript>: Ce single crystals

The results of the study of nano- and microdomain structure evolution in single crystals of relaxor ferroelectric strontium barium niobate Sr_0.61Ba_0.39Nb₂O₆ doped by cerium are presented. It was shown that the initial nanodomain structure represents a self-similar three-dimensional maze. The fractal dimension and average period were revealed. It was demonstrated that application of series of alternating electric field pulses makes it possible to produce a single-domain state in the surface layer. The features of the growth and “merging,” as well as the shape of ensembles of isolated nanodomains formed during switching from the single-domain state, were investigated. The formation of the nanodomain ensembles was considered as a result of the self-organized discrete switching controlled by determined nucleation.     

Electric field-induced birefringence in Pb0.94Ba0.06Sc0.5Nb0.5O3 (PBSN-6) solid-solution single crystals

The temperature dependence of birefringence induced by an electric field in PBSN-6 relaxors was studied. It was shown that the birefringence measured under different conditions of applying an electric field reveals a number of features not characteristic of the behavior of birefringence in conventional relaxors, more specifically, additional anomalies observed under sample cooling and heating in an electric field, a decrease in the thermal hysteresis width with an increase in the electric field strength, and the absence of birefringence saturation (impossibility to reach single-domain state of the sample during the experiment duration) at low temperatures in fields of up to 3 kV/cm. The results obtained are attributed to inhomogeneity of the cubic paraelectric relaxor phase caused by the coexistence of various polar regions having different dimensions and properties.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

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.     

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.     

Electromechanical properties of Pb<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> piezoelectric crystals grown from solution in a melt

Single crystals of lead gallium germanate Pb₃Ga₂Ge₄O₁₄ are grown from their own solution melts. The propagation of bulk acoustic waves is investigated, and the elastic, piezoelectric, and dielectric constants are calculated. The temperature dependences of the dielectric constants of this compound are analyzed.     

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.     

Dielectric investigations in Sr<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> relaxor ferroelectric thin films

The dielectric properties of Sr_0.75Ba_0.25Nb₂O₆ relaxor ferroelectric thin films were carefully analyzed. In contrast to bulk samples which present three distinct dielectric relaxation phenomena Sr_0.75Ba_0.25Nb₂O₆ thin films present only two of them. The suppression of the third anomaly can be mainly attributed to the narrow grain size distribution of nanograins and weak tensile strains imposed to the film from the substrate. The whole set of results point to the interpretation of a dielectric response characteristic of mesoscopic structure, which is composed of clusters and nanodomains.     

Low-frequency relaxation processes in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric crystals

Measurements and analysis of the temperature and frequency dependences of permittivity and losses and of the electrical resistivity of Pb₅Ge₃O₁₁ ferroelectric crystals at temperatures of 100 to 600 K and frequencies of 0.1 to 100 kHz are reported. The dielectric characteristics of the crystals exhibit, in addition to clearly pronounced anomalies near the Curie point T_C=450 K, less distinct anomalous features of the relaxation character in the range 230–260 K. The data obtained on the effect of various factors (degree of crystal polarization, crystal annealing at different temperatures and in different environments, etc.) on the low-temperature anomalies serve as a basis for discussing the possible mechanisms responsible for these anomalies. It is concluded that the low-temperature dielectric anomalies originate from thermal carrier localization in defect levels in the band gap, which entail the formation of local polarized states.     

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.     

Transport characteristics of phonons and the specific heat of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:ZrO<Subscript>2</Subscript> solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y₂O₃:ZrO₂ solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y₂O₃ content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y₂O₃:ZrO₂ solid solution.     

Fabrication and dielectric properties of modified calcium (Pb<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>)(Zr<Subscript>0.7</Subscript>Ti<Subscript>0.3</Subscript>)O<Subscript>3</Subscript> ceramics

The aim of the present work is to report investigations concerning the influence of homovalent modificators on relaxor properties of PBZT 25/70/30 ceramics. The selection of the proper homovalent additive was very important. Literature reports as well as data taken from the periodic table indicated, that calcium ions substitute themselves for lead ions with high likelihood of occurrence. The investigations showed that the substitution significantly changed the microstructure of ceramics – with grains of calcium modified ceramics decreasing and density increasing. The XRD measurements indicate that the pure PBZT ceramics as well as calcium dopant were characterised by tetragonal structure with space group I4 /mmm. Addition of calcium leads to a slight decrease in the lattice constant and crystal structure. The calcium modification also changes the dielectric properties. The temperature characteristic of the dielectric constant achieved a broadened maximum at temperature T_m, which decreases with increasing Ca content. The properties typical for ferroelectric relaxors weaken with increasing calcium dopant.     

Physical properties of FeSe<Subscript>0.5</Subscript>Te<Subscript>0.5</Subscript> single crystals grown under different conditions

We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe_0.5Te_0.5 single crystals grown by self-flux and Bridgman methods. The lowest values of the susceptibility in thenormal state, the highest transition temperature T _c of 14.4 K, and the largest heat-capacity anomaly at T _c were obtained for pure (oxygen-free) samples. The criticalcurrent density j _c of 8.6 × 10⁴A/cm² (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the anion sites. The samples containing an impurity phase of Fe₃O₄ show increased j _c up to2.3 × 10⁵A/cm² due to additional pinning centers. The upper critical field\(H_{c2}\)of ~500 kOe is estimated from the resistivity studyin magnetic fields parallel to the c-axis using a criterion of a 50%drop of the normal state resistivity R _n . The anisotropy ofthe upper critical fieldγ _{H c2} =H ^ab _c2/H _c2 ^c reaches a value ~6 at\(T\longrightarrow T_c\). Extremely low values of the residualSommerfeld coefficient \(\gamma_r\) of about 1 mJ/mol K²,compared to the normal state Sommerfeld coefficient γ _n = 25mJ/mol K² for pure samples indicate a high volume fraction of thesuperconducting phase (up to 97%). The electronic contribution to the specific heat in thesuperconducting state is well described within a single-band BCS model with a temperature dependent gapΔ(0 K) = 27(1) K. A broad cusp-like anomaly in the electronic specific heat observed at low temperatures in samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the Fe²⁺ ions at the 2c sites. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.     

Thermoelectric properties of Pb<Subscript>0.22</Subscript>Sn<Subscript>0.78</Subscript>Te<Ge> solid solution

Temperature dependences of thermoelectric parameters of the Pb_0.22Sn_0.78Te〈Ge(0.5 at%)〉 solid solution in the temperature range 140–440 K are investigated with the purpose to determine the perspectiveness of these solutions as a material for thermoelements.