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...

We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe electron valence undergoes no observable spectroscopic change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ∼5-10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.

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