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.     

Polar regions in the paraelectric phase in a PbZr0.992Ti0.008O3 single crystal

Anomalous behaviour of ε and tan δ in the paraelectric phase was found for PbZr0.992Ti0.008O₃ single crystal. In the vicinity of temperatures 280–290°C (T _c = 236°C) the pyroelectric effect was also observed. Using the data obtained, the existence of an additional polarization (polar regions oriented in an external electric field) in the paraelectric phase is discussed.     

On the role of a weak hydrogen bond OH…O in the formation of the anomalous dielectric response of crystals and polymers near 40°C

In some crystals, polymers, and gels that contain hydrogen bonds OH…O, NH…O of length 2.8–3 Å or water molecules, gigantic anomaly of dielectric permittivity (? ～ 10³–10⁶) is observed in certain circumstances at frequencies of 1–10⁶ Hz, which is accompanied by peculiarities in conductivity σ and dielectric losses tanδ. In crystals this effect appears after a sudden cooling to ?50°C and it is observed at slow heating in the range of 20–40°C. At the return temperature course from 40°C dependences ?(T), σ(T), and tanδ(T) have their usual values. Anomalies in objects that differ by their compositions are unified by their temperatures, which are all close to 40°C. Authors have made an attempt to explain the similarity of these phenomena by the features of hydrogen bonds that are present in the objects.     

Dielectric loss as an indication of the kinetics of the ferroelectric phase transition

The dependences tanδ(f, T) for triglycine sulfate and diglycine nitrate single crystals are investigated in the frequency range f = 1?2 × 10⁷ Hz at temperatures from ?150 to 50°C for triglycine sulfate and from ?150 to ?60°C for diglycine nitrate. The results are discussed in the framework of the model according to which two or more types of media with different dielectric properties are formed in the ferroelectric phase. The polar volumes of domains themselves and the volumes of domain walls with different symmetries are treated as individual media. The dependences tanδ(f, T) make it possible to separate the contribution of a particular medium and to trace its evolution with temperature. The results of the analysis of the relaxation properties confirm that the phase transitions lead to the formation of two media (the polar medium and domain walls) in triglycine sulfate and three media (the polar medium and two types of domain walls) in diglycine nitrate. It is demonstrated that the relaxation properties are not steady-state characteristics of the samples but depend on their prehistory.     

Second harmonic generation in the paraelectric phase in powders and ceramics of BaTiO3

A second harmonic signal has been clearly observed in powders and ceramics of BaTiO₃ in the high-temperature phase. The effect manifests itself at temperatures 500 K above the temperature of the phase transition from the tetragonal phase to the cubic phase, which is also significantly higher than the Burns temperature of BaTiO₃. The temperature dependence of the second harmonic generation signal I _2?? in the paraelectric phase becomes a straight line when plotted in the Arrhenius coordinates lnI _2??(1/T). The experimental results obtained indicate the presence of noncentrosymmetric local regions in the paraelectric phase over a wide range of temperatures. The contribution from these polar regions to the second harmonic generation increases as the phase transition temperature is approached. The temperature dependence of the second harmonic generation in barium titanate ceramics is significantly affected by the technique used for pressing and annealing of the samples.     

High-temperature X-ray diffraction and Raman scattering studies of Ba-doped (Na0.5Bi0.5)TiO3 Pb-free piezoceramics

The structural changes in (100 ? x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ (0 ≤ x ≤ 10) ceramics were investigated as a function of composition and temperature by X-ray diffraction and Raman spectroscopy. As Ba concentration increases, the structure changed from rhombohedral to tetragonal (x ≥ 6.5) across a morphotropic phase boundary like phase coexistence at x ～ 5.5, which is further evidenced by phonon anomalies observed in composition-dependent Raman spectra. On heating, the disappearance of peak splits in {111} (x ≤ 5) and {200} (x ≥ 6.5) Bragg peaks and the changes in their 2θ-positions indicated temperature-driven structural changes: ferroelectric to antiferroelectric (≈T_d, depolarization temperature) at 220 °C and antiferroelectric to paraelectric (rhombohedral to tetragonal) at 320 °C. In addition, Raman spectral analysis suggested that at elevated temperatures, two tetragonal phases with slightly different space groups coexisted at x ≥ 6.5 and most of the phase transition temperatures shifted towards left with increasing x.     

Preparation,Structure, and Dielectric and Magnetic Properties of SrFe<Subscript>2/3</Subscript>W<Subscript>1/3</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline samples of SrFe_2/3W_1/3O₃ (SFWO) ceramic were obtained by solid-phase reactions with subsequent sintering using conventional ceramic technology. X-ray diffraction analysis showed that at room temperature, the SFWO ceramic is single-phase and has a perovskite-type structure with tetragonal symmetry and parameters a = 3.941(9) Å, c = 3.955(6) Å, and c/a = 1.0035. In studying the magnetic properties and the Mössbauer effect in SFWO ceramics, it is found that the material is a ferrimagnet, and the iron ions are only in the valence state of Fe³⁺. It is suggested that in the temperature range of T = 150–210°C, a smeared phase transition from a cubic (paraelectric) phase to a tetragonal (ferroelectric) phase takes place in SFWO with decreasing temperature.     

Spinodal ordering in Cu3Au

Results from, X-ray diffuse scattering and inelastic neutron scattering demonstrate the presence of long-lived fluctuations in order up to 5.0°C above the first-order phase transition, T_c = 383.2°C. The temperature for continuous ordering has been determined to be 358.2°C. The coherent phase boundary is 3–13°C below T_c.     

Structure and dielectric response of (1–<Emphasis Type="Italic">x</Emphasis>)NaNO<Subscript>2</Subscript> + <Emphasis Type="Italic">x</Emphasis>BaTiO<Subscript>3</Subscript> composites at <Emphasis Type="Italic">x</Emphasis> = 0.05 and 0.1

The (1–x)NaNO₂ + xBaTiO₃ composites of two compositions (x = 0.05 and 0.1) have been studied by powder neutron diffraction and broadband dielectric spectroscopy (frequency region 10^–1–10⁷ Hz). The temperature dependences of the ferroelectric order parameter of NaNO₂ in the composites and pure NaNO₂ have been measured. The frequency dependences of the real and imaginary parts of the permittivities of the composites and pure NaNO₂ have been analyzed in the temperature range 25–187°C. The anomaly of the dielectric response observed at T ~ 147°C is assumed to be related to the processes of accumulation and “resolution” of charges at the BaTiO₃ particle boundaries.     

Hyper-Raman scattering spectra in the low frequency range in cubic BaTiO3 and the mechanism of the phase transition

The hyper-Raman spectra in the cubic phase of BaTiO₃ are studied with special emphasis on the lowest-freauency phonon mode. The spectral structure is found to change as the temperature T is raised from T_c (130 °C) to 400 °C. It is shown that the overall experimental results can be well accounted for in terms of the critical dynamical theory based on pseudospin-phonon coupled system developed by Y. Yamada et al. As the consequence, it is found that the coupling between TO phonons and pseudospins presumably constituted by flipping motion of Ti ions causes the cubic-tetragonal phase transition, and plays an important role in understanding the dynamical aspects including the anomalous increase in dielectric constant as T approaches T_c from above.     

Structure-mediated transition in the behavior of elastic and inelastic properties of beach tree bio-carbon

Microstructural characteristics and amplitude dependences of the Young modulus E and of internal friction (logarithmic decrement δ) of bio-carbon matrices prepared from beech tree wood at different carbonization temperatures T _carb ranging from 600 to 1600°C have been studied. The dependences E(T _carb) and δ(T _carb) thus obtained revealed two linear regions of increase of the Young modulus and of decrease of the decrement with increasing carbonization temperature, namely, ΔE ～ AΔT _carb and Δδ ～ BΔT _carb, with A ≈ 13.4 MPa/K and B ≈ ?2.2 × 10^?6 K^?1 for T _carb < 1000°C and A ≈ 2.5 MPa/K and B ≈ ?3.0 × 10^?7 K^?1 for T _carb > 1000°C. The transition observed in the behavior of E(T _carb) and δ(T _carb) at T _carb = 900–1000°C can be assigned to a change of sample microstructure, more specifically, a change in the ratio of the fractions of the amorphous matrix and of the nanocrystalline phase. For T _carb < 1000°C, the elastic properties are governed primarily by the amorphous matrix, whereas for T _carb > 1000°C the nanocrystalline phase plays the dominant part. The structurally induced transition in the behavior of the elastic and microplastic characteristics at a temperature close to 1000°C correlates with the variation of the physical properties, such as electrical conductivity, thermal conductivity, and thermopower, reported in the literature.     

Dielectric relaxation and magnetic characteristics of the Bi0.5La0.5MnO3 ceramics

The complex permittivity ?* of ceramics of bismuth-lanthanum manganite Bi_0.5La_0.5MnO₃ has been measured in ranges of temperatures T = 10–200 K and frequencies f = 10²–10⁶ Hz. Clearly pronounced regions of the non-Debye dielectric relaxation have been revealed at low temperatures (T < 90 K). To describe them, the possible mechanisms have been proposed and discussed. The temperature dependences of magnetization, the anomalous behavior of which can be associated with the phase transition from the paramagnetic phase into the ferromagnetic phase occurring at T ～ 40–80 K, have been measured in the temperature range T = 10–120 K.     

Temperature dependent polariton measurements in BaTiO3

Polariton techniques are used to determine the temperature dependence of all three A₁ modes between room temperature and T_c = 134°C in the ferroelectric phase of BaTiO₃. The results shows that there is a large difference between the clamped dielectric constant, ?(0), determined from these results and ?(0) determined from microwave capacitance techniques. It is argued that, while soft modes in displacive ferroelectrics are the driving force for the transition, other oscillators play an important role in low frequency dielectric behavior.     

Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T _carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T _carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T _carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T _carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T _carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.     

Structural Phase Transition of Perovskite-Type N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>CdBr<Subscript>3</Subscript> Studied by MAS NMR and Static NMR

The structural geometry change in the perovskite-type N(CH₃)₄CdBr₃ single crystal near the phase transition temperature of T _C = 390 K was investigated using magic angle spinning nuclear magnetic resonance techniques. For ¹H and ¹³C nuclei, the temperature dependences of their chemical shift, spectral intensity, and spin–lattice relaxation time (T _1ρ) in the rotating frame were obtained and analyzed. While the chemical shift and T _1ρ of ¹H showed change near T _C, those of ¹³C did not. In addition, the ¹¹³Cd spin–lattice relaxation time T ₁ in the laboratory frame near T _C show no evidence of anomalous change near the phase transition temperature, which coincides with the measured changes in the ¹H T _1ρ. The driving force for this phase transition was connected to the ¹H in the CH₃ groups.     

Effect of domain wall on the dielectric properties of the BaTiO3/SrTiO3 superlattices

Based on a phenomenological thermodynamical theory, the effect of the domain wall on the dielectric properties of the polydomain BaTiO₃/SrTiO₃ superlattices with 180° electric domains in the BaTiO₃ layer is investigated. Theoretical analysis indicates that complete polarization suppression and the largest dielectric response take place at approximately 72% and 53% of the critical volume fraction of the SrTiO₃ layer for the domain wall energy parameter A=3×10⁷ and 5×10⁷, respectively. The dielectric properties largely depend on both the volume fraction of the SrTiO₃ layer and the domain wall energy parameter. Moreover, the gigantic dielectric response which occurs in single-domain BaTiO₃/SrTiO₃ superlattices cannot appear due to the 180° polydomain state in the BaTiO₃ layer, which is in agreement with the previous prediction. The high domain wall energy parameter results in the stabilization of the paraelectric state in the BaTiO₃ layer, however, the enhancement of the domain wall energy parameter within a certain scale increases the dielectric properties greatly.     

X-Ray diffraction and Raman spectroscopy studies of superlattices BaTiO3/(Ba0.5,Sr0.5)TiO3/SrTiO3

The structural characteristics of the BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃/SrTiO₃ superlattice on a (001) MgO substrate have been studied using X-ray diffraction. The modulation period and unit cell parameters of layers forming the superlattice have been measured. The sizes of coherent scattering regions and average microstrains in the direction perpendicular to the surface have been estimated. The obtained characteristics are compared to those of the two-layer BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃ superlattice. The Raman spectra demonstrate a substantial shift of the soft E(TO) mode in the three-layer superlattice as compared to the position in the two-layer superlattice. The effects observed are associated with a substantial increase in the temperature of the phase transition of the three-layer superlattice to the paraelectric phase.     

Amplitude dependences of dielectric permittivity and dielectric losses in a mixed K0.88(NH4)0.12H2PO4 crystal

Amplitude dependences of dielectric permittivity ? and dielectric losses tanδ in K_0.88(NH₄)_0.12H₂PO₄ mixed crystal near the ferroelectric phase transition temperature T _C are investigated. At measurement fields E _～ exceeding some critical value, dependence tanδ(E _～) can be described within a model that includes the depinning of domain walls from point lattice defects. The activation field is found to grow appreciably when approaching T _C from below, due to the presence of nonpolar regions.     

Electrical conductivity and dielectric properties of β-BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal in the temperature range 90–300 K

The electrical conductivity σ and dielectric properties (?, tanδ) of β-BaB₂O₄ were studied in the temperature range 90–300 K. The quantities σ, ?, and tanδ were measured at frequencies of 0.1, 1, and 10 kHz and 1 MHz. The dielectric permittivity and electrical conductivity were found to grow with increasing temperature at all frequencies. The permittivity decreases and the electrical conductivity increases (by several orders of magnitude) with increasing frequency. Maxima were observed in the σ=f(T) and tanδ=f(T) curves for all frequencies; the maxima shift toward higher temperatures with increasing frequency.     

C-oriented SrBi2Nb2O9 films grown on YBa2Cu3O7−δ /SrTiO3 and NdGaO3

Epitaxial SrBi₂Nb₂O₉ films were grown by laser ablation on (001)YBa₂Cu₃O_7?δ /(100)SrTiO₃ and (001)NdGaO₃, with c axis normal to the substrate plane. The SrBi₂Nb₂O₉ films were grown in a layer-by-layer regime on NdGaO₃ substrates in 25-Å steps at a condensation temperature of 700 °C. Microinclusions of secondary phases and a-oriented grains were observed to exist on the surface of (001)SrBi₂Nb₂O₉ films grown on (001)YBa₂Cu₃O_7?δ /(100)SrTiO₃. The dielectric permittivity of the SrBi₂Nb₂O₉ films measured along the c axis is 123 (T=300 K, f=100 kHz), and tan δ≈0.04.