O. A. Bunina

Random-Site Cation Ordering and Dielectric Properties of PbMg1/3Nb2/3O3-PbSc1/2Nb1/2O3

(1 − x)PbMg1/3Nb2/3O3-(x)PbSc1/2Nb1/2O3 (PMN-PSN) solid solution crystals have been grown by the flux method in the whole concentration range. X-ray supercell reflections due to B-cation ordering were observed for as-grown crystals from the 0.1 ≤ x ≤ 0.65 compositional range. Though the ordered domains are rather large (∼50 nm) the relaxor-like dielectric behavior is observed for compositions with x < 0.6. The diffusion of the dielectric permittivity maximum in as-grown crystals is the lowest at x = 0.6 and increases towards the end members of solid solution. Such behavior is explained within a Bragg-Williams approach by employing the random layer model. At x ∼ 0.6 the excitation energy determined from the Vogel-Fulcher relation exhibits a jump which we regard to changing the kind of the polar regions from PbMg1/3Nb2/3O3 to PbSc1/2Nb1/2O3 related type.

Phase transitions in PbMg1/3 Nb2/3 O3-PbTiO3 system

Abstract The result of the roentgenographic studies of (1-x) PbMg1/3 Nb2/3 O3-xPbTiO3 (x<0.4) solid solution system in the temperature range 290-770 K are presented.

Field-induced relaxation of crystal structure in dipole-glass phase of PMN — PT single crystal: X-ray study

Abstract X-ray studies of the electric field-induced structural changes in 0.8PMN — 0.2PT single crystal were made in the temperature range 300–400 K including the points of the supposed dipole-glass (T DG ≈ 352 K) and ferroelectric (T F ≈ 324) transitions. The changes in the form and intensities of the 005 and 224 reflexes were studied after the application the different electric fields (E T DG and T < T F the instantaneous changes in the form and the intensity of the 224 reflex are in accordance with the cubic and rhombohedral symmetries of the parent and ferroelectric phases correspondingly, while at T F < T < T DG the field above some (temperature dependent) threshold value induces the smearing of this Bragg peak followed by the slow (during 30–40 min.) formation of the broad multiplet structure. Also the influence of the slow ac field (frequency 0.01 Hz) on the averaged over the period Bragg peak form was studied at 300 K < T < 400 K.

Electric-field-induced phase transition in the relaxor ceramics based on PMN-PT

The mechanisms of formation of a plateau in dependences of the reversible permittivity on the external electric field strength in a ceramic sample of the yPZN-mPMN-nPNN — xPT (x = 0, 3; y = 0.0982; m = 0.4541; n = 0.1477) system have been analyzed using X-ray diffraction and atomic force microscopy. It has been shown that, in electric fields corresponding to the permittivity anomaly, there occurs an induced phase transition from the heterophase (tetragonal + pseudocubic) state to the single-phase (tetragonal) state. This leads to significant strains of the tetragonal unit cell (up to 0.1%) and the termination of 90° domain switching. The proposed qualitative model of the formation of the observed dielectric anomaly is based on two processes occurring with an increase in the electric field strength: the growth of polar regions of the tetragonal phase and their switching.

Dielectric, magnetoelectric, structure, and dissipative properties and the Mössbauer effect in PbFe1/2Nb1/2O3 ceramics in wide frequency and temperature ranges

High-quality fine-grained ceramic samples of classical multiferroics PbFe1/2Nb1/2O3 (PFN) were synthesized. Their dielectric, magnetoelectric, and magnetic characteristics, including the Mössbauer effect, were measured over wide ranges of temperatures (10–1000 K) and field frequencies (from 25 Hz to 1 MHz). The temperature dependence of the dielectric loss exhibits a maximum between 150 and 170 K, likely due to magnetic ordering. The dependence of ɛ on the magnetic field displays an anomalous increase near the Curie temperature (370 K) that rises to 40% upon heating.

Cation Ordering and Dielectric Properties of PbMg 1/3 Nb 2/3 O 3 -PbSc 1/2 Nb 1/2 O 3 Solid Solution Crystals

(1 m x )PbMg 1/3 Nb 2/3 O 3 -( x )PbSc 1/2 Nb 1/2 O 3 solid solution crystals have been grown by the flux method. X-ray supercell reflections due to B-cation ordering were observed for crystals in the 0.1 < x < 0.65 compositional range. In as-grown crystals diffuseness of dielectric permittivity maximum is the lowest for x = 0.65 composition and increases toward the end members of solid solution. Such behavior is assumed to be due to the random-site ordering of B-site cations and non-monotonic dependence of the compositional order-disorder phase transition temperature on concentration.

Direct transition from the rhombohedral ferroelectric to the paraelectric phase in a (Ba,Sr)TiO3 thin film on a (111)MgO substrate

A heteroepitaxial (200 nm thick) (BST) thin film was deposited on a (111)MgO substrate using rf sputtering. X-ray diffraction examinations confirmed epitaxial growth and rhombohedral symmetry of the film ( and ) at room temperature. Polarized Raman spectra of the film were studied in the temperature range from 100 to 420 K. In contrast to a BST thin film grown on (001)MgO, the observed linear temperature dependence of the squared soft mode frequency suggests the displacive character of the ferroelectric-paraelectric phase transition in a BST thin film on (111)MgO.

X-ray Diffraction Study of BaTiO3/(Ba1-x Sr x )TiO3 Artificial Superlattices

We report x-ray diffraction investigation of two sets of artificial BaTiO3/Ba1-x Sr x TiO3 (BT/BST-x) superlattices (SLs) grown on MgO substrates. The first set of SLs was deposited with the constant periodicity (about 150 Å) and the variable Ba/Sr content in BST layers. In the second set, the modulation period Λ was varied between 30 Å < Λ < 110 Å while Ba content was equal 0 in BST layers. It is shown that, fabrication of the artificial SLs with variable composition of the constituting layers even with large modulation period may provide enhanced variation of the ferroelectric properties comparing to the SLs with variable layers thickness.

Use of various growth catalysts for laser sputtering of ZnO micro- and nanorods

Pulsed laser deposition and various catalysts are used to fabricate ZnO micro- and nanorods at temperatures close to the optimum temperature for each catalyst. A comparative analysis of the optical and structural properties of the rods shows that, as the temperature of growth on Al2O3(11–20) substrates decreases, the internal stresses in the rods decrease, which improves their structure and optical properties. This effect is not observed for GaN/Si substrates because of the high stresses induced by the difference in the lattice parameters. An increase in the synthesis temperature leads to an increase in the lattice strains and the concentration of point defects, the relaxation of selection rules, and the appearance of numerous phonon A1(LO) overtones. The lattice strains calculated from unit cell parameter a and the shift in the phonon A1(LO) frequency agree qualitatively. The study of the photoluminescence of the rods shows that a decrease in the synthesis temperature decreases the imperfection of the ZnO rods and improves their optical properties for UV applications.

X-ray and dielectric studies of hot-pressed K2Sr4Nb10O30 ceramics

ABSTRACT X-ray diffraction (XRD) and dielectric studies of dense hot-pressed K2Sr4Nb10O30 ceramics with a tetragonal tungsten bronze–type structure revealed texturing of the samples and substantial anisotropy of the dielectric permittivity. XRD studies of the K2Sr4Nb10O30 powder show a diffuse phase transition in the 140 – 160°C range while a diffused and frequency-independent maximum of the dielectric permittivity is observed at about 120–125°C for ceramic samples. Substantial anisotropy of the dielectric permittivity (more than 50% in the phase transition region) as well as samples texturing and a difference between phase transition temperatures of the powder and ceramic samples are supposed to be caused by anisotropic residual stresses.