Anharmonic Lattice Dynamics in Sodium Ion Conductors

TL;DR

This study combines Raman spectroscopy and first-principles calculations to reveal anharmonic lattice dynamics in sodium ion conductors Na3PS4 and Na3PSe4, linking lattice behavior to their high ionic conductivity.

Contribution

It provides new insights into the anharmonic lattice dynamics and phase transition mechanisms in sodium ion conductors, highlighting the role of coupled lattice and ion dynamics.

Findings

Na3PSe4 exhibits displacive phase transition with soft modes disappearing in cubic phase.

Na3PS4 shows order-disorder transition with soft modes active in cubic phase.

Coupled lattice-ion dynamics are crucial for understanding high ionic conductivity.

Abstract

We employ THz-range temperature-dependent Raman spectroscopy and first-principles lattice-dynamical calculations to show that the undoped sodium ion conductors Na$_3$PS$_4$ and isostructural Na$_3$PSe$_4$ both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice -- Na$^+$ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na$_3$PSe$_4$ shows almost purely displacive character with the soft modes disappearing in the cubic phase as the change of symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na$_3$PS$_4$ instead shows order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining active in Raman in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice -- mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity in these Na$^+$ ion conductors.