Phase transitions in KNO3 embedded in MCM-41 films with regular nanopores

This paper reports on a comparative study of phase transitions in nanocomposites made up of KNO₃ embedded in 10-μm-thick MCM-41 films with unidirectional pores 4.0 nm in size on an aluminum substrate and of nanocomposites prepared in the form of potassium-nitrate-filled pressed MCM-41 powders with 3.7-nm pores. The temperature dependences of linear permittivity and the amplitude of third harmonic generation have been measured under heating and cooling. The structural transition from phase II to phase I shifts under heating relative to that occurring in bulk KNO₃ toward lower temperatures for potassium nitrate in the film and toward higher temperatures for the pressed MCM-41-based nanocomposite. A significant difference has been observed also within the region of existence of ferroelectric phase III. The data obtained suggest that the shifts of phase transition temperatures observed in the conditions of nanoconfinement are influenced markedly not only by pore size and geometry but also by other factors.     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.     

Effect of confined geometry on linear and nonlinear dielectric properties of triglycine sulfate near the phase transition

The temperature dependence of the linear permittivity and the third harmonic generation amplitude of nanocomposites representing nanoporous silica matrices (opal matrix and SBA-15) with triglycine sulfate embedded in pores has been studied in the region of the ferroelectric phase transition. A broadening of the phase transition and an increase its temperature in comparison with bulk triglycine sulfate have been revealed. The latter becomes more significant as the pore size decreases. It has been shown that the nonlinear dielectric properties of nanocomposites near the phase transition differ significantly from the properties of bulk triglycine sulfate.     

Dielectric properties of crystalline binary KNO3—AgNO3 mixtures embedded in nanoporous silicate matrices

The results of dielectric studies of MCM-41 sili cate matrices with cellular pores (37.0 and 26.1 ? in diameter) filled by binary mixtures of K_1−x Ag _x NO₃ (x=0, 0.05, 0.10) are reported in comparison with those obtained for bulk salts of the same composition in the temperature range of structural phase transitions. It has been revealed that, upon heating, the nanocomposites undergo transitions from crystal modification II to phase I, as in bulk KNO₃, whereas the bulk mixtures with x=0.05 and 0.10 have a complex structure in accordance with the phase diagram. It has been shown that embedding binary salts and pure potassium nitrate in pores with a diameter of 26.1? results in the formation of an intermediate ferroelectric phase upon cooling. The permittivity and electrical conductivity are found to increase with increasing AgNO₃ concentration in bulk samples, as well as with decreasing pore size in the nanocomposites for all values of x.     

Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH₄)₂SO₄ ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH₄)₂SO₄ nanocomposite as compared to bulk (NH₄)₂SO₄ was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.     

Dielectric losses in the triglycine sulfate crystal under heating and cooling

Temperature dynamics of the dielectric spectra and domain structure in the triglycine sulfate (TGS) hydrogen-containing ferroelectric under heating and cooling has been studied using the dielectric spectroscopy and atomic-force microscopy methods. The dielectric spectra are analyzed by the temperature-frequency dependences of the losses ε″ and by the temperature behavior of the maximum losses ε″_max in the dispersion region at frequencies ranging from 102 to 106 Hz. A dynamic conductivity model is proposed for calculating temperature dependences of losses ε″_max. The domain structure dynamics during the heating and cooling of the TGS crystal near the phase transition is studied using the in situ piezoresponse force microscopy. It is experimentally and theoretically shown that the relaxation dispersion is governed by the binding of strongly correlated dipoles with the main lattice that serves as a thermostat.     

Nuclear magnetic resonance study of potassium dihydrophosphate

A powder sample of potassium dihydrophosphate KH₂PO₄ has been studied by the ³¹P NMR method in a wide temperature range covering the ferroelectric phase transition. Changes in the position and shape of the resonance line at the transition to the ferroelectric phase have been revealed. The parameters of the chemical shift tensor of ³¹P (isotropic shift, anisotropy, and asymmetry) in the ferroelectric phase have been calculated from the experimental data. A sharp increase in the anisotropy of the tensor at the phase transition has been demonstrated. Dielectric measurements have also been carried out to verify the transition temperature.     

Heat capacity of PMN near an electric-field-induced phase transition

The heat capacity of a single crystal of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in an electric filed with E = 3 kV/cm applied along the [111] direction has been measured using adiabatic calorimetry over the temperature range 170–250 K. Anomalies in C _p have been found, which correspond to a field-induced phase transition from a relaxor to a ferroelectric state at 225 K under field cooling conditions or at 235–240 K on the subsequent field heating. The field-induced ferroelectric phase persists in a metastable state at low temperatures and is destroyed on zero-field heating at 210 K. The small entropy change ΔS = 0.028R in the field-induced phase transition suggests an insignificant change in the volume fraction of existing polar nanoregions.     

The influence of defects in the pb and o sub-lattices on dielectric properties and phase transitions in PbZrO3

From studies of the influence of controlled introduction of defects into the Pb and O sub-lattices of PbZrO₃ it was ascertained that marked changes are caused in the dielectric parameters and the transitions between the antiferroelectric and ferroelectric states and the ferroelectric and paraeiectric states. With increasing amount of defects the temperature range in which the ferroelectric state occurs is narrowed during the heating process and widened during the cooling process. An attempt has been made to explain the observed effects within the context of existing theoretical models.     

Low‐temperature Raman spectroscopic studies in NaNbO3

Raman spectroscopic measurements were carried out in the temperature range 10–300 K to understand the low‐temperature antiferroelectric (AFE)–ferroelectric (FE) phase transition in NaNbO₃. Several modes in the low wavenumber range were found to disappear, while some new modes appeared across the transition. The temperature dependence of mode wavenumbers suggests that, during cooling, the AFE–FE phase transition begins to occur at 180 K, while the reverse transition starts at 260 K during heating. During cooling, the two phases were found to coexist in the temperature range of 220–160 K. Upon heating, the FE phase is retained up to 240 K and both FE and AFE phases coexist in the temperature range 240–300 K. In contrast to the earlier reports, the present results suggest a different coexistence region and the reverse transition temperature. The reported relaxor‐type FE behaviour over a broad temperature is consistent with the observed coexistence of phases during cooling and heating cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Size effects upon phase transitions in vanadium oxide nanocomposites

A phenomenological model has been proposed for the formation of the major thermal hysteresis loop of optical parameters upon the semiconductor-metal phase transition in vanadium oxide nanocomposites. It has been demonstrated that the effects associated with the influence of nanocrystallite sizes on the phase transition temperature in an individual nanocrystallite, which is determined by the width of the elementary hysteresis loop and the position of the phase equilibrium temperature on the temperature scale, manifest themselves in the form of pronounced features in the loop shape. In particular, the size effects for VO₂ polycrystalline films are observed in the formation of a trapezoidal shape of the optical thermal hysteresis loop with a wide lower base, whereas these effects for nanocomposites based on porous glasses with VO₂ nanocrystals are revealed in the form of the optical hysteresis loop with a narrow lower base (wide upper base). The proposed model also explains the symmetric shape of the major hysteresis loop for vanadium oxide nanocomposites based on opals. The size effects in opal nanocomposites with strictly fixed sizes of pores and, therefore, nanocrystallite sizes manifest themselves in the form of well-defined steps in the heating and cooling branches of the major hysteresis loop.     

铌酸锂钠在低温时的介电铁电和热电性

在300—20K的温度范围内观测了用丘克拉斯基法生长的Li_0.025Na_0.975NbO₃晶体的介电、铁电和热电特性。介电常数,极化强度和热电性的反常表示该晶体在低温时发生相变。此相变有特别大的热滞(约80K),降温时发生于180K附近,升温时发生于260K附近。测量了晶体的室温结构,指出了可能的低温相点群。观测了热电电荷的时间响应,报道了热电电荷随时间改变符号的特异现象,认为其起因是相变过程中两相共存。 关键词：     

Dielectric Properties of Nanoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Films Filled with Ferroelectric SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>

Temperature dependences of permittivity ε′ and third harmonic amplitude γ_3ω of nanocomposites obtained by embedding ferroelectric SC(NH₂)₂ in porous alumina films with pore sizes of 60 and 100 nm are studied. A substantial increase in the temperatures of ferroelectric phase transition T_c1 and T_c2 and that of phase transition T_i from incommensurate phase to paraphase are also observed. The temperatures of all phase transitions are found to rise as pore diameters shrink.     

Effect of Restricted Geometry on the Structure and Phase Transitions in Potassium Nitrate Nanoparticles

The effect of restricted geometry and thermal prehistory of sample preparation on phase transitions in nanocomposites on base of porous glasses with average pore diameters 7 and 46 nm filled by potassium nitrate have been studied by X-rays and neutron diffraction. The nanoparticle sizes have been determined and phase diagrams (on cooling) for these nanoparticles have been described. It is shown that there is a critical nanoparticle size in the interval from 30 till 20 nm, at which in nanocomposite the ferroelectric phase is realized only regardless of preparation method.     

Bias field effects on the dielectric properties of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals with different orientations

The domain states and phase transitions in 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ single crystals were investigated by studying their relative permittivity under various dc bias at constant heating and cooling rates. The orientation dependence of the bias field effect was revealed by examining the temperature dependence of relative permittivity as a function of crystal orientation (the 〈111〉, 〈011〉 and 〈001〉 directions) and dc bias field. The crystals basically have a macrodomain rhombohedral ferroelectric state in the ferroelectric phase under zero dc bias. External bias field could modulate the domain state and induce a stable macrodomain state in the crystals. Also, it is proposed that the dc bias applied along the 〈001〉 or 〈011〉 direction could induce a tetragonal ferroelectric phase or an orthorhombic ferroelectric phase, respectively, in an intermediate temperature range.     

Linear and nonlinear dielectric properties of nanocomposites based on the organic ferroelectric of diisopropylammonium bromide

ABSTRACT

The present work aims at investigating linear and nonlinear dielectric properties of nanocomposites based on diisopropylammonium bromide (C₆H₁₆NBr, DIPAB) embedded into Al₂O₃ films having honeycomb structure with pores of 100?nm in diameter, and into opal matrices with three-dimensional structure containing pores of 100 and 60?nm. The obtained results indicated the shift of Curie point toward lower temperatures for DIPAB in porous aluminum oxide and the appearance of two phase transitions, detected upon heating and cooling for DIPAB in opal matrices. In addition, a ferroelectric phase was found to form between these two phase transitions without significant change of Curie temperature.     

液晶相变的微结构研究

用正电子湮没和差示扫描量热(DSC)变温测量液晶(EBBA)样品的正电子湮没寿命谱及DSC曲线,结果表明正电子湮没的短寿命(τ_s)基本不变,而长寿命(τ₁)和强度(I₁)明显地有两次跳变,其跳变的温度范围与DSC所测定的相转变温度范围基本一致,在加热和冷却过程中样品在晶体相←→向列相之间的相互转变的相转变温度范围显著不同。用正电子湮没的ORE模型讨论了由于液晶相转变所引起的微结构变化,从而提出正电子是研究液晶(EBBA)相变的探针。 关键词：     

Specific Features of the Domain Structure of BaTiO<Subscript>3</Subscript> Crystals during Thermal Heating and Cooling

This paper presents the results of the study of the domain structure of barium titanate crystals in a wide temperature range including the Curie point (T_C) using the polarization-optical method in the reflected light and the force microscopy of the piezoelectric response. It is shown that a new a–c domain structure forms during cyclic heating of the crystal above T_C and subsequent cooling to the ferroelectric phase. The role of uncompensated charges appeared on the crystal surface during the phase transition and their influence on the formation of the domain structure during cooling are discussed.     

The temperature dependence of the first,second and third harmonics of the non-linear pyroelectric current in monoaxial ferroelectrics having a second-order phase transition

The theoretical temperature dependences of the first, second and third harmonics of the non-linear pyroelectric current (NPC) are given. It has been established that the higher the order of the harmonic, the faster its magnitude rises, whereas the contribution of the appropriate harmonic to the total NPC decreases with rising harmonic order in all temperature ranges. The results obtained may provide useful data for the study of the phase transitions in ferroelectric media.     

Non ergodic aging in potassium niobo-tantalate crystals

Slow dynamics has been studied in various potassium niobo-tantalate crystals (KTN) by recording the complex dielectric constant after several thermal histories: isothermal evolution following controlled cooling or rapid quenching, positive or negative temperature cycles. The results reveal most of the behaviours of aging already found in lithium-potassium tantalate (KLT): effective ergodicity breaking (the asymptotic value of the dielectric constant varies as the logarithm of the cooling rate R), quasi-independence of the isothermal evolution with respect to a sojourn at lower temperature. But some differences between KTN and KLT are noticeable: coefficients of with opposite signs, role of the quenching temperature on the subsequent evolution, no overshot after a temperature jump. In order to explain the results we propose to extend the model initially developped for KLT taking into account the different nature of the low temperature phase, paraelectric for KLT and ferroelectric for KTN. In this model the variations of the dielectric constant are attributed to the slow movements of polarization domain walls hindered by static random fields. By measuring the dielectric constant during cooling and immediate heating at the same rate, an illustrative comparison is provided, showing that the evolution of the domain size is reversible in KLT and not in KTN. Received 17 November 1999