A Dynamically Induced Phase Transition in Na 4 P 2 S 6 Ultrafast Na + Mobility Triggering Rotor Phase Formation

TL;DR

This study shows that fast sodium ion movement in Na4P2S6 triggers a structural phase transition, leading to a rotor phase with high ion conductivity.

Contribution

The paper demonstrates that Na+ diffusion initiates a phase transition uncoupled from polyanion rotations in Na4P2S6.

Findings

Na+ diffusion precedes the transformation of the P2S6 framework to a rotor phase.

Rapid P2S6 motions occur in a molten Na+ substructure without significantly affecting Na+ hopping.

The study reveals a new perspective on phase transitions in fast-ion conductors.

Abstract

The physical properties
of any crystalline solid, such
as the irregular
movement of ions or atoms, are closely linked to its structure. Changes
in local structure or local defect chemistry are typically attributed
to changes in ion hopping. Conversely, one might also ask whether
fast ionic diffusion can cause structural changes, finally initiating
an overall phase transition. By using high-resolution 23Na and 31P nuclear magnetic resonance (NMR) carried out
at temperatures as high as 650 °C, we show that changes of the
local Na+ environment in the Na+-conducting
model compound Na4P2S6 indeed precede
the transition of the anionic framework from β-Na4P2S6 to the fast-conducting γ-phase.
While rapid 2D Na+ diffusion governs ionic conductivity
in the β-phase of Na4P2S6,
the high-temperature γ-phase has been theoretically predicted
and experimentally shown to be a rotor phase with high dynamics of
both the mobile Na+ cations and the anionic framework.
Here, we provide evidence that Na+ diffusion and the initial
transformation of the Na substructure precede the transition of the
P2S6 units to a rotating framework. NMR spectra
and relaxation times of both 23Na and 31P reveal
that rapid P2S6
4– motions
occur in a molten Na+ substructure, but these motions do
not influence Na+ hopping much. Hence, we suggest that
Na+ hopping while first initiating the transformation to
the rotor phase is indeed uncoupled from polyanion rotations at high
temperatures. Our study provides a new perspective on the details
governing phase transitions in fast-ion conductors and may lead to
a deeper understanding of these phenomena.

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