Symmetry Breaking in the Superionic Phase of Silver-Iodide

Hajibabaei, Amir; Baldwin, William J; Csányi, Gábor; Cox, Stephen J

Abstract

In the superionic phase of silver iodide, we observe a distorted tetragonal structure characterized by symmetry breaking in the cation distribution. This phase competes with the well known bcc phase, with a symmetric cation distribution at an energetic cost of only a few meV/atom. The small energy difference suggests that these competing structures may both be thermally accessible near the superionic transition temperature. We also find that the distribution of silver ions depends on the low-temperature parent polymorph, with memory persisting in the superionic phase on the nanosecond time scales accessible in our simulations. Furthermore, simulations on the order 100 ns reveal that even at temperatures where the bcc phase is stable, significant fluctuations toward the tetragonal lattice structure remain. Our results are consistent with many "anomalous" experimental observations and offer a molecular mechanism for the "memory effect" in silver iodide. Solid electrolytes offer great promise as materials for energy storage owing to their excellent ionic conductivity , with relatively high energy densities while remaining safe to use. But the atomic mechanisms that govern their behavior are far from simple [1]. For example, nanoscale diffusion in superionic conductors can substantially differ from the typical Brownian motion [2-8]. The archetypal type I solid electrolyte used as a model to understand these systems is silver iodide [9], yet many of its properties remain poorly understood [10]. At ambient conditions, AgI assumes hexagonal/cubic close-packed structures with many possible polymorphs resulting from variations in the stacking sequence. The main polymorphs with ordered stacking sequences are wurtzite (β) and zincblende (γ) [11-13]. Heated above 147 • C, a β/γ mixture transitions into the superionic α phase with a bcc I-framework. It is known, from nearly a century ago [14], that the α phase can retain a memory of its parent structure evident from the β/γ composition obtained upon cooling. This "memory effect" was systematically investigated more recently [15] by heating and cooling samples with well-controlled degrees of stacking disorder, which showed that the degree of persisting memory also depends on kinetic factors such as the cooling rate. As the β/γ → α transition occurs rapidly and with no remaining traces of the low temperature phases, mechanisms such as nucleation and crystallization are generally ruled out. Although a clear explanation for this memory effect has remained elusive, there is ample evidence to suggest that the picture painted by the notion of a straightforward β/γ → α phase transition is too simplistic [14-16]. For example, it has been proposed that Ag + ions preferentially occupy certain sites within the I-bcc framework , and that the degree of preference for certain sites depends upon the β/γ stacking composition of the low-temperature parent phase [16]. In this seminal study of the memory effect, Burley proposed that such preferential site occupation was responsible for sample-dependent variations in integrated intensities of the diffraction pattern in the α phase. Moreover, he also noted that any memory is irreversibly lost when temperatures exceed 170-175 • C. Experiments at higher temperatures further demonstrate AgI's complex phase behavior. For example, at approximately 427 • C, AgI undergoes a further order-disorder transition, which, in purely stoichiometric samples , exhibits an anomalous heat capacity [18, 19]. In a subsequent theoretical analysis, Perrott and Fletcher attribute this observation to entropic changes, of which the configurational entropy of Ag + plays a major role [20]. Early Raman spectroscopy experiments generally support this scenario [21-24]. More recent Ra-man polarization-orientation measurements [25] on single crystals of the α phase found crystal-like features that could not be accounted for solely by the bcc I-host lattice, nor by assuming a crystal-like average distribution of the mobile Ag +. Instead, this observation was attributed to strongly anharmonic I-lattice vibrations that are coupled to Ag + diffusion. Molecular simulations are in principle well-placed to provide insight at the microscopic level to help understand such experimental observations. Indeed, classical molecular dynamics (MD) simulations employing the empirical Parrinello-Rahman-Vashista (PRV) force field [26, 27] support the notion of Ag + preferentially occupying sites in the α phase [28-32]. Despite considerable constraints on the time and length scales that can be probed, insights from ab initio MD (AIMD) simulations [33] elucidate a dynamic bonding behavior that is challenging to capture with conventional empirical force fields. In particular, recent work has shown that iodide's lone pair electrons, represented by maximally localized Wannier centers, have a rotational motion that couples to diffusion of Ag + [34], in an analogous manner to the "paddle-wheel" mechanism associated with molecular su-perionic solids. To overcome the limits on accessible time and length

Citation

Hajibabaei, A., Baldwin, W. J., Csányi, G., & Cox, S. J. (in press). Symmetry Breaking in the Superionic Phase of Silver-Iodide. Physical Review Letters,

• Journal Article Type: Letter
• Acceptance Date: Nov 19, 2024
• Deposit Date: Dec 16, 2024
• Journal: Physical Review Letters
• Print ISSN: 0031-9007
• Electronic ISSN: 1079-7114
• Publisher: American Physical Society
• Peer Reviewed: Peer Reviewed
• Public URL: https://durham-repository.worktribe.com/output/3222432
• Publisher URL: https://journals.aps.org/prl/accepted/99072Ya2Gdb1e59bb1187be47295a7ad6de515fe8