Lattice dynamics of the cluster chain compounds M2Mo6Se6

TL;DR

This study investigates the lattice vibrations of M2Mo6Se6 compounds with various guest ions using experimental and computational methods, revealing insights into phonon behavior and superconductivity.

Contribution

It provides a comprehensive analysis combining experiments and ab-initio calculations to understand lattice dynamics and their relation to superconductivity in M2Mo6Se6.

Findings

Good agreement between calculations and experiments for phonon frequencies.

No evidence of rattling phonon modes in Tl2Mo6Se6.

Lower Tc in In2Mo6Se6 partly due to reduced phonon energies.

Abstract

The lattice dynamics of members of the M2Mo6Se6 family of materials with guest ions M = K, Rb, Cs, In, and Tl has been studied using inelastic x-ray scattering and Raman spectroscopy at room temperature, as well as by ab-initio calculations. We find a good match between calculations and experiment, both for structure factors (Eigenvectors) and for the calculated phonon frequencies. The observed lattice dynamics for Tl2Mo6Se6 show no signs of anharmonicity or absence of avoided dispersion crossings, thus ruling out previously hypothesised rattling phonon modes. The reduced mode energies for In2Mo6Se6 are identified as only partially responsible for the lower superconducting transition temperature Tc in this material when compared to Tl2Mo6Se6.