Time relaxation of permittivity in [(CH3)2NH2]3CuCl5 crystal under external electric field

Continuation of our previous studies connected with dielectric properties of tris-dimethylammoniumpentachlorocuprate crystal [(CH₃)₂NH₂]₃CuCl₅ crystal under the influence of electric field are presented in this article. We studied the time dependence of the permittivity under the influence of electric field at constant temperatures above T _c. In experiment, slow but significant decrease of permittivity was observed, which can be described with two main relaxation processes. The observed dielectric behavior can be related to the coexistence of phases and slow transition to ferroelectric one through long-time metastable states connected with the presence of incommensurate phase in the studied crystal. The coexistence of phases and slow transition was confirmed from the appearance of induced polarization under external electric field, measured as depolarization current.     

Influence of the dc and ac field intensity on electric permittivities of (Na0.5Bi0.5)0.88Ba0.12TiO3 ceramics

The electric permittivity measurements under different ac or dc field amplitudes on lead-free (Na_0.5Bi_0.5)_0.88Ba_0.12TiO₃ ceramics were performed. A significant increase or decrease in permittivities was observed with increasing ac or dc fields intensity, respectively. It was found that temperature T _m decreases or increases with increasing ac or dc fields, respectively. When ac field amplitude increased, stronger frequency relaxation around T _m and greater shift in T _m was observed. This was in opposite to the effect of dc field intensity. The obtained results were discussed in terms of polar region behaviour in external electric fields.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

Effect of uniaxial stress on the dielectric properties of BaTiO3+0.1wt.%Eu2O3 ceramics

ABSTRACT

BaTiO₃+0.1wt.%Eu₂O₃ ceramics were prepared by a solid-state reaction method. The dielectric behavior of these ceramics as a function of uniaxial pressure has been systematically studied. The external stress showed obvious effects on these properties. An increase of the Curie point (T_c) and decrease of the Curie–Weiss temperature (T₀) was observed with increasing pressure, resulting in an increase in the first-order nature of the phase transformation (T_c–T₀ increases). Broadening and flattening of the permittivity versus temperature curves near their maximum was found. The pressure behavior of thermal hysteresis and the ??/?T vs. T plot suggests that the phase transition changes to second-order type with increasing pressure. Furthermore, 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.     

Successive phase transitions in tetra(isopropylamine)decachlorotricadmate (II) [(CH3)2CH NH3]4Cd3Cl10] crystal - Dielectric and dilatometric studies

Single crystal of a new [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ compound was grown and its properties have been characterized by dielectric and dilatometric investigations. Dielectric measurement evidenced the phase transitions at T₁ = 352.8 K, T₂ = 293.5 K and T₃ = 261.5 K on cooling run. Dilatometric measurement of thermal expansion showed clearly two successive phase transitions at T₁ and at T₂. No temperature hysteresis was observed for phase transitions at T₁ and T₂. Large temperature hysteresis was observed at T₃ in dielectric studies. Transitions at T₁ and T₃ are classified as a first order and at T₂ as a continuous one. Anomalies of electric permittivity and expansion connected with the transitions are observed at practically the same temperatures and close to those observed earlier in DSC (Differential Scanning Calorimetry) studies. Results of dilatometric studies were applied to estimate critical coefficient β for continuous phase transition at T₁ which is equal to 0.40 ± 0.01.     

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP₂Se₆ crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T _c?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.     

Properties of (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 analysed within dielectric permittivity,conductivity, electric modulus and derivative techniques approach

The (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ ceramics with perovskite structure were sintered. The XRD test proved that the samples are cubic (a?=?3.920?±?0.001?Å). Microstructure and atomic composition were determined with a SEM (JSM-5410) equipped with energy dispersion X-ray analyser (ISIS-300). The fluctuation in the chemical composition was found indicating on local disorder. Broadband dielectric spectroscopy in the range 10^?1–3?·?10⁷?Hz was applied within the range of 100–650?K. The real, ?′(f,?T), and imaginary, ?″(f,?T), parts of complex dielectric permitivity characteristics, both in the temperature and frequency domain, show relaxation processes partially covered by electric conductivity. At high temperatures the electric conductivity exhibits a thermally activated behaviour σ(f,?T)?∝?exp(?E _a/kT) but the variable range hopping (VRH) dependence σ?∝?exp[?(T ₀/T)^1/4] is manifested at low temperatures. The derivatives technique in the frequency (??log??/??log?ω) and temperature (??log??/?T) domain enabled various relaxation processes to be distinguished. The data converted to electric modulus representation, M*(f,?T)?=?1/?*, exhibited clearly resolved relaxation peaks. The relaxation times obtained from the peaks position show a slightly non-Arrhenius temperature behaviour with the activation energy varying in 0.4–0.6?eV range and characteristic time of the electric conductivity relaxation of the order of 10^?12?s. The relaxation times can be fitted at better accuracy with the VRH dependence where T ₀ are of the order of 10⁸?K. It is shown that the low frequency ac-conductivity converges to dc-conductivity and the relation σ(0)?～?ω_m?～?τ_m ^?1 typical for the disordered solids applies. The conduction current relaxation relationship behaves in accord with the VRH system: σ_dc?∝?(T/T ₀)^q (e ²/kT) ω_c, where ω_c?=?ν_ph exp[?(T ₀/T)^1/4] is valid for the locally disordered (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ compound.     

The influence of the lithium concentration on the low-temperature phase transition in phases LixV2O5 with shcherbinaite-structure

With high-resolution Guinier diffractometry the low-temperature stability of the intercalated lithium vanadium oxide bronzes ε - Li _x V₂O₅, with 0.32 ≤ x ≤ 0.80, was studied. In the course of these investigations a line of thermally induced structural phase transitions was detected in the x-T field. The reversible continuous phase transitions can be described in terms of a classical Landau-type theory. They are proper (potentially) ferroelastic with an one-dimensional order parameter transforming as the Γ⁺ ₃ irreducible representation of space group Pmmn. The symmetry is orthorhombic P2₁/m2₁/m2/n above T_c , but monoclinic P2₁ /m11 below T_c .

The transition temperatures depend strongly on the stoichiometry of the intercalated lithium ions. The critical exponent β, and, hence, the character of the phase transition do not change significantly with the lithium concentration. However, for x ≤ 0.52 corresponding phase transitions could not be detected, supporting the idea that the accepted stability field of ε - Li _x V₂O₅(0.32 ≤ x ≤ 0.80) should be subdivided into two fields, ε- and ε'-phase, according to whether the phase transition occurs, or not. The influence of the reduction rate of the intercalated phases on the critical parameters will be discussed in terms of a mean-field theory.     

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σ_ac) and dielectric permittivity (?) are determined in the temperature range 300?K?T3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T _c?=?(486?±?1)?K. A second structural phase transition was observed around 345?K. The conductivity varies with temperature range and for T?>?428?K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) ^s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.     

The effect of hydrostatic pressure on the ferroelectric phase transition in [NH2(CH3)2]3[Sb2Cl9]

The effect of hydrostatic pressure on the ferroelectric phase transition temperature in [NH₂(CH₃)₂]₃[Sb₂Cl₉] (DMACA) has been studied by electric permittivity measurements at pressures up to 400 MPa. The pressure-temperature phase diagram is given. The phase transition temperature (T_c) increases with increasing pressure up to 150 MPa, passes through a maximum and then decreases with a further increase of pressure. The unexpected nonlinear decrease in T_c with pressure increasing above 150 MPa suggests that the mechanism of ferroelectric phase transition in DMACA is different from hitherto assumed.     

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.     

Uniaxial pressure influence on dielectric properties of Pb(Zr0.99Ti0.01)O3 single crystals

The effect of uniaxial pressure (0–1000 bars) applied parallel to AC electric field on dielectric properties of Pb(Zr_0.99Ti_0.01)O₃ single crystals has been investigated. It was found that uniaxial pressure significantly influences these properties. With increasing pressure: (i) peak ?(T) decreases, becomes diffuse and shifts to higher temperature, (ii) the thermal hysteresis is reduced, (iii) the hump in ?(T) profile connected with antiferroelectric–intermediate phase transition vanishes, and (iv) the local anomaly in ?(T) profile connected with polar microregions existence above T_m is protruded and slightly shifts towards higher temperature. It was concluded, that applied uniaxial pressure or increasing Ti-ions content in lead zirconate titanate system induces similar effects. The results were discussed in terms of domain switching under pressure.     

Dielectric behavior and low temperature phase transition in NH4IO3

The electrical properties namely ac conductivity σ(ω,?T) and the complex dielectric permittivity (ε*) are measured at selected frequencies (5–100?kHz) as function of temperature (95?K?T?4IO₃. The ferroelectric hysteresis loops and the X-ray diffraction pattern are also measured. The analysis of the data indicates that the compound undergoes a structural phase transition at ～103?K and the behavior of σ(ω,?T) obeys the power law. The trend of the temperature dependence of the angular frequency exponent s (0?s?4IO₃; (2) the data indicate that the compound undergoes a structural phase transition at 103?K; (3) the originality of this transition has been confirmed by X-ray diffraction; (4) no evidence for the existence of a ferroelectric transition at 103?K as mentioned earlier; and (5) the quantum mechanical tunneling is proposed as the main mechanism of the electric conduction.     

Proton NMR study of molecular dynamics and phase transitions in methylammonium hexachloro plumbate, [NH3CH3]2PbCl6

Proton spin lattice relaxation time (T₁), measured as a function of temperature in the range 375–77 K, shows slope changes at 333, 221 and 111 K, in addition to a first order phase transition at 150K. The observed T₁ behaviour and second moment (M₂) variation with temperature are explained on the basis of the different possible motions of CH₃ and NH₃ groups.     

Effect of Hydrostatic Pressure on the Dielectric Response of Pb(Mg1/3Nb2/3)O3 Relaxor

The effect of hydrostatic pressure on the dielectric response of Pb(Mg_1/3Nb_2/3)O₃ relaxor single crystal was studied. An increase in the dispersion of the dielectric anomaly, characteristic of the relaxors, was observed: pressure-induced downward shift of the temperature T _m of permittivity maximum at 1 kHz amounts to dT _m /dp = ? 3.8 K/100 MPa, whereas that measured at 1 MHz equals ? 3.3 K/100 MPa. Analysis of the dielectric response of the highly polarizable Pb(Mg_1/3Nb_2/3)O₃ shows that hydrostatic pressure results in a decrease of the correlation radius r _c of polar nanodomains, as well as their activation energy.     

HighT c superconductivity in YBa2Cu3O7−x studied by PAC and PAS

HighT _c superconductivity has been investigated in YBaCuO by both perturbed angular correlation and positron annihilation spectroscopy techniques as a function of temperature from 77 to 300 K. An abrupt change has been observed in the positron lifetime and Doppler broadening and the electric field gradient and its asymmetry parameter acrossT _c, indicating a transition of two- to one-dimensional Cu-O-Cu chain structure and a charge transfer from CuO layers to CuO chains. An anomaly of the normal state has been demonstrated around 125 K, which is attributed to the structural instability.     

Dielectric properties of Sn2P2S6 crystals as a function of their growth conditions

The behavior of the permittivity near a phase transition in Sn₂P₂S₆ crystals of different technological quality is studied. It is established that, in high-resistance crystals, where an internal electric field is formed by the screening of spontaneous polarization in the polar phase, long-time relaxation of ɛ is observed in a temperature range ∼2 K above T _max. This relaxation and change in the form of the maximum of ɛ′(T) at a phase transition are attributed to an internal electric field induced by the volume space charge formed in regions near the surface. It is established that the existing differences in the properties of Sn₂P₂S₆ crystals are due to deviations from stoichiometry, arising during growth and synthesis of the crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1456–1461 (August 1999)     

E-T phase diagrams of a solid solution of the multicomponent PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbNi1/3Nb2/3O3-PbTiO3 system near the morphotropic phase boundary

The specific features of the dielectric and pyroelectric responses of a solid solution in the barium-doped multicomponent yPbZn_1/3Nb_2/3O₃-mPbMg_1/3Nb_2/3O₃-nPbNi_1/3Nb_2/3O₃-xPbTiO₃ system of the composition y = 0.0982, m = 0.4541, n = 0.1477, and x = 0.3 near the morphotropic phase boundary have been investigated. It has been assumed that the maxima revealed in the dependences of the reversible permittivity on the electric field strength with both the forward and backward changes in the field (E ₌) are associated with the induced phase transition. Based on the experimental results, the E-T phase diagrams are constructed in the temperature range from ?100 to 150°C for different temperature-field regimes: (i) variation in the electric field E ₌ at a fixed temperature of the sample and (ii) variation in the temperature of the sample at a constant value of E ₌. It has been found that there is a singular point in the E-T phase diagram and that, in the vicinity of this point, the dielectric and pyroelectric responses of the studied ceramics exhibit specific features.     

Dielectric relaxation in the weak ferroelectric Li2Ge7O15 near the ferroelectric phase transition

The dielectric permittivity of Ni-doped Li₂Ge₇O₁₅ crystals was studied in the vicinity of the ferroelectric phase transition. Introduction of Ni has been shown to suppress the dielectric anomaly and to reduce substantially the transition temperature. A temperature hysteresis in ɛ (T) has been observed in nominally pure and Ni-doped Li₂Ge₇O₁₅ crystals near the transition point. Measurements performed under cooling from the paraphase reveal dispersion of dielectric permittivity at Debye relaxation frequencies of the order of 10⁴–10⁵ Hz at T _c . It is proposed that the hysteresis phenomena and the low-frequency dispersion are caused by residual defects (of the type of random local fields), which become polarized in the ferroelectric phase and become disordered above T _c . Fiz. Tverd. Tela (St. Petersburg) 40, 2198–2201 (December 1998)     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.