Raman spectroscopic studies of disordered ferroelectric oxides

Relaxational properties of compositionally disordered AB03 perovskite oxides were studied. These oxides are the prototypical soft ferroelectric (FE) mode systems, and their interesting dipolar relaxational properties are determined by their long, strongly temperature-dependent correlation lengths for the dipolar interactions. The simple cases involve dilute chemical substitutions in the incipient ferroelectrics KTaO3 and SrTiO3, which exhibit relatively weak, low-temperature Debye-type relaxations. More complicated dipolar interactions are seen in B-site disordered Nb-doped KTaO3, which exhibits glass-like relaxor and relaxor-to-ferroelectric crossover behaviors at low temperatures. Finally, there is a class of more complex perovskites represented by PMN, PZN-PT and the PLT that exhibit strong, high-temperature relaxor and/or ferroelectric properties.; The renewed interest in the KTa1-xNbxO (KTN) mixed perovskite materials, especially in high quality thin films, is connected with their remarkable dielectric properties in the dilute compositions. Off-center Nb ions in the highly polarizable KTaO3 lattice provide a drastic increase in the dielectric peak, up to 20 times in comparison with the pure KTaO3 and KNbO3. The effects of the substrate and the symmetry-breaking defects on their vibration spectra were studied by micro-Raman spectroscopy. An anomalous residual intensity of the forbidden first-order scattering modes in the cubic paraelectric phase of the KTN films was connected with the formation of polar microregions even far above the bulk Tc. On the whole, the KTN film behavior shows the existence of specific defects enhancing the perovskite unit cell in the film so that the activity of off-center Nb ions increases in producing larger electric dipoles and extending the precursor phase above Tc. In diluted compositions with low Nb concentrations KTN materials exhibit formation of polar nano regions and relaxor like behavior. This behavior is analogous with behavior of well know relaxor systems, such as PbMg1/3Nb2/3O3 (PMN) and Pb1-x LaxTiO3 (PLT).; Raman scattering was studied in model relaxor ferroelectrics PbSc 1/2Ta1/2O3 (PST) and PMN and in related low-permittivity materials SrAl1/2Ta1/2O3 (SAT), SrAl 1/2Nb1/2O3 (SAN) and BaMg1/3Ta 2/3O3 (BMT). Comparative analysis of the Raman spectra gives evidence that complex perovskites of this type consist of nanoscale clusters with the 1:1 B-site order, irrelevant to whether a stoichiometric composition for the B cations is 1:1 or 1:2. This result is in agreement with the recent microstructure studies of both PST and PMN by direct methods. The low-permittivity compounds reveal the static basic spectrum for the corresponding 1:1 or 1:2 compositions. The basic spectrum is modified in relaxors by dynamic effects in the course of evolution to the ferroelectric state. Model relaxors PST and PMN show clearly such dynamic effects as mode broadening and a central peak.; In A-site substituted complex perovskite PLT systems micro-Raman results indicate that the crystal structure of the PLT films was strongly influenced by the La contents. The dielectric properties of PLT thin films were studied in the temperature range 80--700 K and results indicate that PLT thin films undergo normal-to-relaxor ferroelectric transformation with 30 at% La content in PLT films. The observed behavior is evaluated in terms of diffuseness and Vogel-Fulcher relationship, which is typical for relaxor ferroelectrics. It is consistent with typical relaxor behavior of disordered materials with polar nanoregions.