Our work reveals facile [PX4]3- anion rotation in superionic Na11Sn2PS12 and Na11Sn2PSe12, and greatly hindered [SbS4]3- rotational dynamics in their less conductive analogue, Na11Sn2SbS12. Along with introducing dynamic frustration in the energy landscape, the fluctuation caused by [PX4]3- anion rotation is firmly proved to couple to, and facilitate long range Na+-cation mobility, by transiently widening the bottlenecks for Na+-ion diffusion.
Significance and Impact
The combined analysis developed in our work resolves the role of the long-debated paddle-wheel mechanism, and is the first direct evidence that anion rotation significantly enhances cation migration in room temperature rotor phases. These findings deliver important insights into the fundamentals of ion transport in solid electrolytes, and may be particularly important to the design of divalent ion conductors where static anion frameworks rarely support good ion transport.
- Helmholtz free energy surfaces of the S/Se ligands of [PnX4] show that S/Se ligands of [PS4]3- and [PSe4]3- exhibit a very shallow free energy landscape and low rotation barrier of 0.12-0.24 eV, while [SbS4]3- shows a deeper free energy landscape and a higher barrier (0.6-0.86 eV) for the polyanion to rotate.
- The time-joint correlation analysis developed in our work confirms the dynamical coupling between anion rotation and cation migration, providing clear evidence of the synchronization of the velocity of the cations and polyanions, while showing that hindered rotor anions can retard cation migration.