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تسجيل مستخدم جديدColossal dielectric constant and Maxwell-Wagner relaxation in $Pb(Fe_{1/2}Nb_{1/2})O_{3-x}PbTiO_3$ single crystals
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Recently, materials exhibiting colossal dielectric constant ($CDC$) have attracted significant attention because of their high dielectric constant and potential applications in electronic devices, such as high dielectric capacitors, capacitor sensors, random access memories and so on.
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A neutron scattering investigation of the magnetoelectric coupling in PbFe_{1/2}Nb_{1/2}O_{3} (PFN) has been undertaken. Ferroelectric order occurs below 400 K, as evidenced by the softening with temperature and subsequent recovery of the zone center
transverse optic phonon mode energy (hbar Omega_{0}). Over the same temperature range, magnetic correlations become resolution limited on a terahertz energy scale. In contrast to the behavior of nonmagnetic disordered ferroelectrics (namely Pb(Mg,Zn)_{1/3}Nb_{2/3}O_{3}), we report the observation of a strong deviation from linearity in the temperature dependence of (hbar Omega_{0})^{2}. This deviation is compensated by a corresponding change in the energy scale of the magnetic excitations, as probed through the first moment of the inelastic response. The coupling between the short-range ferroelectric and antiferromagnetic correlations is consistent with calculations showing that the ferroelectricity is driven by the displacement of the body centered iron site, illustrating the multiferroic nature of magnetic lead based relaxors in the dynamical regime.
We have characterized the dynamics of the polar nanoregions in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) through high-resolution neutron backscattering and spin-echo measurements of the diffuse scattering cross section. We find that the diffuse scatterin
g intensity consists of emph{both} static and dynamic components. The static component first appears at the Curie temperature $Theta sim 400$ K, while the dynamic component freezes completely at the temperature T$_{f} sim 200$ K; together, these components account for all of the observed spectral weight contributing to the diffuse scattering cross section. The integrated intensity of the dynamic component peaks near the temperature at which the frequency-dependent dielectric constant reaches a maximum (T$_{max}$) when measured at 1 GHz, i. e. on a timescale of $sim 1$ ns. Our neutron scattering results can thus be directly related to dielectric and infra-red measurements of the polar nanoregions. Finally, the global temperature dependence of the diffuse scattering can be understood in terms of just two temperature scales, which is consistent with random field models.
We investigate the low temperature behaviour of Pb(In$_{1/2}$Nb$_{1/2}$)O$_{3}$-Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-PbTiO$_{3}$ using dielectric permittivity measurements. We compare single crystal plates measured in the [001] and [111] directions with a
polycrystalline ceramic of the same composition. Poled crystals behave very differently to unpoled crystals, whereas the dielectric spectrum of the ceramic changes very little on poling. A large, frequency dependent dielectric relaxation seen in the poled [001] crystal around 100 K is much less prominent in the [111] crystal, and doesnt occur in the ceramic. Preparation conditions and the microstructure of the material play a role in the low temperature dynamics of relaxor-ferroelectric crystals.
High-temperature (T) and high-electric-field (E) effects on Pb[(Zn_{1/3} Nb_{2/3})_{0.92} Ti_{0.08}]O_3 (PZN-8%PT) were studied comprehensively by neutron diffraction in the ranges 300 <= T <= 550 K and 0 <= E <= 15 kV/cm. We have focused on how phas
e transitions depend on preceding thermal and electrical sequences. In the field cooling process (FC, E parallel [001] >= 0.5 kV/cm), a successive cubic (C) --> tetragonal (T) --> monoclinic (M_C) transition was observed. In the zero field cooling process (ZFC), however, we have found that the system does not transform to the rhombohedral (R) phase as widely believed, but to a new, unidentified phase, which we call X. X gives a Bragg peak profile similar to that expected for R, but the c-axis is always slightly shorter than the a-axis. As for field effects on the X phase, we found an irreversible X --> M_C transition via another monoclinic phase (M_A) as expected from a previous report [Noheda et al. Phys. Rev. Lett. 86, 3891 (2001)]. At a higher electric field, we confirmed a c-axis jump associated with the field-induced M_C --> T transition, which was observed by strain and x-ray diffraction measurements.
We investigated the dielectric properties of Pb(Fe1/2Nb1/2)1-xTixO3 single crystals below room temperature. Two dielectric anomalies were detected in sample A while only one was detected in sample B in the temperature range 90~300 K. A Debye-like rel
axation with strong frequency dispersion was detected in both samples. The pre-edge XAFS suggests that this dielectric anomaly is induced by the hopping conductivity between Fe2+ and Fe3+. The EXAFS results give us a clear picture of the local structure of iron ions. The weak frequency dependent dielectric anomaly only observed in sample A is supposed to be due to the dipole glass behavior.
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