Broken Local Symmetry in ParaelectricBaTiO3Proved by Second Harmonic Generation

Abstract

Precursor dynamics of a cubic to tetragonal ferroelectric phase transition in BaTiO3 is studied by the accurate measurement of the second harmonic generation (SHG) integral intensities. A finite signal holds for the SHG integrated intensity above the ferroelectric Curie temperature T(c)=403 K. Above the Burn's temperature T(d)≈580 K, the power law with the exponent γ=1 shows normal SHG nature originating from the hyper-Raman scattering by dynamical polar excitations, while, below T(d), a SHG signal from polar nanoregions becomes dominant with the larger exponent γ=2. Such a crossover of the power law exponent near T(d) is discussed on the basis of the effective Hamiltonian method and Monte Carlo simulation.

Upper bounds for flexoelectric coefficients in ferroelectrics

“…In particular, using experimental data from Refs. 12-14, one finds for BaTiO 3 jf 44 j < 3:3V; jf 11 À f 12 j < 7 V (17) and for SrTiO 3 jf 44 j < 2:4V; jf 11 À f 12 j < 10 V:…”

“…These experimental results correspond to the flexocoupling constants of the order of a few hundred volts 6 and customarily discussed in terms of the static bulk flexoelectric effect. At the same time, the upper bounds (17) and (18) suggest that so high values of the flexocoupling constants would have lead to the formation of the incommensurate phase in these materials. 15 Since no incommensurate phase been reported, either in BaTiO 3 or in SrTiO 3 , the high flexoelectric response in perovskite ceramics can hardly be due to the static bulk flexoelectric effect.…”

“…Recent developments in the field indicate that possible explanation may be found in manifestations of the surface piezoelectricity 16 or it may also be related to the local symmetry breaking in the paraelectric phase. 17 While the inequalities obtained indicate the upper bounds for flexoelectric constants in materials where incommensurate phase does not form, they may also be used to understand materials where such phases indeed form. 8,18 …”

Classical ferroelectriclike behavior of highly orderedPb(Sc1/2Nb1/2Sivasubramanian1, Subramanian2, Kojima3 2016Phys. Rev. BSelf Cite7150View full textAdd to dashboardCite

“…The cubic to tetragonal phase transition was found to be an order-disorder type associated with the relaxation dynamics of pretransitional polar clusters formed in the paraelectric phase [39][40][41][42]. The existence of such pretransitional polar clusters in these systems was clearly revealed through various studies [43][44][45][46]. Relaxor like properties of these polar clusters, such as the existence of T B and the intermediate temperature T * , were revealed by AE studies in the paraelectric phase [47].…”

Second harmonic generation measurements at high pressures on powder samples

“…due to the presence of surfaces, strains or defects, nominally centrosymmetric crystals may show SHG. For example, SHG measurements have been presented for surfaces [8], nanocrystals [9] or BaTiO 3 [10]. Furthermore, crystals which belong to a centrosymmetric space group may have an acentric magnetic structure, and it has been shown by Fiebig et al [11] that this then allows the detection of a temperature dependent magnetic phase transition, for example in centrosymmetric Cr 2 O 3 , by observation of the temperature dependence of the SHG signal.…”