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.     

Atomic structure of Sr0.75Ba0.25Nb2O6 single crystal and composition-structure-property relationin (Sr,Ba)Nb2O6 solid solutions

The structure of the Sr_0.75Ba_0.25Nb₂O₆ single crystal has been investigated by x-ray diffraction. The occupancies of the Ba and Sr sites in two structural channels are determined. It is found that these sites are split in the large (pentagonal) channel. A qualitative correlation is revealed between the smearing of the phase transition and the displacement of the Sr atom from the m symmetry plane in the pentagonal channel at different [Sr]/[Ba] ratios. The degree of acentricity of the NbO₆ octahedra is analyzed as a function of the [Sr]/[Ba] ratio.     

Fluorescence of rare-earth ions in dirty displacive ferroelectrics

The phase transition of ferroelectric Sr_1?xBa_xNb₂O₆ was studied by measuring the fluorescence emitted from the rare-earth ions doped as a probe. The fluorescence spectral width and the wavelength showed singular behavior around the transition temperature. The samples possess a diffuse transition with characteristic optical properties near the inflection point of the temperature-dependence curve of the spontaneous polarization.     

Optimizing conditions of fabrication and the properties of BaNb2O6-SrNb2O6 binary ceramics

Conditions for fabricating (Ba_0.5Sr_0.5)Nb₂O₆ ceramics are optimized. It is shown that materials synthesized at T = 1250°C and sintered at T = 1375–21400°C exhibit the best ceramic characteristics. The (Ba_0.5Sr_0.5)Nb₂O₆ solid solution is found to be a ferroelectric with a diffuse phase transition. The possibility of fabricating Ba_0.5Sr_0.5Nb₂O₆ thin films from ceramic targets prepared in the resulting optimum regimes is shown.     

Diffuse phase transition in rare earth ion doped SBN

The diffuse transition of a rare earth ion doped SBN (Ba_xSr_1?xNb₂O₆) was studied by measuring the spontaneous polarization P_s, and the dielectric constant ?. The phase transition of SBN can be regarded as a special case of a second order transition.     

铁电钾钠铌酸锶钡薄膜电光性能的研究

采用溶胶-凝胶法在氧化镁单晶衬底上制备了符合化学计量比的完全填充型铁电钾钠铌酸锶钡(KNSBN)薄膜，通过X射线衍射，摇摆曲线，X射线Φ扫描，扫描电子显微镜等方法研究了薄膜的微结构，采用Adachi法研究了薄膜的电光特性. 实验发现，KNSBN薄膜在氧化镁(001)单晶衬底上沿c轴外延生长，K⁺，Na⁺的引入有效地提高了薄膜的横向电光系数r₅₁. 成分为K_0.2Na_0.2Sr_0.24Ba_0.56Nb₂O₆, K_0.2Na_0.2Sr_0.6Ba_0.2Nb₂O₆, K_0.2Na_0.2Sr_0.72Ba_0.08Nb₂O₆的三种KNSBN薄膜的r₅₁值分别为108.52pm/V, 119.98pm/V, 126.96pm/V，r₅₁的数值随Sr²⁺含量增加而增大. 关键词： 横向电光系数 钾钠铌酸锶钡 外延生长     

Effect of Ni doping on ferroelectric and dielectric properties of strontium barium niobate crystals

The influence of Ni doping on the ferroelectric and dielectric properties have been examined in Sr_0.61Ba_0.39Nb₂O₆ (SBN:61) relaxor crystals. The dopants introduced into SBN:61 crystals promote the switching process by reducing the value of threshold nucleation field, and thus coercive field. We present real-time studies of domain nucleation and growth processes in doped SBN:61 by the nematic liquid crystal (NLC) decoration technique. The broad phase transition and low-frequency dielectric dispersion that are exhibited by doped SBN:61 samples have a strong link to the configuration of the ferroelectrics microdomains, which in turn is strongly determined by Ni ions concentration.     

Critical slowing down in the single crystalline relaxor SBN75

The dielectric susceptibility of strontium-barium niobate, Sr_0.75Ba_0.25Nb₂O₆, reveals strong relaxor behaviour. It is shown that the relationship between the frequency of the ac probing field and the temperature, at which the real part of the susceptibility passes through a maximum, satisfactorily follows the activated dynamic scaling law predicted for the random-field Ising model. Applicability of a power law as suggested by the dynamic scaling theory is discussed.     

Optical spectroscopy of neodymium-doped calcium barium niobate ferroelectric crystals

In this work, the optical absorption, luminescence and Raman spectra of a single Ca_0.28Ba_0.72Nb₂O₆:Nd³⁺ ferroelectric crystal have been measured and compared to those obtained for the self-frequency solid-state laser converter Sr_0.61Ba_0.39Nb₂O₆:Nd³⁺ crystal. The calcium-niobate system displays a much higher transition temperature (∼150 °C) than the strontium-niobate one (∼80 °C) and so it appears as an excellent candidate for a self-frequency converter solid-state laser, more stable to pumping radiation that than based on the strontium-niobate one.     

Evolution of the dielectric and acoustic parameters of chromium-doped SBN single crystals with variations in temperature

The temperature dependences of the dielectric characteristics of Sr_0.61Ba_0.39Nb_{2 − x} O₆: Cr _x ³⁺ single crystals with x = 0.0, 0.005, and 0.010 are investigated in the low-frequency and infrared ranges. It is revealed that the behavior of the dielectric response exhibits specific features in the vicinity of the transition from the ferroelectric phase to the paraelectric phase and in the low-temperature range. The measurements are performed along the [001] (polar axis) and [100] directions. The temperature dependences of the relative sound velocity and the attenuation of elastic waves are also studied, and the temperature ranges of the anomalous behavior of these parameters are determined. The specific features revealed in the behavior of the dielectric and acoustic properties are considered from the viewpoint of the structural transformations occurring in the SBN system upon doping with a chromium impurity. Original Russian Text ? A.S. Pilipenko, A.I. Burkhanov, L.I. Ivleva, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 543–546.     

Dielectric permittivity dynamics of Ba1−x SrxTiO3 epitaxial films (x=0.75): Microstructure and depolarization effects

Trilayer epitaxial heterostructures including metal oxide electrodes (SrRuO₃, 200 nm) and a sandwiched dielectric layer (Ba_0.25Sr_0.75TiO₃, 700 nm) were grown by laser ablation on (001)LaAlO₃ substrates. The maximum permittivity of the Ba_0.25Sr_0.75TiO₃ layer (?′/?₀≈3700) was obtained at T _M =160 K and an external electric field E≈10⁶ V/m. The ?′(T) dependence for the Ba_0.25Sr_0.75TiO₃ layer in the paraelectric phase is well fitted by the Curie-Weiss relation, with the Curie constant and the Weiss temperature differing only insignificantly from the corresponding bulk values. The change in the permittivity of the Ba_0.25Sr_0.75TiO₃ layer induced by the application of a ±2.5 V bias voltage to the electrodes reached as high as 85%. The electric-field dependence of the polarization retained clearly pronounced saturated hysteresis loops up to temperatures 10–15 K above T _M .     

Dielectric investigation of a new TKWB lead free relaxor

A new lead-free tungsten bronze (TKWB) type relaxor derived from Ba_1−xSr_xNb₂O₆ and containing Sn was prepared via classical solid state route. A 2.5% Sn:Nb ratio was enough to both induce the relaxor effect and bring the transition temperature close to room temperature. Promising dielectric characteristics were evidenced, in particular when compared to perovskite-type lead-free relaxors. Chemical bonding arguments are used to explain the role of tin in the structure-dielectric properties relationships.     

Diffuse phase transition in Ba5NdTi3−xZrxNb7O30 ferroelectric ceramics

Polycrystalline samples of Ba₅NdTi_3−xZr_xNb₇O₃₀ (x=0, 1, 2, 3) compounds of the tungsten-bronze (TB) structural family were prepared by a high-temperature solid-state reaction technique. X-ray study of the compounds shows the formation of single phase compounds in the orthorhombic crystal system at room temperature. Detailed studies of the dielectric properties (dielectric constant and loss tangent) as a function of temperature (−50 to 350°C) at four different frequencies, 1, 10, 100 and 1000 kHz show relaxor behavior and diffuse phase transition of the compounds. Study of temperature dependence of resistivity shows that these compounds have negative temperature coefficients of resistance (NTCR).     

SBN thin films growth by RF plasma beam assisted pulsed laser deposition

SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr_0.6Ba_0.4Nb₂O₆ and ceramic Sr_0.5Ba_0.5Nb₂O₆ stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31° in-plane orientation relative to the MgO (100) axis is identified.     

Dielectric response of Ba0.75Sr0.25TiO3 epitaxial films to electric field and temperature

The structure and dielectric parameters of the intermediate ferroelectric layer in the (001)SrRuO₃ ∥ (100)Ba_0.75Sr_0.25TiO₃ ∥ (001)SrRO₃ heterostructure grown by laser ablation on (001)La_0.294Sr_0.706Al_0.647Ta_0.353O₃ were studied. Tensile mechanical stresses accounted for the polar axis in the ferroelectric, being oriented predominantly parallel to the substrate plane. The remanent polarization in the Ba_0.75Sr_0.25TiO₃ layer increased approximately linearly with decreasing temperature in the interval 320–200 K. The real part of the dielectric permittivity of the intermediate ferroelectric layer reached a maximum ?′/?₀=4400 at T _M≈285 K (f=100 kHz). The narrow peak in the temperature dependence of the dielectric loss tangent for the Ba_0.75Sr_0.25TiO₃ ferroelectric layer, observed for T<T _M, shifted toward lower temperatures with decreasing frequency and increasing bias voltage applied to the electrodes.     

Effect of electric field on the smeared phase transition in crystals PbIn1/2Nb1/2O3-28PbTiO3 and PbIn1/2Nb1/2O3-37PbTiO3

The effect of a direct-current electric field (0 < E < 3 kV/cm) on the smearing of the phase transition and the Curie-Weiss exponent (γ) for two PbIn_1/2Nb_1/2O₃-28PbTiO₃ and PbIn_1/2Nb_1/2O₃-37PbTiO₃ crystals lying at different distances from the morphotropic phase boundary has been investigated. The universal Curie-Weiss law has been used to approximate the temperature dependences of the dielectric permittivity. It has been shown that the more remote is the crystal from the morphotropic phase boundary, the larger is the exponent γ and, consequently, the more smeared is the phase transition (in the zero field, γ = 1.67 and 1.49 for PbIn_1/2Nb_1/2O₃-28PbTiO₃ and PbIn_1/2Nb_1/2O₃-37PbTiO₃, respectively). It has been found that a weak electric field (no greater than 2–3 kV/cm) in the case of the more smeared phase transition almost does not affect the Curie-Weiss exponent, whereas for the PbIn_1/2Nb_1/2O₃-37PbTiO₃ crystal, this exponent decreases with increasing electric field strength and approaches γ = 1, which is characteristic of the conventional ferroelectric.     

Time evolution of polar regions in (Sr0.65Ba0.35)2NaNb5 O 15 single crystal

The linear, second-order and third-order dielectric susceptibilities χ₁, χ₂, χ₃ of a (Sr_0.65Ba_0.35)₂NaNb₅O₁₅ (SBNN) single crystal have been measured. Strong relaxation was found in all investigated dielectric susceptibilities near the phase transition. The time dependence of linear and third-order dielectric susceptibility can be explained as the effect of polar nano-clusters evolution. Near the phase transition, the net polarization of the crystal, monitored by second-order dielectric susceptibility, changes both its value and orientation. Measurements of the pyroelectric effect indicate that the non-zero net polarization in SBNN crystals appears spontaneously. In addition, the ferroelectric phase transition in SBNN showed many features of a first-order phase transition.     

Photoinduced electrical properties of rare-earth ion doped SBN

Photocurrents, pyroelectric currents and photovoltaic currents in ferroelectric Sr_0.5Ba_0.5Nb₂O₆ (SBN50) were investigated. These quantities were measured as a function of temperature. By doping the rare-earth ion Pr³⁺ into the sample, SBN changed to a relaxor ferroelectric with a diffuse phase transition. The relation between the diffuse phase transition and the photoexcited phenomena were explained in terms of the role of the dopant ions     

Templated Grain Growth of Textured Piezoelectric Ceramics

Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Sr_0.53Ba_0.47Nb₂O₆, and (Na_1/2Bi_1/2)TiO₃-BaTiO₃. Highly oriented Bi₄Ti₃O₁₂, Sr₂Nb₂O₇, CaBi₄Ti₄O₁₅, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Sr_0.53Ba_0.47Nb₂O₆ and (Na_1/2Bi_1/2)TiO₃-BaTiO₃ ceramics have been produced by TGG. The resulting ceramics show texture levels up to 90%, and significant enhancements in the piezoelectric properties relative to randomly oriented ceramics with comparable densities. For example, piezoelectric coefficients of textured piezoelectrics are from 2 to 3 times higher than polycrystalline ceramics and as high as 90% of the single crystal values. In textured PMN-PT, a low field (< 5 kV/cm) piezoelectric coefficient (d ₃₃) of ～1600 pC/N was obtained with > 0.3% strain (at 50 kV/cm). The high field dielectric and electromechanical properties of textured perovskites are more hysteretic than those of single crystals, probably as a result of clamping by the residual template particles, residual random grains, the presence of non-ferroelectric second phases, and a wide orientation distribution. Lateral clamping of one grain by another may also be an important factor in fiber-textured samples. Means to further improve the quality of texture and thus properties of textured piezoelectric ceramics by TGG are presented.     

Evolution of microstructure and dielectric properties of (LiCe)-doped Na0.5Bi2.5Nb2O9 Aurivillius type ceramics

Aurivillius type (NaBi)_0.5?x(LiCe)_xBi₂Nb₂O₉ ceramics were prepared by the standard ceramics route. The single crystal structural ceramics were achieved for all compositions and lattice distortion was decreased by (LiCe) dopants. The temperature dependent dielectric properties revealed that all compositions possess a high Curie-temperature (>780 °C). A modified Curie–Weiss relationship is used to study the diffuseness behavior of a ferroelectric phase transition indicating the degree of diffuseness of NBN-based ceramics increased with (LiCe) modifications. The degradation of resistance implied a plausible model that Ce⁴⁺ ions entered into the B-site of the pseudo-perovskite structure and acted as acceptor doping. Further investigation demonstrated that both electrical conduction and dielectric relaxation processes were associated with the oxygen vacancies produced by the substitution of Nb⁵⁺ ions by the Ce⁴⁺ ions.