Effect of fluoropolymer composition on topochemical synthesis of SrMnO3−δFγ oxyfluoride films

Abstract

We report the synthesis of SrMnO 3- F  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 accommodate [11]. Early synthesis efforts of transition metal oxyfluorides were carried out by solid-state reactions at around 1000°C [12]. In order to reduce the synthesis temperature, topochemical fluorination of metal oxide precursors with fluorine sources including F 2 , NH 4 F, MF 2 (M = Ba, Cu, Ni, Zn), and XeF 2 was utilized, significantly reducing the reaction temperature [5]. This fluorination method has been reported in producing powder samples of copper, titanium, iron, manganese and other metal oxyfluorides [3][4][5][6]13].An alternative approach to fluorination at low temperatures was introduced by Slater [14], who demonstrated that polyvinylidene difluoride (PVDF) can be used as a fluorine vapor source when decomposed in close proximity to metal oxide powders. Following the work of Slater, PVDF has been applied in fluorination of other metal oxide samples in powder form, especially in producing the perovskite-related oxyfluoride materials [9,[15][16][17]. The use of fluoropolymers can further reduce the fluorination temperature to 180°C for PVDF and 330°C for polytetrafluoroethylene (PTFE) [11], and mitigate the formation of secondary phases.Additionally, polymer-based fluorination has been applied to the synthesis of oxyfluoride thin films, carried out as post-growth reaction on as-grown films [18][19][20][21]. To date, these fluorination studies of thin films have all utilized PVDF as the fluorine source, while PTFE has been only investigated in producing bulk oxyfluorides [22]. To the best of our knowledge, polyvinyl fluoride (PVF) has not been reported as a fluorinating agent yet. Thus, there has been no systematic report of how the choice of fluoropolymer influences the fluorination reaction and resultant oxyfluoride films.We have synthesized epitaxial SrMnO 3- F  (SMOF) oxyfluoride thin films by fluorinating the as-grown SrMnO 2.5 (SMO) films with PVF, PVDF, and PTFE. We find that the use of PVDF and PVF results in more F incorporation than PTFE; however, the crystalline...

Strain-Induced Anion-Site Occupancy in Perovskite Oxyfluoride Films

“…33 The incorporation of F into SrMnO2.5-δFγ via the topochemical reaction used herein leads to a partial reduction of Mn, with previous x-ray absorption spectroscopy experiments revealing the presence of a mixed Mn 2+ /Mn 3+ valence. 19 Thus, the photon-energy dependence of the resonant linear dichroism spectrum shown in the near-EF line-cut plot indicates that the Mn 3d electrons near the Fermi level prefer to occupy states with an in-plane orbital character at the Mn 3+ sites and states with out-of-plane orbital character at the Mn 2+ sites.…”

On the Solubility and Stability of Polyvinylidene Fluoride

“…It was accepted that the principal mechanism for the degradation of PVDF involves the expulsion of HF from the PVDF chain, with a concomitant formation of C=C double bonds. PVDF has actually been used as a source of F in high-temperature reactions [251,252], and therefore, any recycling process that involves pyrolysis of the PVDF must take into account the fact that HF will be produced. The stability of the residue above 440°C is attributed to the C=C bonds along the backbone of the polymer chain.…”

High capacity manganese layered-perovskite cathode for fluoride ion batteries involving cationic and anionic redox reaction

“…In the pre-edge region of F K-edge spectra (Figure 5d and Supplementary Figure 15), a small peak attributed to metal-fluorine bond 51 ions to fill these sites. Note that similar molecular-like O2 state are formed in electrode materials of sodium ion battery during charge (i.e., Na-deintercalation) process 5 .…”

“…In the pre-edge region of F K-edge spectra (Figure 5d and Supplementary Figure 15), a small peak attributed to metal-fluorine bond 51 appears with the insertion of fluoride ions, and its intensity increases significantly when x > 2, indicating that the formation of O2 is accompanied by the formation of the Mn-F bond in the perovskite blocks of La1. 2Sr1.8Mn2O7-δFx.…”