Cooperative excitations in superionic PbF2. (29th November 2021)

• Record Type: Journal Article
• Authors: Mohn, Chris E.; Krynski, Marcin; Kob, Walter; Allan, Neil L.
• Is Part Of: Philosophical transactions. Volume 379:Number 2211(2021)
• Publication Date: 2021-11-29
• Subjects: β-PbF2 -- superionc conductivity -- DFT -- collective diffusion -- molecular dynamics; Physical sciences -- Periodicals; Engineering -- Periodicals; Mathematics -- Periodicals; 500
• DOI: 10.1098/rsta.2019.0455
• Languages: English
• ISSNs: 1364-503X

Abstract

Abstract : Links between dynamical Frenkel defects and collective diffusion of fluorides in β -PbF2 are explored using Born–Oppenheimer molecular dynamics. The calculated self-diffusion coefficient and ionic conductivity are 3.2 × 10 −5 cm 2 s −1 and 2.4 Ω −1 cm −1 at 1000 K in excellent agreement with pulsed field gradient and conductivity measurements. The calculated ratio of the tracer-diffusion coefficient and the conductivity-diffusion coefficient (the Haven ratio) is slightly less than unity (about 0.85), which in previous work has been interpreted as providing evidence against collective 'multi-ion' diffusion. By contrast, our molecular dynamics simulations show that fluoride diffusion is highly collective. Analysis of different mechanisms shows a preference for direct collinear 'kick-out' chains where a fluoride enters an occupied tetrahedral hole/cavity and pushes the resident fluoride out of its cavity. Jumps into an occupied cavity leave behind a vacancy, thereby forming dynamic Frenkel defects which trigger a chain of migrating fluorides assisted by local relaxations of the lead ions to accommodate these chains. The calculated lifetime of the Frenkel defects and the collective chains is approximately 1 ps in good agreement with that found from neutron diffraction. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.