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.     

Dielectric dispersion and magnetic properties of Ba-modified Pb(Fe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>

The polycrystalline samples of Ba-modified Pb(Fe_1/2Nb_1/2)O₃ (i.e., (Pb_1-xBa_x)(Fe_1/2Nb_1/2)O₃ PBFN, with x=0,0.05,0.07) were synthesized by a mechanosynthesis (i.e., high-energy ball milling) route followed by a mixed oxide method. Structural analysis provides the information on formation of single-phase orthorhombic structure on substitution of a small amount (x=0.07) of Ba at the Pb-site of Pb(Fe_0.50Nb_0.50)O₃ (PFN). The ferroelectric–paraelectric phase transition in PFN was observed at 383 K, which decreases on increasing Ba-concentration in PBFN. Detailed studies of dielectric properties of PBFW show the following: (i) diffuse phase transition, (ii) low loss tangent, (iii) low activation energy, and (iv) low frequency dielectric dispersion. An anomaly in the ac conductivity was found very close to phase transition temperature. The activation energy is found to decrease from 0.19 to 0.01 eV on increasing Ba-concentration to 7% (x=0.07). Temperature field-dependent magnetization measurements of all the samples showed antiferromagnetic transition at ∼15 K (for x=0.07). PBFN sample showed a slight increase in the coercivity (i.e., from 400 Oe (PFN) to 500 Oe (PBFN, for x=0.07) at 2 K. PACS 61.10.Nz; 68.37.Hk; 75.50.Ss; 75.60.Ej; 77.22.Ch; 77.22.Gm     

Microstructure,dielectric and piezoelectric properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-Ba(Ti<Subscript>0.95</Subscript>Zr<Subscript>0.05</Subscript>)O<Subscript>3</Subscript> lead-free ceramics with CuO sintering aid

Using an ordinary ceramic fabrication technique, we fabricated lead-free (1-x)(K_0.5Na_0.5)NbO₃-xBa(Ti_0.95Zr_0.05)O₃ ceramics with CuO sintering aid . Ba(Ti_0.95Zr_0.05)O₃ diffuses into (K_0.5Na_0.5)NbO₃ to form a new solid solution. The ceramics with perovskite structure possess orthorhombic phase at x≤0.04 and become tetragonal phase at x≥0.06. Both the paraelectric cubic–ferroelectric tetragonal and the ferroelectric tetragonal–ferroelectric orthorhombic phase transition temperatures decrease with increasing the concentration of Ba(Ti_0.95Zr_0.05)O₃. The doping of CuO effectively promotes the densification of the ceramics. The coexistence of the orthorhombic and tetragonal phases at 0.04<x<0.06 and the improvement in sintering performances of the ceramics significantly enhance the piezoelectric and dielectric properties at room temperature. The ceramics with x=0.04–0.06 and y=0.75–1.50 possess excellent properties: d₃₃=119–185 pC/N, k_P=37–44%, k_t=35–49%, ε=341–1129, cosδ=0.7–4.4% and T_c=312–346 °C. PACS 77.65.-j; 77.84.Dy; 77.84.-s     

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.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

Relations between physical properties of the biferroic Pb(Fe<Subscript>1-x</Subscript>Nb<Subscript>x</Subscript>)O<Subscript>3</Subscript>ceramics and their composition change

The work shows basic features of ferromagnetic ceramics Pb(Fe_{1 - x}Nb_x)O₃ with various contents of Fe/Nb (x=1/3, 1/2, 2/3). The synthesis were carried out by powders sintering and calcination whereas densification by hot uniaxial pressing and conventional sintering methods. Optimum conditions of PFN obtention with various contents were based on X-ray, microstructural, dielectrical and conductivity tests together with increase of iron contents in Pb(Fe_{1 - x}Nb_x)O₃ usage parameters of those ceramics worsen. Optimum parameters are shown by the ceramics with contents Pb(Fe_{1 / 2}Nb_{1 / 2})O₃ synthetized using calcination method and densified by hot uniaxial pressing method.     

Dielectric relaxation in BiLi<Subscript>0.6</Subscript>W<Subscript>0.4</Subscript>O<Subscript>3</Subscript> ceramics

The low-frequency process of dielectric relaxation in the new lead-free compound BiLi_0.6W_0.4O₃ prepared by conventional ceramic technology is studied. The features of dielectric relaxation are discussed in terms of a model of interaction between the domain boundaries and point defects of a crystalline lattice.     

Polymethylmethacrylate/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> composite nanofiber membranes with ultra-low dielectric permittivity

Ultra-low dielectric permittivity poly (methyl methacrylate)/Fe₃O₄ composite fiber membranes have been successfully prepared using electrospinning. The composite membranes were characterized by SEM (scanning electron microscopy), TEM (transmission electron microscopy), FT-IR (Fourier transform infrared), XRD (X-ray diffraction) and a radio frequency (RF) impedance/capacitance material analyzer. The magnetic measurement showed that the composite membranes displayed the super-paramagnetic property. The results showed that the dielectric permittivity of the composite fiber membranes was decreasing with increasing Fe₃O₄ nanoparticle content.     

The role of the secondary pyroelectric effect in a ferroelectric relaxor 0.72Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-0.28PbTiO<Subscript>3</Subscript>

The depth profile of the pyroelectric coefficient effective values of a 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-0.28PT) single crystal is studied. It was shown that owing to high pyroelectric coefficients the secondary pyroelectric effect significally contributes (∼40%) to the total pyroelectric coefficient.     

Dielectric and optical properties of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)<Subscript>0.8</Subscript>Ti<Subscript>0.2</Subscript>O<Subscript>3</Subscript> (PMNT-0.2) ferroelectric relaxor single crystals

The dielectric and optical (optical transmission, small-angle light scattering, birefringence) properties of PMNT-0.2 single crystals and their variation induced by a dc electric field have been studied. The birefringence was found to increase anomalously at the transition from the rhombohedral ferroelectric to the inhomogeneous relaxor phase (the spontaneous ferroelectric transition temperature T_sp). Below T_sp, the dielectric and optical properties were observed to exhibit anomalies originating from reorientation and growth of domains in size. Unlike ferroelectric relaxors of the type of PbB′_1/3B′_2/3O₃ and PbB′_1/2B′_1/2O₃, in PMNT-0.2 neither induction of the ferroelectric phase by an electric field nor thermally stimulated destruction of the ferroelectric state occurs through the percolation mechanism (i.e., they are not accompanied by anomalously narrow maxima in small-angle light scattering). This is attributed to the inhomogeneous structure of the relaxor phase, as a result of which the phase transition does not take place simultaneously in various regions of the crystal.     

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.     

Dielectric and polar order behaviors of BaTiO<Subscript>3</Subscript>-Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics

New perovskite solid solution ceramics of (1−x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T _m of their dielectric constant maximum ε _m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Magnetic and dielectric response of cobalt-chromium spinel CoCr<Subscript>2</Subscript>O<Subscript>4</Subscript> in the terahertz frequency range

The nature of the phonon and magnon modes in the CoCr₂O₄ multiferroic with a cubic spinel structure has been studied using submillimeter spectroscopy and infrared Fourier spectroscopy. This paper reports on the first measurement of the evolution with temperature of the exchange optical magnon in the ferrimagnetic (T _C = 94 K) and two low-symmetry (T _S ≈ 26 K, T _lock-in = 14.5 K) phases of CoCr₂O₄ down to T = 5 K in zero magnetic field. It has been shown that the detected magnon is not a ferrimagnetic order parameter and originates, most probably, from spin precession in the cobalt sublattices. At the points of the magnetic phase transitions, the oscillator parameters of the two lowest-frequency phonon modes reveal an anomalous temperature behavior, thus evidencing the presence of significant interaction between the magnetic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originating from vibrations of the CoO₄ tetrahedra during the transition of CoCr₂O₄ to the multiferroic state (T < T _S ) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.     

Investigations of dielectric enhancement in (Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>)<Subscript>1-x</Subscript>(TiO<Subscript>2</Subscript>)<Subscript>x</Subscript> ceramics prepared by laser-sintering technique

The maximum dielectric permittivity of Ti-doped Ta₂O₅ ceramics may reach 450 by a laser-sintering technique. The aim of this study is to investigate the mechanisms of the dielectric enhancement based on the unique structural and morphological properties of the laser-sintered ceramics. The reason for the dielectric enhancement is due to the crystal structure distortion in the high-temperature phase, the oriented grain growth taking place in a direction deviating from [001] in the laser-sintered ceramics. The concurrent nature of quenching effects, a sharp temperature gradient and mass transfer in liquid phase originated from laser high energy irradiation with strict directivity leads to the structural and morphological properties. PACS 81.40.Tv; 61.80.Ba; 77.22.-d; 77.22.Ch     

Microwave dielectric properties of Ba(Zn<Subscript>1/3</Subscript>Ta<Subscript>2 / 3</Subscript>)O<Subscript>3</Subscript> for application in high power waveguide window

Higher dielectric constant, low dielectric loss and good transmission characteristicshave been the goal for developing the ceramic waveguide window for high power windowapplications. The choice of materials having high k with low dielectric lossand reduced window size is key parameters to achieve maximum microwave transmissionwithout unleashing microwave dissipation. The microwave dielectric properties ofsynthesized Ba(Zn_1/3Ta_{2 /3})O₃ (BZT) ceramics have been studied for high power windowapplications. The structural studies are correlated with microwave dielectric propertiesof BZT. The maximum values of dielectric constant ?_r =30, Q ×f₀ = 102 THz and near zero temperaturecoefficient of resonance frequency were obtained for BZT ceramics sintered at thetemperature of 1550 °Cfor 4 h. The measured results are used to design a tapered transition from air filledwaveguide to narrow (reduced width and height) dielectric filled waveguide using Heckenslinear taper at a specific frequency. The simulation result shows that the lowerreflection loss is obtained for the tapered transition of the narrow BZT window ascompared to the standard waveguide BZT window. The return loss of –34 dB is obtained forS-bandwaveguide window with a bandwidth of 675 MHz. The return loss observed in the narrow BZTwindow is –46 dB with a bandwidth of 570 MHz at a center frequency of 3.63 GHz. Most ofthe disadvantages in conventional windows will be rectified using the design of the tapertransion employing narrow waveguide window in high power applications.     

Polar nanoregions in Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> (PMN): insights from a supercell approach

We report construction of a model of polar nanoregions in the PMN relaxor ferroelectric based on first-principles lattice dynamics for chemically ordered supercells [S.A. Prosandeev et al., Phys. Rev. B 70, 134110 (2004)], combined with invariance under permutations and dipole-dipole interaction as a source supporting randomly oriented residual polarization. Representative analytical estimates of polar nanore-gion — supercell mapping reproduce both nonzero local and zero macroscopic polarization of the structure, as well as the temperature change of the supercell anisotropy at cooling and field cooling.     

Phase transition in Bi<Subscript>8</Subscript>Fe<Subscript>6</Subscript>Ti<Subscript>3</Subscript>O<Subscript>27</Subscript> multiferroic ceramics

The polycrystalline Bi₈Fe₆Ti₃O₂₇ compound was prepared by a high-temperature solid-state reaction technique. Preliminary structural analysis by X-ray diffraction (XRD) confirms the formation of a single-phase compound in an orthorhombic crystal system at room temperature. The elemental content of the compound was analyzed by EDAX microanalysis. Microstructural analysis by scanning electron microscopy (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the pellet sample. Detailed studies of temperature-dependent dielectric response at various frequencies show dielectric anomalies at 380, 389 and 403°C for 10 kHz, 100 kHz, and 1 MHz respectively. The hysteresis loop observed by applying an electric field of 12 kV/cm on the poled sample with smaller remanent polarization supports the existence of ferroelectricity in this material. The value of d₃₃ of the compound was found to be 19 pC/N.      

Study of the anisotropy of the dielectric response of Na<Subscript>1/2</Subscript>Bi<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript> relaxor ferroelectric

The dielectric response, conductivity, and domain structure of (Na_1/2Bi_1/2)TiO₃ single crystals are studied in the temperature range of 290–750 K for the [100], [110], and [111] crystallographic directions. It is shown that the region of optical isotropization is observed in polarized light in the temperature range of 570–620 K. In this case, the birefringence (Δn) decreases and disappears (together with the image of the domain structure) for the [100] directions. The region of optical isotropization in the [111] directions is characterized by the disappearance of the image of the domain structure and by the existence of individual regions with partial quenching. The domain structure in the [110] directions remains distinguished against the background of a significant decrease in Δn in the indicated temperature range. The region of isotropization is also manifested in the temperature dependence of the imaginary part of the dielectric response and is determined by the isotropic character of the conductivity in the range of 570–620 K. The bulk conductivity has a thermally activated character with activation energies E _a = 50?60 meV at T < 500 K and E _a = 700?900 meV for T > 620 K. The low-frequency dispersion of the dielectric response is determined by the Maxwell–Wagner mechanism and is due to an increase in the ionic conductivity at temperatures above 620 K. The anisotropy of the susceptibility holds in the entire studied ranges of frequencies (25 Hz–1 MHz) and temperatures.