Critical exponents of ferroelectric transitions in modulated SrTiO_3:Consequences of quantum fluctuations and quenched disorder

The ferroelectric transitions of several SrTiO3-based ferroelectrics are investigated experimentally and theoretically, with special attention to the critical scaling exponents associated with the phase transitions, in order to understand the competition among quantum fluctuations （QFs）, quenched disorder, and ferroelectric ordering. Two representative systems with sufficiently strong QFs and quenched disorders in competition with the ferroelectric ordering are investigated. We start from non-stoichiometric SrTiO3（STO） with the Sr/Ti ratio deviating slightly from one, which is believed to maintain strong QFs. Then, we address Ba/Ca co-doped Sr1-x（Ca0.6389Ba0.3611）xTiO3（SCBT） with the averaged Sr-site ionic radius identical to the Sr2＋ ionic radius, which is believed to offer remarkable quenched disorder associated with the Sr-site ionic mismatch. The critical exponents associated with polarization P and dielectric susceptibility ε, respectively, as functions of temperature T close to the critical point Tc, are evaluated. It is revealed that both non-stoichiometric SrTiO3 and SCBT exhibit much bigger critical exponents than the Landau mean-field theory predictions. These critical exponents then decrease gradually with increasing doping level or deviation of Sr/Ti ratio from one. A transverse Ising model applicable to the Sr-site doped STO （e.g., Sr1-xCaxTiO3） at low level is used to explain the observed experimental data. It is suggested that the serious deviation of these critical exponents from the Landau theory predictions in these STO-based systems is ascribed to the significant QFs and quenched disorder by partially suppressing the long-range spatial correlation of electric dipoles around the transitions. The present work thus sheds light on our understanding of the critical behaviors of ferroelectric transitions in STO in the presence of quantum fluctuations and quenched disorder, whose effects have been demonstrated to be remarkable.

Paraelectric-ferroelectric interface dynamics induced by latent heat transfer and irreversibility of ferroelectric phase transitions

The temperature gradients that arise in the paraelectric-ferroelectric interface dynamics induced by the latent heat transfer are studied from the point of view that a ferroelectric phase transition is a stationary, thermal-electric coupled transport process. The local entropy production is derived for a ferroelectric phase transition system from the Gibbs equation. Three types of regions in the system are described well by using the Onsager relations and the principle of minimum entropy production. The theoretical results coincides with the experimental ones.

Shape-induced phase transition of vortex domain structures in ferroelectric nanodots and their controllability by electrical and mechanical loads

Shape-induced phase transition of vortex domain structures （VDSs） in BaTiO3 （BT） nanodots under open circuit boundary condition have been investigated using an effective Hamiltonian method. Our calculation indicates the tetragonal VDS missing in cubic BT nanodots can be induced by varying the shape of a nanodot from cube to platelet. Interestingly, a novel VDS is found in BT nanoplatelets in our simulations. Further investigation shows that it is a result of compromise between the ground state and the symmetry of the shape of the nanodot. Furthermore, based on the novel VDS, routes of controlling VDSs governed by homogeneous electric field and uniform stress are discussed. In particular, our results show the possibility of designing multi-states devices based on a single VDS. ~ 2017 The Authors. Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics.

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO2-terminated BaTiO3（001） surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3（001） surface.

Binary perovskite solid solutions and trends in their phase transitions temperatures variation

Some of the known binary ABO_(3)perovskites with ferro-(FE)or antiferroelectric(AFE)at T_(C)and different magnetic phase tran-sitions(PT):ferro-and antiferromagnetic at T_(N)and also some of their solid solutions are considered.Some correlations between their FE or AFE and/or magnetic PTs temperatures,on the one hand,and their interatomic bond A-O strains,on the other hand,have been constructed.It is shown that in the plotted diagrams these temperatures change with a change inδAO values as follows:classical FEs are followed by multiferroics,starting with BiFeO_(3)and ending with YVO_(3),followed by classical AFEs.At the same time,the temperatures T_(C)and T_(N)experience maxima at the corresponding points for BiFeO_(3),then quickly decrease,and the difference between them,T_(C)-T_(N),basically also decreases,slightly increasing along the way to the point EuTiO_(3).which made it possible to systematize these T_(C)and T_(N).

Phase formation,structure and dielectric properties of ceramics(Na_(0.5)Bi_(0.5))TiO_(3)-(K_(0.5)Na_(0.5))NbO_(3)-BiFeO_(3)

Influence of BiFeO_(3)(BF)on phase formation,unit cell parameters,microstructure,dielectric and ferroelectric properties of solid solutions close to the morphotropic phase boundary in the(Na_(0.5)Bi_(0.5))TiO_(3)–(K_(0.5)Na_(0.5))NbO_(3) system additionally modified by the low-melting KCl additives has been studied.The formation of pure perovskite structure samples decrease in the unit cell parameters and increase in the TC value stimulated by the BF addition have been revealed.It was proved that modification of compositions by small amounts of the BF and KCl additives leads to improvement of dielectric parameters.