Pseudo Jahn-Teller effect in permittivity of ferroelectric perovskites

Abstract

Abstract. Microscopic origin of dielectric susceptibility and permittivity of cubic ferroelectric perovskites ABO 3 (BaTiO 3 , KNbO 3 , etc.) in their high-symmetry paraelectric phase is explored. Ferroelectricity in these crystals is due to the strong pseudo Jahn-Teller effect (PJTE) resulting in off-centre instability of the transition metal ion B. In addition to the noticed earlier big enhancement of the permittivity by the local PJTE, it is found that the latter induces also anisotropy, contrary to widespread expectation for isotropic polarization in cubic crystals. The temperature and field dependence of polarization, dielectric susceptibility, and permittivity, as well as their angular dependence, are revealed as resulting from the local PJTE instability by numerical diagonalization of the Hamiltonian including the vibronic coupling and temperatureaveraged interaction with the external electric field. IntroductionWe consider the dielectric susceptibility and permittivity of ferroelectric perovskites ABO 3 , like BaTiO 3 , KNbO 3 , etc., in which the polar instability of the elementary-cell octahedron [BO 6 ] is due to the pseudo Jah-Teller effect (PJTE) [1][2][3]. The adiabatic potential energy surface (APES) of this octahedron was shown to have eight equivalent wells situated along the four trigonal axes С 3 (figure 1), in which the atom B is off-center shifted making a local dipole moment, with potential barriers between the wells. At higher temperatures, in the paraelectric phase the local polar displacements in different centers have arbitrary (incoherent and dynamic) orientations that make the macroscopically averaged crystal seen as cubic. At a lower temperature T = T C, the ferroelectric phase transition results in orientation ordering of the local dipoles (partial or full) resulting in a polarized crystal, and the multi-valley APES explains the temperature-dependent formation of all the four ferroelectric phases [1].A detailed presentation of this vibronic coupling theory is given in a series of papers [1, 2] and books [3] including a recent more elaborate theoretical description based on the Green's functions approach [1]. Its main conclusions are confirmed in dozens of experimental observations forming a pattern (see one of them in [4] and many others cited in [1][2][3]). Recently the theory was expanded to also include the possibility of coexisting PJTE-induced ferroelectric and open-shell magnetic properties in perovskites ABO 3 , and a table was given listing the B atoms with electron configurations

The Jahn–Teller and Pseudo-Jahn–Teller Effects: A Unique and Only Source of Spontaneous Symmetry Breaking in Atomic Matter

“…A similar effect takes place under a local strain gradient leading to enhanced flexoelectricity [32]. The orientational polarization is confirmed experimentally as being by orders of magnitude stronger, than the displacive polarization, expected in the absence of JTE-induced SSB [32][33][34][35].…”

“…The study of such SSB-induced novel properties is far from being exhausted. In particular, quite recently, a novel effect is predicted, based on these studies: orientational polarization of solids [32][33][34][35]. The tunneling between the differently oriented dipolar distortions in the equivalent minima of the SSB makes the crystals with such centers similar to polar liquids (the possible existence of crystals with free rotating dipoles above the freezing temperatures was predicted by Debye in 1912, but not found until the present works [32][33][34][35]).…”

“…In particular, quite recently, a novel effect is predicted, based on these studies: orientational polarization of solids [32][33][34][35]. The tunneling between the differently oriented dipolar distortions in the equivalent minima of the SSB makes the crystals with such centers similar to polar liquids (the possible existence of crystals with free rotating dipoles above the freezing temperatures was predicted by Debye in 1912, but not found until the present works [32][33][34][35]). Under the influence of external electric fields, the equivalent minima of the APES become nonequivalent with all the dipole displacements frozen in the minima along the electric field, meaning orientational polarization, which results in strongly enhanced permittivity and electrostriction [33,34].…”

“…It was shown that such dipolar distortions may occur in centrosymmetric systems under the PJTE, and they lead to a variety of properties of crystals with such centers (e.g., BaTiO 3 ), confirmed experimentally [28,29]. The PJTE was shown to be the unique origin of the whole series of properties of such perovskite crystals, including ferroelectricity, multiferroicity, flexoelectricity, permittivity, and polar nanoregions [28][29][30][31][32][33][34][35].…”

“…For this reason, the cases of small SSB in polyatomic systems were ignored for a long time. However, in more recent works, it was shown that the SSB in systems with relatively small lifetimes τ in the minima wells of the multiminimum APES are of special interest, they became most challenging in view of the recently revealed novel properties in their cooperative interactions and interactions with external perturbations (see [27][28][29][30][31][32][33][34][35] and references therein).…”