The relaxor-like dynamics of the cubic-tetragonal ferroelectric phase transition was studied by Brillouin and Raman scattering in K(Ta 1Àx Nb x)O 3 (KTN) crystals with x ¼ 0.40 (KTN40). The local symmetry breaking by the polar nanoregions (PNRs) was observed in a paraelectric phase by Raman scattering on the A 1 (z) mode of the PNRs with R3m symmetry. Upon cooling from a high temperature, the remarkable increase in the LA phonon damping starts at 45 K above the cubictetragonal phase transition temperature of T C-T ¼ 308 K, which is defined as the intermediate temperature, T* $ 353 K, indicating the start of the rapid growth of the PNRs. The coupling between the LA mode and fluctuation of the PNRs caused a remarkable elastic anomaly in the vicinity of T C-T. The analysis of the temperature dependent central peak shows a critical slowing down towards T C-T , which is the evidence for the order-disorder nature of a ferroelectric phase transition. The evolution of the dynamic PNRs is discussed by the estimation of their length scale, and it is found that it starts to increase near T* and gradually grows towards T C-T. V
The low-frequency infrared-active optical modes were studied in a barium zirconate, BaZrO3, single crystal with the perovskite structure using terahertz (THz) time-domain spectroscopy (TDS). The real and imaginary parts of the dielectric constants were accurately determined in the frequency range between 0.2 and 2.7 THz. Upon cooling from room temperature to 8 K, the lowest-frequency TO1 mode at 2.32 THz showed a pronounced softening to 1.94 THz. The real part of the dielectric constant at 0.5 THz determined by THz-TDS obeys Barrett's relation, and the existence of a plateau confirms that the quantum effects lead to saturation of the soft mode frequencies of the TO1 and TO2 modes below ≈20 K. This is reminiscent of incipient ferroelectrics with the perovskite structure such as CaTiO3.
A B S T R A C TThe elastic, electronic and optical properties of MNNi 3 (M= Zn, Sn and Cu) have been calculated using the plane-wave ultrasoft pseudopotential technique which is based on the first-principles density functional theory (DFT) with generalized gradient approximation (GGA). The optimized lattice parameters, independent elastic constants (C 11 , C 12 , and C 44 ), bulk modulus B, Compressibility K, shear modulus G, and Poisson's ratio υ, as well as the band structures, total and atom projected densities of states and finally the optical properties of MNNi 3 have been evaluated and discussed. The electronic band structures of the two hypothetical compounds show metallic behavior just like the superconducting ZnNNi 3 . Using band structures, the origin of features that appear in different optical properties of all the three compounds have been discussed. The large reflectivity of the predicted compounds in the low energy region might be good candidate materials as a coating to avoid solar heating.
In relaxor ferroelectrics, the role of randomly orientated polar nanoregions (PNRs) with weak random fields (RFs) is one of the most puzzling issues of materials science. The relaxation time of polarization fluctuations of PNRs, which manifests themselves as a central peak (CP) in inelastic light scattering, is the important physical quantity to understand the dynamics of PNRs. Here, the angular and temperature dependences of depolarized and polarized CPs in 0.44Pb(Mg1/3Nb2/3)O3-0.56PbTiO3 single crystals with weak RFs have been studied by Raman and Brillouin scattering, respectively. The CPs observed in Raman scattering show the very clear angular dependence which is consistent with the local tetragonal symmetry. It is different from the well-known local rhombohedral symmetry with strong RFs for Pb(Mg1/3Nb2/3)O3. In Brillouin scattering, depolarized and polarized CPs show two relaxation processes corresponding to transverse and longitudinal fluctuations of PNRs. The remarkable slowing down towards the Curie temperature was observed for transverse fluctuations in local tetragonal symmetry.
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