Abstract
In a semi-review paper, we show that the local pseudo-Jahn–Teller effect (PJTE) in transition metal B ion center of ABO3 perovskite crystals, notably BaTiO3, is the basis of all their main properties. The vibronic coupling between the ground and excited electronic states of the local BO6 center results in dipolar distortions, leading to an eight-well adiabatic potential energy surface with local tunneling or over-the-barrier transitions between them. The intercenter interaction between these dipolar dynamic units results in the formation of the temperature-dependent three ferroelectric and one paraelectric phases with order–disorder phase transitions. The local PJTE dipolar distortion is subject to the presence of sufficiently close in energy local electronic states with opposite parity but the same spin multiplicity, thus limiting the electronic structure and spin of the B(dn) ions that can trigger ferroelectricity. This allowed us to formulate the necessary conditions for the transition metal perovskites to possess both ferroelectric and magnetic (multiferroic) properties simultaneously. It clarifies the role of spin in the spontaneous polarization. We also show that the interaction between the independently rotating dipoles in the paraelectric phase may lead to a self-assembly process resulting in polar nanoregions and relaxor properties. Exploring interactions of PJTE ferroelectrics with external perturbations, we revealed a completely novel property—orientational polarization in solids—a phenomenon first noticed by P. Debye in 1912 as a possibility, which was never found till now. The hindered rotation of the local dipole moments and their ordering along an external field is qualitatively similar to the behavior of polar molecules in liquids, thus adding a new dimension to the properties of solids—notably, the perovskite ferroelectrics. We estimated the contribution of the orientational polarization to the permittivity and flexoelectricity of perovskite crystals in different limiting conditions.
Highlights
There are innumerable publications devoted to the study of crystals with perovskite ABO3 structure, reflecting the rich variety of their properties with wide-ranging applications in physics, chemistry, and materials science
The Jahn–Teller effect (JTE) in degenerate states and the pseudo-Jahn–Teller effect (PJTE) in nondegenerate states are the only source of spontaneous symmetry breaking (SSB) in polyatomic systems [8,9,10], while degeneracy and pseudodegeneracy are the only source of SSB in the whole spectrum of transformations from elementary particles to nucleus, to atoms, molecules, crystals, and phase transitions [12]
Note that the treatment of the ferroelectric and superconductivity phenomena based on the local vibronic coupling significantly deviates from the traditional band theory of crystals, because the disordered local (JTE and PJTE) distortions violate the translational symmetry
Summary
There are innumerable publications devoted to the study of crystals with perovskite ABO3 structure, reflecting the rich variety of their properties with wide-ranging applications in physics, chemistry, and materials science. As illustrated, the crystal is centrosymmetric, and for some of the electronic configurations of the B ion, it has a nondegenerate ground state with a relatively low-lying excited state of opposite parity (but the same spin multiplicity), which mix under vibronic coupling to produce the PJTE dipolar distortions (note that the JTE does not produce dipolar distortions in centrosymmetric crystals) This B center dipolar instability plays a key role in all the main properties of the appropriate perovskites (in some perovskites, the A center may play a similar role, not fully explored so far). Note that the treatment of the ferroelectric and superconductivity phenomena based on the local vibronic coupling significantly deviates from the traditional band theory of crystals, because the disordered local (JTE and PJTE) distortions violate the translational symmetry
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