Jahn-Teller effect in cubic fullerides A$_{3}$C$_{60}$

TL;DR

This study investigates how lattice vibrations influence the Jahn-Teller effect in cubic fullerides, revealing slight increases in stabilization energy and weak interactions between distortions at different sites, supporting independent JT rotations.

Contribution

The paper provides a detailed analysis of lattice effects on the Jahn-Teller distortions in cubic fullerides using DFT calculations and develops an expression for the adiabatic potential energy surface considering these interactions.

Findings

Jahn-Teller stabilization energy slightly increases in fullerides.

Warping of the potential energy trough occurs at a few meV.

Interactions between neighboring JT distortions are comparable in magnitude.

Abstract

Compared to isolated C$_{60}^{3-}$ ions, characterized by a threedimensional equipotential trough at the bottom of the lowest adiabatic potential energy surface (APES), the Jahn-Teller (JT) effect in cubic fullerides is additionally influenced by the interaction of JT distortions at C$_{60}$ sites with vibrational modes of the lattice. This leads to modification of JT stabilization energy and to the warping of the trough at each fullerene site, as well as to the interaction of JT distortions at different sites. Here we investigate these effects in three fcc fullerides with A=K,Rb,Cs and in Cs$_3$C$_{60}$ with bcc (A15) structure. DFT calculations of orbital vibronic coupling constants at C$_{60}$ sites and of phonon spectra have been done for fully ordered lattices (1 C$_{60}$/u.c.). Based on them the elastic response function for local JT distortions has been evaluated and the lowest APES investigated. To this end an expression for the latter in function of trough coordinates of all sites has been derived. The results show that the JT stabilization energy slightly increases compared to an isolated C$_{60}^{3-}$ and a warping of the trough of few meV occurs. The interaction of JT distortions on nearest- and next-nearest-neighbor fullerene sites is of similar order of magnitude. These effects arise first of all due to the interaction of C$_{60}$ sites with the displacements of neighbor alkali atoms and are more pronounced in fcc fullerides than in the A15 compound. The results of this study support the picture of weakly hindered independent rotations of JT deformations at C$_{60}$ sites in cubic A$_3$C$_{60}$.