Four modifications of the Jahn-Teller effects. The problem of observables: spin-orbit interaction, tunneling splitting, and orientational polarization of solids.

In a semi-review paper, we first show that Landau's fundamental idea of the origin of spontaneous symmetry breaking (SSB) in atomic matter due to electronic degeneracy, termed the Jahn-Teller effect (JTE) and further developed into the pseudo-JTE (PJTE), was appended recently with two more modifications, the hidden JTE (h-JTE) and hidden PJTE (h-PJTE). All four versions of JTEs are defined in the adiabatic approximation by their adiabatic potential energy surfaces (APES), which possess a common feature - the lack of a minimum in the high-symmetry configuration, thus confirming (and extending) the Landau idea of SSB. However, although serving as a qualitative indication of the SSB and consequent possible (virtual) properties of the system, the APES by themselves are not experimentally observable directly, and this important feature of JTEs is often ignored.

Jahn-Teller and Pseudo-Jahn-Teller Effects: From Particular Features to General Tools in Exploring Molecular and Solid State Properties.

In a generalization of the latest achievements in this field, and as a pattern of massive applications, we present here the Jahn-Teller effect (JTE) and pseudo-JTE (PJTE) as general tools in the study of physical and chemical phenomena related to structural properties of polyatomic systems. We show that the JTE and PJTE are no more specific features of particular (rare) systems (as it was assumed earlier), but virtual properties of all molecular and solid state formations. They occur as a result of vibronic coupling that compensates for the error (inadequacy) introduced in semi-classical definitions of polyatomic configurations by their high-symmetry nuclear positions, thus appending the basic understanding of related phenomena with a new dimension.

Sub-lattice of Jahn-Teller centers in hexaferrite crystal.

A novel type of sub-lattice of the Jahn-Teller (JT) centers was arranged in Ti-doped barium hexaferrite BaFeO. In the un-doped crystal all iron ions, sitting in five different crystallographic positions, are Fe in the high-spin configuration (S = 5/2) and have a non-degenerate ground state. We show that the electron-donor Ti substitution converts the ions to Fe predominantly in tetrahedral coordination, resulting in doubly-degenerate states subject to the [Formula: see text] problem of the JT effect.

Sudden polarization and zwitterion formation as a pseudo-Jahn-Teller effect: a new insight into the photochemistry of alkenes.

We show that the intermediates of photochemical reactions-sudden polarization and zwitterion formations-are consequences of the pseudo-Jahn-Teller effect (PJTE), which facilitates a better understanding, rationalization, prediction, and manipulation of the corresponding chemical and biological processes. This statement is illustrated on a series of alkenes, including ethylene, butadiene, diallyls, stilbene, and ring-halogenated stilbenes, but the formulated methodology and results are applicable to any photochemical reaction controlled by sudden polarization (and can be extended to other chemical reactions with short-lived intermediates). All these molecules have many polar modes, significantly complicating the PJTE multimode problem.

Geometry, Electronic Structure, and Pseudo Jahn-Teller Effect in Tetrasilacyclobutadiene Analogues.

We revealed the origin of the structural features of a series of tetrasilacyclobutadiene analogues based on a detailed study of their electronic structure and the pseudo Jahn-Teller effect (PJTE). Starting with the D4h symmetry of the Si4R4 system with a square four-membered silicon ring as a reference geometry, and employing ab initio calculations of energy profiles along lower-symmetry nuclear displacements in the ground and several excited states, we show that the ground-state boat-like and chair-like equilibrium configurations are produced by the PJT interaction with appropriate excited sates. For Si4F4 a full two-mode b1g-b2g adiabatic potential energy surface is calculated showing explicitly the way of transformation from the unstable D4h geometry to the two equilibrium C2h configurations via the D2h saddle point.