Dynamic disorder in Bi sub-lattice of $\delta$-Bi$_2$O$_3$

TL;DR

This study reveals that in $ extdelta$-Bi$_2$O$_3$, Bi ions oscillate within local minima maintaining monoclinic symmetry, despite the average cubic structure, impacting our understanding of its high ionic conductivity.

Contribution

The paper provides new insights into the dynamic disorder of Bi ions in $ extdelta$-Bi$_2$O$_3$, showing local monoclinic symmetry persists despite a cubic average structure.

Findings

Bi ions oscillate between local minima of the potential energy surface.

The local symmetry of Bi sub-lattice remains monoclinic up to melting.

Implications for ion conduction mechanisms in $ extdelta$-Bi$_2$O$_3$.

Abstract

$\delta$-Bi$_2$O$_3$ is one of the fastest known solid oxide ion conductors owing to its intrinsically defective fluorite-like structure with 25\% vacant sites in the O sub-lattice. Numerous diffraction measurements and molecular dynamics simulations indicate that the Bi ions construct a cubic, fcc lattice, and the O ions are %distributing and migrating through it. Nonetheless, in this study we present Raman scattering measurements that clearly show that the Bi sub-lattice preserves the monoclinic symmetry of the low temperature phase ($\alpha$-Bi$_2$O$_3$) up to the melting temperature of the crystal. The apparent contradiction between our observations and previous findings suggests that Bi ions oscillate between local minima of the effective potential energy surface. These minima represent the monoclinic phase while the time-averaged structure is the cubic phase. We discuss the implication of these low-frequency oscillations on ion conduction.