Lattice-gas model for alkali-metal fullerides: body-centered-cubic structure

TL;DR

This paper develops a Coulomb lattice-gas model with screening to describe ordering in alkali-metal fullerides with a body-centered-cubic structure, analyzing phase diagrams via mean-field approximations.

Contribution

It introduces a novel lattice-gas model incorporating screening effects and electronic energies to study ordering phenomena in alkali-metal fullerides.

Findings

Phase diagrams vary with shell radii.

Effective pair interactions are derived from screening assumptions.

Mean-field approximations reveal ordering transitions.

Abstract

A Coulomb lattice-gas model with a host lattice screening mechanism is adapted to describe the ordering phenomena in alkali-metal fullerides of body-centered-cubic structure. It is assumed that the electric charge of an alkali ion residing at a tetrahedral interstitial site is completely screened by its first-neighbor C$_{60}$ molecules. The electronic energy of the C$_{60}^{x-}$ ion is also taken into consideration as a charged spherical shell. By means of these assumptions an effective (short range) pair interaction is obtained between two alkali ions. The resultant lattice-gas model is analyzed by using two- and six-sublattice mean-field approximations. The thermodynamic properties are summarized in phase diagrams for different shell radii.