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The complex dielectric response of ultrafine grain barium titanate ceramics is investigated using broadband dielectric spectrometer. Extrinsic effects like conduction, space charge associating with grain boundaries are discussed. One dielectric relaxation ascribes to the interaction of defects with grain boundaries, for which the activation energy...
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... titanate BTO -based ceramics is one of the most attractive ferroelectric material for the use of multilayer 1 capacitors. However, the BTO-based ceramics show unusual dielectric characteristics when its grain size down to nanoscale. Comparing with coarse BTO ceramics, the conductivity of 35 nm nanocrystalline enhances one to two orders of mag- 2 nitude due to a greatly reduced oxidation enthalpy. Colossal permittivity present in ultrafine grain nonstoichiometric BTO ceramics, which is ascribed to small polarons based on elec- tronic hopping of Ti 3+ / Ti 4+ . 3 And the dielectric properties are more susceptible to grain boundary for nanocrystalline 4,5 BTO ceramics, so called “dilute the effects of grain size.” Dielectric response mechanism, effectively deduced by dielectric relaxation investigation, is closely to dielectric 6 7 like fluctuation of dipolar, ferroelectric domain, interfacial 8,9 polarization, and structural defect in ferroelectric 10,11 ceramics, etc. For nanocrystalline BTO ceramics, the ef- fective permittivity will be controlled by the combination of the intrinsic size effect and of the “dilution” effect due to the 5 nonferroelectric grain boundaries, at which an interfacial 12 polarization could be induced, thus, the electrical properties were susceptible to the space charge and chemical defects. Here we investigated the dielectric properties of 15 nm BTO ceramics. Dielectric relaxation and its dominant extrinsic effects like conductions, space charge between grains were discussed. BTO ceramics with an average grain size of 13 15 nm were prepared as reported elsewhere. The relative density determined by Archimedes’ method is 98.6%. X-ray diffraction ͑ XRD ͒ , atomic force microscopy ͑ AFM ͒ , x-ray photoelectron spectroscopy ͑ XPS ͒ , and transmission electron microscopy ͑ TEM ͒ were used to characterize the microstruc- ture of the samples. Dielectric properties were studied on a broadband dielectric spectrometer. Figure 1 ͑ a ͒ demonstrates that the crystal structure is BTO and the average grain size calculated by Scherrer for- mula in ͑ 200 ͒ diffractive plane is 16.7 nm, the value shifts to 19.3 nm deduced from the inner of Fig. 1 ͑ a ͒ after thermal treatment. Thus, the thermal treatment does not induce ap- parent growth of ceramics grain. The grain size determined by AFM topography of the surface is about 15 nm illustrated in Fig. 1 ͑ b ͒ . Binding energy of Ti 2 p 3 / 2 marked in Line 1 is derived 457.43 eV from XPS analysis, which is smaller than that reported for standard BTO ceramics spectrum, demon- strating that the valence of Ti in BTO ceramics shifts from Ti 4+ to Ti 3+ in order to balance the oxygen vacancies. However, the oxygen vacancies cannot be efficiently compen- sated after thermal treatment in air atmosphere because binding energy of Ti 2 p 3 / 2 marked in Line 2 is derived 457.13 eV in Fig. 1 ͑ c ͒ . The grain boundary and ferroelectric domain detected by TEM present in the samples ͑ figures are not shown here ͒ . Dielectric response of 15 nm BTO ceramics, examined in terms of dielectric constant, dependence of temperature is showed in Fig. 2. Some interesting features can be observed as follows: ͑ 1 ͒ at approximately megahertz frequency, one dielectric response peak corresponding to phase transition from ferroelectric to paraelectric displays disperse behavior, which consists with our previous works. 14,15 ͑ 2 ͒ With the measured frequency decline to tens to hundreds of kilohertz, dielectric constant increases with further increasing temperature after Curie phase transition, the same results were re- 16 ported for 50 nm dense nanocrystalline BTO ceramics. What’s more, relaxation phenomenon locates in the paraelectric phase. ͑ 3 ͒ With further decreasing frequencies to 1 kHz or even lower, the dielectric constant present two pronounced relaxation processes. The ferroelectric to paraelectric phase transition can be attributed to the relaxation process associated with the do- main reorientation, domain wall emotion, and the dipolar 17 behavior. However, the dielectric abnormal of paraelectric phase indicates that the extrinsic mechanisms like space charge polarization and/or nonignorable ionic conductivity dominate dielectric behaviors. The breadth of dielectric constant in the paraelectric phase propose existing the composi- tional fluctuations of Ti 4+ / Ti 3+ , which indicates the existence of oxygen vacancies and released electron. Figures 3 ͑ a ͒ and 3 ͑ b ͒ illustrated dielectric relaxation and accompanied activation energy, respectively. A sharp incre- ment of the real dielectric constant appears at the frequency lower than 10 Hz, accompanying with a wide and promi- nent dielectric loss peak in Fig. 3 ͑ b ͒ . The peak shifts to higher frequency with increasing temperature, indicating a 18,19 thermally activated relaxation process, which probably 11 relates to the motion of oxygen vacancies. The results of XPS measurements support this assumption. Furthermore, activated energy was confirmed through Arrhenius law fitting, written as follow: = 0 exp − E a k B T , 1 where is relaxation time, E a represents the activated energy, k B is Boltzmann constant, and T corresponds to the absolute temperature. Our excellent fit yields activation energy of re- laxation process to be 0.26 eV, the value increases to 0.41 eV ͑ the relaxation process locates at lower temperature and frequency ͒ with the samples annealed at 873 K for 30 h. Lewis 20 and co-workers reported that the experimental diffusion energy for vacancies of oxygen is 0.4–0.68 eV. So it is reason- able to believe that the dielectric relaxation induced by the oxygen defects. Because grain boundary facilitates the defect reaction, the activation energy was remarkably lower in 2 nanocrystalline BTO ceramics. Thus, the activation energy in the present study is lower than the previous work may be ascribed to the accumulation of electron released by the for- mation of oxygen vacancies at grain boundaries. Another interesting characterization is that the real dielectric constant increases sharply with decreasing frequency and reaches maximum ϳ 70 000 at higher temperature dis- played in Fig. 3 ͑ a ͒ . However, the colossal permittivity is not originated from the intrinsic mechanism but probably results from interfacial polarization model, also called Maxwell– Wagner effect, which was usually adopted to explain the dielectric relaxation with extremely high dielectric constant and found to be intensively reported for BTO-based thin 21,22 films ceramics capacitor and deficient BTO-based 3 ceramics. When an electric current passes through interfaces between two different dielectric media because of their different conductivities, surface charges pile up at the interfaces, and give rise to a Debye-like relaxation process under 23 an external alternating voltage. The imaginary modulus, the reciprocal of the imaginary permittivity ͑ because the value of tan ␦ is rather high at low frequency which makes the relaxation peak difficult to distinguish ͒ depends on measured frequency with the temperature higher than T C ͓ see Fig. 3 ͑ b ͔͒ , using Arrhenius law fitting, written as follows: f m = f 0 − E a k B T , 2 where f m is maximum relaxation frequency, f 0 is the prefac- tor, as shown in the inset of ͑ b ͒ of Fig. 3, indicate that the dielectric responses are Maxwell–Wagner relaxation and the E a values are obtained to be 0.58 eV and 1.01 eV for as- prepared and annealed samples, respectively. The complex impedance spectra can present two semi- circles modeled with an equivalent electrical circuit includ- ing resistor R and capacitor C elements composed of two parallel elements connected in series. One semicircle shows lower impedance corresponding to grain while the other locating at low frequency relates to grain boundary which 24 makes considerable contribution to the impedance. In this work, the inset of Fig. 4 displays only one arc over the frequency range of 1 Hz to 3 MHz, however, the impedance without zero intercept ͑ about 45 ⍀ at 374 K and at 3 MHz ͒ reaches the order of ϳ 10 7 at low frequency, which suggests that a small semicircle with a low impedance exists at higher frequency. However, with measured temperature and frequency increasing, the grain and grain boundary can be sepa- rated. It can be concluded that one rather small high- frequency semicircle for the grain and a rather large arc for the grain boundary. What’s more, the deviation from large semicircle at even low frequency results from interfacial polarization as discussed above. In summary, ultrafine grain BTO ceramics show unusual dielectric response. The extrinsic relaxation mechanisms like interfacial polarization, ionic conductivity polarization is predominant This work for was the supported ultrafine grain by the size 863-project BTO ceramics, under Grant even they No. are 2007AA03Z524, highly-dense and and possess the National of ferroelectric Natural domains. Science Foundation of China ͑ NSFC ͒ under Grant No. 50872094, and Hainan University Research Fund. This work was supported by the 863-project under Grant No. 2007AA03Z524, and the National Natural Science Foundation of China ͑ NSFC ͒ under Grant No. 50872094, and Hainan University Research ...
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... BaTiO 3 -based ceramics have relatively low piezoelectric properties (d 33 ~ 200 pC/N), which limits their applicability [8][9][10][11][12][13][14][15][16][17][18][19][20]. Accordingly, BaTiO 3 -based ceramics are mainly used as dielectric materials rather than as piezoelectric materials. ...
Li+ and Al3+ co-doped BaTiO3 ceramics were prepared by the conventional high-temperature solid reaction. The piezoelectric coefficients of the BaTiO3-based ceramics were obtained from room temperature to the Curie temperature. The influence of doping content on the temperature stability of the doped BaTiO3 ceramics was also discussed. From the X-ray diffraction results, we inferred that the Li+–Al3+ ion pairs preferentially oriented along the [001] direction, the main direction of lattice distortion. After co-doping, the piezoelectric properties of the BaTiO3-based ceramics became more temperature-stable. The d33 piezoelectric coefficient remained high below the Curie temperature (TC ≈ 125 °C). After normalizing the data, we observed that the BaTiO3-based ceramic doped with 2 mol% Li+–Al3+ was nearly independent of temperature. Therefore, when doped at 2 mol%, the Li+ and Al3+ ion pairs fixed the polarization direction of the pinning domain and achieved high-temperature stability. The proposed co-doping method is potentially up-scalable to mass production for future electronic devices.
... Thus, to balance the oxygen vacancies, there is a shifting of Ti 4+ to Ti 3+ due to which there is a shift in the binding energy, as shown in Figure 4D. 44 Thus, the XPS data The bandgap of the synthesized samples was evaluated using Tauc's plot, and the bandgap was found to be ∼3.2, 2.12, and 1.67 eV for Ba (x = 0, 0.1, and 0.2, respectively) as shown in Figure 5. ...
BaTi1−xFexO3 compositions (for x = 0, 0.1, and 0.2) were prepared via a solid‐state reaction route. The presence of iron (Fe) in barium titanate (BaTiO3) eventually decreased the energy bandgap; thus, its utilization for water cleaning application through photocatalysis process was explored (using methylene blue [MB] dye as an indicative pollutant in water). Characterization of the synthesized powder was performed through scanning electron microscopy, X‐ray diffraction, Raman spectroscopy, and X‐ray photoelectron spectroscopy. The bandgap of the synthesized powder was calculated as 3.2, 2.12, and 1.67 eV for BaTi1−xFexO3 compositions (for x = 0, 0.1, and 0.2), respectively. BaTi0.8Fe0.2O3 powder showed excellent results, and ∼71% of the MB dye (∼5 mg/L concentrated) was degraded using the photocatalysis process under visible light. To check the potentiality of BaTi1−xFexO3 compositions (for x = 0, 0.1, and 0.2), the photocatalysis process was carried out by changing the concentration of MB dye (2.5–10 mg/L with a step of 2.5 mg/L) and the amount of BaTi0.8Fe0.2O3 powder (0.05–0.2 g with a step of 0.05 g) for ∼5‐mg/L concentrated MB dye. The treated water was further used as a growth parameter and phytotoxicity analysis through germination index on the wheat seeds. Lastly, the BaTi1−xFexO3 compositions (for x = 0, 0.1, and 0.2) were explored for water cleaning applications under real‐time solar irradiation.
... Each semi-circle in the Cole-Cole plots corresponds to a different relaxation mechanism. In general, a low-frequency semicircle corresponds to the grain boundary contribution, and highfrequency semi-circles correspond to the relaxation mechanisms originating within the grains [80,81]. ...
The major challenge limiting the application of inorganic perovskite multiferroics is the presence of high conductivity, giving rise to extrinsic factors that suppress the intrinsic ferroelectric response. Therefore, it is vital to identify, control, and understand conductivity effects in these perovskite oxides. For this purpose, the electrical and dielectric responses of a well-known multiferroic, BiFeO3 (BFO), have been extensively studied. In the present study, the role of oxygen vacancies has been thoroughly investigated through their manipulation by synthesis techniques, heat-treatment protocols, and specifically selected high-valent B-site dopants. Both Mn substitution and the sol–gel preparation technique significantly reduce oxygen vacancy formation. Comprehensive dielectric, impedance, and conductivity analyses have shown that in the low-temperature region, direct hopping of charge carriers between Fe²⁺ and Fe³⁺ can be classified as short polaronic hopping. However, the hopping of electrons generated because of singly- and doubly-charged oxygen vacancies and the conduction of space charges at the grain boundaries exhibits the characteristics of long polaron hopping. A polydispersive nature of the various hopping mechanisms has been observed in the present study. The temperature dependences of grain resistance (arising from two different intra-grain hopping mechanisms) and grain boundary resistance have been studied and quantified through equivalent circuit analysis. Based on the activation energies calculated for the three identified thermally activated processes, a potential well diagram is proposed.
... The binary or ternary chemical elements have been added into the lattice sites or the interstitial sites to modify its properties and thus obtaining excellent dielectric and mechanical properties that can meet the market demand. The chemical elements being added into BaTiO 3 ceramic can not only suppress the grain growth of the doped system, reduce the rate of temperature change, and there by adjusting its Curie point and expanding its Curie peak, but also improve the piezoelectric, dielectric coefficient and reduce the resistivity of the doped system at room temperature [24][25][26][27]. ...
The electronic structures and the optical properties of Fe/Co–doped cubic BaTiO3 ceramics in which the Ti sites replaced partly and uniformly by Fe or Co chemical elements have been investigated using first–principles method based on the density functional theoretical framework. The calculated energy band structures indicate that Fe/Co–doped cubic BaTiO3 system has electrically conductivity while the purely BaTiO3 features the characteristic of semi–conductivity. In consistent with the energy band structures, some new peaks correlated to the impurity bands have arisen in the total density of state of the Fe/Co–doped BaTiO3 system due to the 3d orbital of Fe/Co atoms making an evidently contribution and a strong orbital hybridization incurred by d–d interaction and p–d interaction near the Fermi energy level being introduced by Fe/Co–doping. The analysis of optical properties, including the calculation of dielectric function, reflection spectrum and absorption spectrum of Fe/Co–doped BaTiO3 system, implied that their photoelectric properties can be improved by Fe/Co–doping due to the required activation energy for the photoelectron transition being reduced effectively. These phenomena such as red–shift phenomenon, the bridge effect of impurity bands and the strong orbital hybridization suggested that they can be as a potentially candidate for optoelectronic materials applied in the field of energy conversion.
... Charges can hop through the defects and contribute to the conduction [23,24], while in the latter case charges accumulate at the grain boundaries, which first contribute to the dielectric constant generating colossal values. With a further increase in temper ature, these charges participate in conduction and lead to thermally activated Maxwell Wagner type relaxation in the dielectric response [25]. The overall dielectric response of the material becomes diffused with broad frequency dispersion, just like that observed in relaxor ferroelectrics in which the frequency-dependent dielectric response arises due to the formation of polar regions [25,26]. ...
... With a further increase in temper ature, these charges participate in conduction and lead to thermally activated Maxwell Wagner type relaxation in the dielectric response [25]. The overall dielectric response of the material becomes diffused with broad frequency dispersion, just like that observed in relaxor ferroelectrics in which the frequency-dependent dielectric response arises due to the formation of polar regions [25,26]. This type of behavior has been reported for many oxides such as CaCu 3 Ti 4 O 12 and CoFe 2 O 4 [23,24,27,28]. ...
... This model leads to Cole-Cole plots, in which each semicircle corresponds to a distinct relaxation mechanism, where a lowfrequency semicircle belongs to the contributions from the grain boundaries. Semicircles in the high-frequency region, however, correspond to the relaxation mechanisms originating from the orientational or hopping polarization within the grains [11,25,27]. ...
We presented detailed frequency and temperature dependent dielectric response in Eu1-xBaxTiO3 (0 < x < 0.5). Excluding grain boundary effects, four relaxation mechanisms have been observed. Relaxation dynamics were observed to arise due to hopping conduction associated with defects namely, oxygen vacancies, Eu3+ and Ti3+ ions. Dielectric relaxation analysis led to identification of Ti ions in two different environments with different relaxation rates in the overall EuTiO3 perovskite structure. Emergence of another relaxation mechanism associated with ferroelectric order as a consequence of formation of polar regions have also been observed for higher Ba concentrations. Addition of Ba led to the identification of relaxation dynamics associated with hopping conduction between Eu ions, Ti ions (in the regions with and without oxygen vacancies) and with the formation of ferroelectric polar regions. Furthermore, polydispersivity and relaxation times have been extracted within the frame work of modified Debye model. Relaxation times have been observed to increase with decrease in temperature while larger values of polydispersivity revealed wide distribution of relaxation times due to the presence of lattice parameter and energy barrier distributions.
... As defect chemistry is highly dependent on the microstructure and the density of grain boundaries, there is great motivation to tailor the annealing process to achieve the desired electrical properties. While SPS is attributed to a decrease in grain-boundary resistivity, the fundamental effects of SPS on electrical properties remain not yet clarified [20,22,23,27,28]. ...
The re-oxidation kinetics of BaTiO3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5μm to 10μm. The re-oxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20h of exposure to an ambient environment at only 600°C. The residual density of charge carriers is reduced when using finer starting powders. SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10μm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO3 sintered using the conventional route with 10μm size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7μm in standard ceramics to 1.5μm in SPS ceramics.
... In case of materials having nano-sized grains, colossal dielectric constant arises due to the large number of grain boundaries. As it contains more contact points, charge accumulation and conduction in high temperature regime strongly effects the insulating nature and hence weakens the ferroelectricity [13]. In Nicklates case, it is reported that hopping of the carriers effects the dielectric response, such as in La 1.5 Sr 0.5 NiO 4 and Nd 1.5 Sr 0.5 NiO 4 , presence of Ni +3 ions forms localized small polarons. ...
... In addition to the hopping of the carriers, another extrinsic factor which affects the dielectric response, is grain boundary conduction. For example, in BaTiO 3 (BTO), composed of dense grains having size less than 50 nm, finite size of the grains strongly effect the dielectric response due to grain boundary conduction [13]. In this particular case, presence of large interfacial area led to an increase in conductivity, which was an order of magnitude higher than bulk counter-part. ...
... In this particular case, presence of large interfacial area led to an increase in conductivity, which was an order of magnitude higher than bulk counter-part. Here increase in conductivity, accompanied colossal rise in the value of dielectric constant in the temperature regime higher than the structural phase transition temperature [13,18]. Dielectric with small grains responds to the alternating electric field and give rise to the relaxation process known as Maxwell-Wagner relaxation [19,20]. ...
Post sintering studies of BaTiO3 (BTO) nanoparticles are presented in detail. Bulk nanostructures were prepared via three different compaction processes, namely, uniaxial cold pressing (UCP), Cold Isostatic Pressing (CIP) and Spark Plasma Sintering (SPS). Effect of compaction technique on microstructures have been investigated and correlated with electrical response for each sample. In addition to the transport properties, temperature and frequency dependent dielectric response of variously sintered samples and bulk counterpart was recorded. Several aspects have been identified that are essential to be taken into account in order to completely understand physical processes. Drastically distinct features were observed in paraelectric (PE) regime well above ferroelectric (FE)-PE transition temperature. These features include intra grain conduction with a reduction in the magnitude of PE to FE peak dielectric constant magnitude. Role of strain, grain boundary conduction associated with observation of Maxwell Wagner relaxation and hopping conduction in dielectric and ferroelectric response have been observed and discussed. Densification with presence of oxygen vacancies, significantly enhances conductivity associated with the hopping of the carriers, in turn deteriorated ferroelectric response.
... 1-4 Complex oxides comprise of a large class of compounds for which the opportunity to combine magnetic and electrical elements continues to grow, leading to interesting physical outcomes, such as giant magnetoresistance, 5 multiferroic, 6 electrocaloric 7 and magnetocaloric 8 properties. Building on our own version of sol-gel/solvothermal synthesis 9,10 and drawing from research of perovskite oxides, [11][12][13][14][15][16][17][18][19] we aim to explore the dielectric properties at the nanoscale. These compounds have special crystal structures, compositions and micro-(nano-) structures, opening a window of opportunity for future materials, and potentially some undiscovered properties to emerge as a consequence of the structure-property relationships. ...
We report the activation field and selective frequency-dependent ferroelectric behavior of BaMn3Ti4O14.25 (BMT-134) at room temperature. BMT-134, a recently discovered multiferroic complex oxide, exhibits antiferromagnetic and ferroelectric behavior and belongs to the hollandite crystal class. The microstructure can be manipulated through processing conditions to prepare a nanocrystalline textured tablet. We measured polarization-electric field (P-E) hysteresis loops and strain-electric field butterfly loops as a function of frequency in order to investigate the AC dynamics of domain switching and strain behavior. Under an electric field loading condition, a clear hysteresis loop of the electric field-displacement curve is obtained at 50 Hz, indicating that room temperature ferroelectricity is attainable under the right processing conditions. When the frequency is increased to 500 Hz, the coercive field also increases, until the frequency reaches 5 kHz, at which point the electric field versus electric displacement becomes linear indicating the limit of domain switching at high frequency.
... The relaxor ferroelectrics belong to the family of perovskites, such as (PMN), Pb(Sc 1/2 Ta 1/2 )O 3 (PST), (PbLa)(ZrTi)O 3 (PLZT) and Ba(Zr 0.25 Ti 0.75 )O 3 [14][15][16][17][18]. However FE materials are reported to exhibit frequency dependent diffused phase transition due to the extrinsic effects such as small grain size [19], thickness of the nanocrystalline thin films [20] and electrode polarization [21]. Similarly as the annealing temperature and applied ac frequency increases a systematic increase in the dielectric peak associated with the C-T transition has been reported for Ba 0.6 Sr 0.4 TiO 3 [22]. ...
... Similarly as the annealing temperature and applied ac frequency increases a systematic increase in the dielectric peak associated with the C-T transition has been reported for Ba 0.6 Sr 0.4 TiO 3 [22]. The observation of artificial relaxor like feature in bulk and thin film FE materials was attributed to the grain size effects, accumulation of charges at the grain boundaries and electrical conductivity [20]. ...
... Morphological analysis reveal the effects of the extrinsic factors on the dielectric response, and these findings are consistent with the literature that indicates frequency dependent diffused phase transition due to the small grain size [19], thickness of the nanocrystalline thin films [20] and electrode polarization [21]. Similarly, as the annealing temperature and applied ac frequency increases, a systematic increase in the intensity of the dielectric peak associated with the C-T transition have been reported for Ba 0.6 Sr 0.4 TiO 3 [22]. ...
... However, there are many causes for size effects in ferroelectrics and is often difficult to separate true size effects from other factors that change with particle/grain size [16][17][18][19]. The stability of the ferroelectric phase can be determined by additional factors like defect chemistry, incorporation of foreign atoms and bulk hydroxyl groups, aggregation level of particles, porosity level, residual stresses, etc. [3,20,21]. In fact, it has been theoretically demonstrated that besides grain size, internal stresses also affect the structural, ferroelectric and dielectric properties of fine grained BTO ceramics [22,23]. ...
... An additional consideration concerns the electrical boundary conditions in ferroelectric materials. The spontaneous polarization creates depolarization fields that require compensating charges near or at the surfaces/interfaces [19][20][21][22][23][24][25][26][27][28]. If for some reason a spontaneous polarization would not be compensated by charge, the consequence of the necessary boundary condition of zero polarization at the free surface would be an internal electrical field oriented in opposition to the polarization itself. ...
