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Register a new userBrillouin scattering and molecular dynamics study of the elastic properties of Pb(Mg1/3Nb2/3)O3
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Muhtar Ahart
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We report the complete set of elastic constants and the bulk modulus for single crystal Pb(Mg1/3Nb2/3)O3 (PMN) at room temperature obtained from Brillouin spectroscopy and molecular dynamics (MD) simulations. The bulk modulus from Brillouin is found to be 103 GPa, in a good agreement with earlier x-ray studies. We also derived the refractive index along all principal axes and found PMN to be optically isotropic, with a refractive index of 2.52 +/- 0.02. PMN shows mechanical anisotropy with A=1.7. The MD simulations of PMN using the random site model overestimate the elastic constants by 20-50 GPa and the bulk modulus is 148 GPa, but the mechanical anisotropy matches the Brillouin results of A = 1.7. We also determined the elastic constants for various models of PMN and we find variation in the elastic constants based on chemical ordering.
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Ferroelectric photovoltaic materials are an alternative to semiconductor-based photovoltaics and offer the advantage of above bandgap photovoltage generation. However, there are few known compounds, and photovoltaic efficiencies remain low. Here, we report the discovery of a photovoltaic effect in undoped lead magnesium niobate-lead titanate crystal and a significant improvement in the photovoltaic response under suitable electric fields and temperatures. The photovoltaic effect is maximum near the electric-field-driven ferroelectric dipole reorientation, and increases threefold near the Curie temperature. Moreover, at ferroelectric saturation, the photovoltaic response exhibits clear remanent and transient effects. The transient-remanent combinations together with electric and thermal tuning possibilities indicate photoferroelectric crystals as emerging elements for photovoltaics and optoelectronics, relevant to all-optical information storage and beyond.
Neutron and x-ray diffraction techniques have been used to study the competing long and short-range polar order in the relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) under a [111] applied electric field. Despite reports of a structural transition from a cubic phase to a rhombohedral phase for fields E $>$ 1.7 kV/cm, we find that the bulk unit cell remains cubic (within a sensitivity of 90$^{circ}$-$alpha$ =0.03$^{circ}$)for fields up to 8 kV/cm. Furthermore, we observe a structural transition confined to the near surface volume or `skin of the crystal where the cubic cell is transformed to a rhombohedral unit cell at T$_{c}$=210 K for E $>$ 4 kV/cm, for which 90$^{circ}$-$alpha$=0.08 $pm$ 0.03$^{circ}$ below 50 K. While the bulk unit cell remains cubic, a suppression of the diffuse scattering and concomitant enhancement of the Bragg peak intensity is observed below T$_{c}$=210 K, indicating a more ordered structure with increasing electric field yet an absence of a long-range ferroelectric ground state in the bulk. The electric field strength has little effect on the diffuse scattering above T$_{c}$, however below T$_{c}$ the diffuse scattering is reduced in intensity and adopts an asymmetric lineshape in reciprocal space. The absence of hysteresis in our neutron measurements (on the bulk) and the presence of two distinct temperature scales suggests that the ground state of PMN is not a frozen glassy phase as suggested by some theories but is better understood in terms of random fields introduced through the presence of structural disorder. Based on these results, we also suggest that PMN represents an extreme example of the two-length scale problem, and that the presence of a distinct skin maybe necessary for a relaxor ground state.
Following the recent discovery of a bulk photovoltaic effect in the Pb[(Mg1/3Nb2/3)0.68Ti0.32]O3 crystal, we report here more than one order of magnitude improvement of photovoltaicity as well as its poling dependence in the related composition of lead magnesium niobate-lead titanate noted Pb[(Mg1/3Nb2/3)0.7Ti0.30]O3. Photocurrent measurements versus light intensity reveal a fascinating hysteretic charge carriers dynamics clearly demonstrating charge generation, trapping and release processes.
Single crystals of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) poled along [001] were investigated by dielectric, x-ray, and polarized light (PLM) and piezo-force microscopy (PFM) methods. PLM revealed {100} macro-domain plates that formed after poling, whose size increased on heating between room temperature and a rhombohedral rightarrow tetragonal phase transition, above which point a break-up of the macro-domain plates was observed. Corresponding PFM studies demonstrated that poling reduced the size of stripe-like domains that were internal to the macro-domain plates, whose size also increased on heating to TR-T. The temperature dependence of both the size of the macro-domain plates and internal sub-domains followed the Arrhenius relation with the activation energy of 0.4-0.5eV. The coercive field displays an abnormal increase on heating below TR-T, different than that for PMN-PT. The anomalously increased coercive field can be ascribed to the Arrhenius-type domain growth, indicating a simple thermally activated process and an important role of hierarchial domains in the improved performance of PIN-PMN-PT.
The temperature dependence of elastic, dielectric, and piezoelectric properties of (65-x)Pb(Mg1/3Nb2/3)O3-xBaTiO335-PbTiO3 ceramics with x=0, 1, 2, 3, and 4 was investigated. Compound with x=2 was found to exhibit a large piezoelectric response (d31=-170 pC/N, d33=530 pC/N at 300 K). Particularly, its d31 value was nearly a constant over a temperature range from 185 to 360 K. A broad ferroelectric phase transition tuned by BaTiO3 doping was deduced from the dielectric constant, elastic compliance constant and Raman spectra. The temperature-stable piezoelectric response was attributed to the counter-balance of contributions from the dielectric and elastic responses.
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