Role of Ca off-Centering in Tuning Ferroelectric Phase Transitions in Ba(Zr,Ti)O3System

We here report the substitution effects of the smaller Ca for the bulky Ba in the (Ba1-xCax)(Ti-1-yZry)O3 perovskite oxides for two systems (Ba1-xCax)TiO3 with y=0 and (Ba1-xCax)(Ti0.9Zr0.1)O3 with y=0.1. Ca off-centering was found to play a critical role in stabilizing the ferroelectric phase and tuning the polarization states in both systems. It was demonstrated that the atomic displacement due to Ca off-centering in the bulky Ba-sites in the perovskite structure provides an effective approach to compensate for the reduction of ferroelectricity due to chemical pressure, which allows to keep the Curie point nearly constant in the (Ba1-xCax)TiO3 system and increase the Curie point in the (Ba1-xCax)( Ti0.9Zr0.1)O3 system. It was commonly observed that the Ca off-centering effects lead to the shift of the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions toward lower temperatures and the ferroelectric stability of the tetragonal phase, resulting in the occurrence of quantum phase transitions with interesting physical phenomena at low temperatures in the (Ba1-xCax)TiO3 system and remarkable enhancement of electromechanical coupling effects around room temperature in the (Ba1-xCax)(Ti0.9Zr0.1)O3 systems over a wide range of Ca-concentrations. These findings may be of great interest for the design of green piezoelectric materials

Ferroelectricity in Silver Perovskite Oxides

There are two silver perovskite oxides: AgNbO3 and AgTaO3. AgNbO3 has a noncentrosymmetric group of Pmc21 at room temperature with a ferri-electric ordering of polarization. Such a ferri-electric state with small polarization can be changed into a ferroelectric state with very large polarization by a high electric field or by a chemical modification. The induced ferroelectric phase shows promising electromechanical response for applications in piezoelectric devices. In contrast, AgTaO3 is a quantum paraelectric, but ferroelectricity also can be induced through chemical substitution. The findings of good ferroelectric and piezoelectric performance in the silver perovskites are hoped to trigger further theoretical and experimental investigations on these systems.Comment: 30 pages, 41 figures; Ferroelectrics - Material Aspects, Mickael Lallart (Ed.), ISBN: 978-953-307-332-3, InTech, Rijeka, Croatia. 2011, pp.413-44

Red photoluminescence in praseodymium-doped titanate perovskite films epitaxially grown by pulsed laser deposition

Intense red photoluminescence (PL) under ultraviolet (UV) excitation was observed in epitaxially grown Pr-doped Ca0.6Sr0.4TiO3 perovskite films. The films were grown on SrTiO3 (100) substrates by pulsed laser deposition, and their epitaxial growth was confirmed by x-ray diffraction and reflected high-energy electron diffraction. The observed sharp PL peak centered at 610 nm was assigned to the transition of Pr3+ ions from the 1D2 state to the 3H4 state. The PL intensity was markedly enhanced by postannealing treatments at 1000 °C, above the film-growth temperature of 600 or 800 °C. Because the excitation and absorption spectra are similar to each other, it was suggested that the UV energy absorbed by the host lattice was transferred to the Pr ions, resulting in the red luminescence

Pb(Mg1/3Nb2/3)O3 (PMN) Relaxor: Dipole Glass or Nano-Domain Ferroelectric?

Combining our comprehensive investigations of polarization evolution, soft-mode by Raman scattering and microstructure by TEM, and the results reported in the literatures, we show that prototypical relaxor Pb(Mg1/3Nb2/3)O3 (PMN) is essentially ferroelectric for T<Tc~225 K. Its anomalous dielectric behavior over a broad temperature range results from the reorientation of domains in the crystal. A physic picture of the structure evolution in relaxor is also revealed. It is found that nanometric ferroelectric domains (gennerally called as polar nano-region (PNR)) interact cooperatively to form micrometric domain. Such multiscale inhomogeneities of domain structure in addition to the well-known inhomogeneities of chemical composition and local symmetry are considered to play a crucial role in producing the enigmatic phenomena in relaxor system.Comment: 16 pages, 10 figures; http://www.intechopen.com/books/advances-in-ferroelectrics/pb-mg1-3nb2-3-o3-pmn-relaxor-dipole-glass-or-nano-domain-ferroelectric