Electron–phonon interaction and Jahn–Teller effect in the SbSI atomic chain

Abstract

The quasi-one-dimensional SbSI atomic chain consisting of up to 60 atoms is considered theoretically. One-electron energies, interatomic bond strengths, and ionic charges of atoms have been calculated using the unrestricted Hartree–Fock method within the Hw basis set employing the pseudopotential. The symmetry of normal vibrational modes of the SbSI chain in the paraelectric and ferroelectric phases has been determined. As shown, the A u and B g symmetry top electronic levels of the highest valence band are degenerate in the paraelectric phase. The A ̄ u symmetry normal mode interacting with the degenerate A u symmetry electronic states in the valence band top induce the Jahn–Teller effect. The same mode interacting with A u symmetry electronic states in the valence band and B g symmetry that states in the conduction band bottom induces the pseudo-Jahn–Teller effect (JTE). Due to both JTE, the normal mode force constant K decreases. The polynomial expansion coefficients of the normal mode A ̄ u total energy dependence upon normal coordinate, E T= E T0+ K( z) 2+ c( z) 4, demonstrate anomalous variation in the phase transition region. The anomalies of E T0, of the harmonic coefficient K and anharmonicity factor c are caused by the anomalies of the unit cell volume V 0, and both the Jahn–Teller and pseudo-Jahn–Teller vibrational constants undergo changes during the phase transition.