Diffusion of heterovalent ions in ionic crystals

Diffusion of divalent and trivalent cations is investigated in NaCl and KCl under conditions of heterodiffusion at infinite dilution. This purpose is achieved by letting radiotracers of very strong specific activity diffuse in the intrinsic range of highly pure single crystals. The importance of the experimental procedures in yielding reliable results is emphasized. Thus, Co⁺⁺, although supposedly characterized by an anomalous diffusion in NaCl and KCl, gives very good diffusion profiles with the carrier-free isotope Co-58. Also Fe⁺⁺⁺, whose diffusion was considered impossible, gives good diffusion profiles with certain experimental precautions. Analysis of the data gives, for the diffusion of Co⁺⁺, the migration enthalpy of 0.76 eV in NaCl and 0.66 eV in KCl and the migration entropy of −2.84 k in NaCl and −1.45 k in KCl. These results are in good agreement with the values for the full set of divalent cations.     

The thickness dependence of the permittivity of barium titanate single crystals

A thickness dependence of the permittivity ofc-domain barium titanate single crystals was found. The experimental results were explained by means of a model of a dielectric with surface layers. The thickness of the layer and the permittivity of the surface layer and bulk of the crystal were determined.

---

iO₃

iO₃, -. . , .

     

The growth of single crystals of barium titanate from melts with barium chloride

On the basis of experiments carried out on the growing of barium titanate, single crystals (BaTiO₃) it is shown that the data given by B. T. Matthias [1] are incomplete and insufficient thus leading in certain cases to negative results. This paper gives the conditions for growing crystals which are suitable for fundamental physical research.     

Uniaxial stress dependence of the dielectric properties of barium titanate single crystals

BaTiO₃ single crystals were grown by the melt-grown method. The effect of uniaxial pressure (0–1700 bar) on the dielectric properties of these crystals has been systematically studied. The external stress showed obvious effects on these properties. An increase in the difference between the Curie T_c and Curie–Weiss T₀ temperatures induced by the applied pressure is observed. This could be ascribed to the inducing of non-ferroelectric cubic islands in the tetragonal phase by the applied compressive stress. On the other hand, the pressure behavior of thermal hysteresis and the ??/?T vs. T plot strongly suggests that the phase transition changes to second-order type with increasing pressure. The Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type. An increase in the difference between the Curie T_c and Burn's T_B temperatures with increasing pressure is observed. This could be ascribed to the narrowing of the temperature range on which the Curie–Weiss law is valid. In general, the obtained results are in good agreement with hydrostatic pressure data. Some kind of relaxation near T_c, which is strongly coupled with strain caused by applied compressive stress, is postulated.     

On the relation between the thickness dependence of optical absorption and the surface layers of barium titanate single crystals

Single crystals of BaTiO₃, prepared by crystallization in the system BaCl₂-BaO-BaTiO₃-show a dependence of the optical absorption coefficient on the sample thickness. The observed thickness dependence of the absorption coefficient can be explained by the existence of a surface layer with optical properties differing from those of the inner part of the crystal. By means of an etching method a direct determination of the thickness of the surface layer of crystals in the thickness range of 0.075 to 0.178 mm was carried out. The value found is (6.0±0.2)×10^-4 cm.

---

iO₃, 1₂-BaO-BaTiO₃ . . 0,075 0,178 mm. (6,0±0,2). 10^-4 cm.

     

Electroluminescence in lead titanate single crystals

Electroluminescence in PbTiO₃ single crystals is studied with variation in applied electric field, frequency (20 Hz to 5 kHz) and temperature. The EL onset depends on the rate at which the dipole switches. Extremely sharp upward rising nature of the pulses of micro second duration suggest that there is a self maintained discharge in the dielectric due to secondaryγ _p mechanism. Frequency dependence of EL suggests that both the secondary mechanisms, viz. theγ _p andγ _i are active after the application of a high field and the critical field at which this occurs decreases with increase in the frequency of the applied voltage. Similarly the onset voltage decreases with increase in frequency. The temperature dependence of EL at the applied frequency of 50 Hz shows that the onset voltage is intimately connected with the coercive field of the crystal and it is minimum at the Curie point. The study suggests that EL occurs in the bulk and there is a breakdown in the dielectric due to an avalanche formation.     

Persistent light-induced absorption in oxidized iron-doped barium titanate crystals

Illumination of oxidized iron-doped BaTiO(3) crystals with visible or ultraviolet light yields absorption changes of as much as 1300 m(-1) that are stable at room temperature. These photochromic effects cannot be erased by light, but heating the crystal to a moderate temperature (100 degrees C) can switch it back to its initial state. These effects, in particular the optical irreversibility, might be of interest, e.g., for optical control of charge-transport properties and for persistent data storage.     

Transformation of the domain structure of niobium-containing barium titanate crystals

This paper reports on the results of investigations into the transformation of the domain structure of niobium-containing barium titanate crystals in electric fields varied in the same manner as in the stepwise measurement of the polarization curve.     

Precursor dynamics in rhodium-doped barium titanate single crystals studied by Brillouin light scattering and birefringence measurements

The dynamics of precursor polar clusters in rhodium(Rh)-doped barium titanate single crystals were examined in the paraelectric phase by Brillouin light scattering and birefringence measurements in a wide temperature range. The longitudinal acoustic (LA) mode and central peaks in the inelastic light scattering spectrum were investigated by Brillouin scattering, while the temperature dependence of the birefringence was accurately determined by means of a birefringence imaging system. In a specific temperature range above the ferroelectric phase transition temperature, the LA mode frequency exhibited a significant softening concomitant with the substantial increase in the hypersonic damping, which was attributed to the formation of polar clusters and their interactions with acoustic waves. In this temperature range where acoustic anomalies were brought about, the birefringence showed non-zero values, indicating the existence of local non-centrosymmetric regions. All these results clearly indicated that the off-centered motions of Ti ions in the oxygen octahedra in Rh-doped BaTiO₃ are correlated in the paraelectric phase inducing slowing down of the precursor dynamics and anomalous birefringence, similar to pure BaTiO₃ (Ko et al., Phys. Rev. B 84, 094123 (2011)). The effect of rhodium doping in BaTiO₃ enhanced the first-order character of the ferroelectric phase transition.     

Effects of oxygen deficiency on the acoustic anomalies and phase transition behaviors of barium titanate single crystals

The acoustic behaviors of oxygen-reduced barium titanate (BaTiO_3-δ) single crystals with δ∼0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO₃ (BTO) showed two pronounced anomalies at approximately 112 °C and −11 °C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 °C compared to those of the pure BaTiO₃ suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO₃ were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point.     

Influence of 90° domain switching on the physical properties of tetragonal barium titanate single crystals

In this paper, we report the influence of 90° domain switching on the physical properties of tetragonal BaTiO₃ single crystals. It is found that the contribution of 90° domain switching to the piezoelectric response is much larger in magnitude than the contribution of the direct piezoelectric effect under large external mechanical stress. Simultaneously, the interconversion of a and c crystallographic axes and the large actuation strain as high as 1% is induced by 90° domain switching in the tetragonal BaTiO₃ single crystal.     

Hydrogen-induced degradation in strontium titanate single crystals

Hydrogen-induced degradation in strontium titanate single crystals was studied through an electrochemical hydrogen charging process, in which the silver electrodes of the crystals were made a cathode in 0.01 M NaOH solution to evolve hydrogen by electrolysis of water. After the process, the resistance of the crystals was decreased by more than one order of magnitude, the dielectric loss was obviously increased, and the capacitance became more dependent on frequency. It is proposed that atomic hydrogen generated by electrolysis of water diffuses into the crystals and exists as a charged interstitial impurity donating electrons to the conduction band of the crystals. Attention should be paid to this hydrogen-induced degradation when the reliability of perovskite-type ferroelectric devices and components is studied. PACS 77.22.Ch; 77.22.Gm; 77.84.Bw     

Oriented surface self-segregation in lead titanate single crystals

The morphological features of the surface nanostructure of lead titanate crystals and lead zirconate titanate ceramics formed due to thermally stimulated surface self-segregation were studied by atomicforce microscopy, Auger electron spectrometry, and reflection high-energy electron diffraction. It was experimentally found that oriented ensembles of titanium dioxide nanoparticles 10–50 nm in size appear on the lead titanate single crystal surface at a certain temperature. A possible mechanism of the detected phenomenon is discussed, which is a new type of epitaxy via selective internal mass transfer of proper atoms from the substrate to the surface.     

Impurity centers in a barium titanate ceramic doped with rare-earth ions

The ceramic BaTiO₃ doped with rare-earth ions Y, La, Nd, Sm, Lu, and Dy to 0.1–0.5 at. % was studied in the temperature range 160<T<480 K by ESR. Several ESR spectra were observed. The most intense spectra have g factors close to 5.5 and 1.96. The dependences of their intensity, g factor, and width on the concentration of rare-earth ions and temperature were studied. Analysis of the data obtained made it possible to determine the critical concentration of the rare-earth ions x _c =0.2–0.3 at. %. It is characterized by the fact that for x<x _c or x>x _c all rare-earth ions, except Lu, replace predominantly Ba⁴⁺ or Ti⁴⁺, respectively. Models of paramagnetic centers were established: Fe³⁺-V _O (g≈5.5) and Ti³⁺-Ln³⁺ (g≈1.96), where V _O is an oxygen vacancy and Ln is a rare-earth ion. An abrupt change in axial symmetry to cubic for a Fe₃₊-V _O center at a phase transition from the tetragonal into the cubic phase was observed for the first time. The role of new centers in the appearance of the posistor effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1838–1842 (October 1999)     

Thermostimulated surface autosegregation in bismuth titanate single crystals

High resolution scanning electron microscopy (HRSEM), atomic force microscopy (AFM), and electron microprobe X-ray analysis (EPMA) were used to study the morphology and local phase composition of (001) faces in single crystals of bismuth titanate Bi₄Ti₃O₁₂ formed as a result of thermostimulated surface segregation (TSAS). One possible mechanism for this phenomenon, generated by the selective internal mass transfer of the matrix’s own atoms to the surface in competition with the processes of selective component evaporation, is discussed.     

On the problem of the permittivity dispersion of barium titanate

The complex permittivity of multidomain single crystals of BaTiO₃ was measured in the decimetre and centimetre wave bands and it was proved that in this frequency region there is dispersion of the permittivity. Apparatus for measuring the permittivity of substances with a high is described and the influence of the inhomogeneity of the field in the sample on the characteristics of the cavity resonator is calculated. Present theories of the high-frequency properties of BaTiO₃ are discussed from the point of view of the experimental results.

---

BaTiO₃ , . . BaTiO₃.

     

Short-term reversible changes in the dielectric dispersion of a barium titanate single crystal

An effect of spontaneous fluctuation of the dispersion frequency was detected during investigations of the dielectric dispersion of a BaTiO₃ single crystal while measuring under the electric field only. A decrease in the dispersion frequency by three orders of magnitude and its subsequent return to the previous value was observed. A possible mechanism of this phenomenon is suggested, which involves the dynamics of the screening process.