Effect of Fluoropolymer Composition on Topochemical Synthesis of SrMnO$_{3-{\delta}}$F$_{{\gamma}}$ Oxyfluoride Films

TL;DR

This study explores how different fluoropolymer sources influence the fluorination process of SrMnO$_{3-{eta}}$F$_{ ext{γ}}$ thin films, revealing insights into fluorine incorporation, lattice expansion, and oxidation state changes relevant for designing oxyfluoride materials.

Contribution

It demonstrates the effect of various fluoropolymers on fluorine incorporation and lattice modifications in SrMnO$_{3-{eta}}$F$_{ ext{γ}}$ films, providing new understanding of topochemical fluorination mechanisms.

Findings

Higher F concentrations with PVF and PVDF compared to PTFE.

Lattice expansion correlates with Mn-F bond elongation.

Fluorination reduces Mn oxidation state, indicating F substitutes on O sites.

Abstract

We report the synthesis of SrMnO$_{3-{\delta}}$F$_{{\gamma}}$ perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown SrMnO$_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene (PE) to isolate the role of carbon in the vapor transport process. The F distribution was probed by X-ray photoemission spectroscopy, which confirmed the incorporation of F into the films and revealed higher F concentrations in films exposed to PVF and PVDF compared to PTFE. The c-axis parameter expands after fluorination, a result consistent with density functional theory calculations that attribute the volume expansion to elongated Mn-F bonds compared to shorter Mn-O bonds. Using X-ray absorption spectroscopy, we show that the fluorination process reduces the nominal Mn oxidation state suggesting that F substitutes on O sites in the lattice as opposed to filling anion vacancy sites, a finding further supported by calculated formation energies of different F site occupancies. These results provide new insights into topochemical fluorination of perovskite oxides, which should enable future synthesis and design efforts focused on oxyfluoride heterostructures.