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تسجيل مستخدم جديدDielectric properties of relaxor-ferroelectric ceramic and single crystal Pb(In$_{1/2}$Nb$_{1/2}$)O$_{3}$-Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-PbTiO$_{3}$ at cryogenic temperatures
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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.
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We report the spontaneous decay of a soft, optical phonon in a solid. Using neutron spectroscopy, we find that specific phonon lifetimes in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ are anomalously short within well-defined ranges of energy and momen
tum. This behavior is independent of ferroelectric order and occurs when the optical phonon with a specific energy and momentum can kinematically decay into two acoustic phonons with lower phase velocity. We interpret the well-known relaxor waterfall effect as a form of quasiparticle decay analogous to that previously reported in quantum spin liquids and quantum fluids.
An atomistic effective Hamiltonian is used to investigate electrocaloric (EC) effects of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) relaxor ferroelectrics in its ergodic regime, and subject to electric fields applied along the pseudocubic [111] direction.
Such Hamiltonian qualitatively reproduces (i) the electric field-versus-temperature phase diagram, including the existence of a critical point where first-order and second-order transitions meet each other; and (ii) a giant EC response near such critical point. It also reveals that such giant response around this critical point is microscopically induced by field-induced percolation of polar nanoregions. Moreover, it is also found that, for any temperature above the critical point, the EC coefficient-versus-electric field curve adopts a maximum (and thus larger electrocaloric response too), that can be well described by the general Landau-like model proposed in [Jiang et al, Phys. Rev. B 96, 014114 (2017)] and that is further correlated with specific microscopic features related to dipoles lying along different rhombohedral directions. Furthermore, for temperatures being at least 40 K higher than the critical temperature, the (electric field, temperature) line associated with this maximal EC coefficient is below both the Widom line and the line representing percolation of polar nanoregions.
Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ (BFN) ceramics are considered to be a potential candidate for technological applications owing to their high dielectric constant over a wide range of temperature values. However, there exists considerable discrepancy o
ver the structural details. We address this discrepancy through a comparative analysis of the earlier reported structures and combined X-Ray Diffraction (XRD) at room temperature and Neutron Powder Diffraction (NPD) measurements in the range of 5K up to room temperature. Our study reveals a cubic structure with space group $Pmbar{3}m$ at all measured temperatures. The local environment of the Fe ions is investigated using X-ray Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) technique. A detailed investigation of the electronic properties of the synthesized BFN ceramics is carried out by combination of theoretical and experimental tools: X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and density functional theory (DFT) within GGA$+U$. The bandgap is estimated using the diffuse reflectance measurements in the UV-Vis-NIR range and an appropriate value of the electron-electron correlation strength $U$ is estimated based on the UV-Vis-NIR and the XAS spectra.
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.
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