## 2 Fluorine Occupation Patterns in Layered Oxylfluoride Perovskite
### Fluorine Occupation of Both the Terminal Apical Sites and the Interstitial Sites  
In most cases, low-temperature fluorination in layered perovskite structure proceeds by anion substitution at the apical sites and fluorine insertion in interstitial sites between perovskite blocks. This type of fluorination, especially in Cu oxides, has already been reviewed in detail by [PERSON] _et al._ and [PERSON] _et al._, independently [33; 34; 35], so we will present the following four examples in this paper, namely Sr\({}_{2}\)CuO\({}_{2}\)F\({}_{2+\delta}\)[8; 36], Ba\({}_{2}\)ZrO\({}_{3}\)F\({}_{2}\)\(x\)H\({}_{2}\)O [37], Sr\({}_{2}\)TiO\({}_{3}\)F\({}_{2}\)[38], and nitride-fluoride Ce\({}_{2}\)MnN\({}_{3}\)F\({}_{2-\delta}\)[39]. The discovery of a superconducting copper oxyfluoride Sr\({}_{2}\)CuO\({}_{2}\)F\({}_{2+\delta}\) by [PERSON] _et al._ triggered the search for new oxyfluoride perovskite materials by a low-temperature route [8]. Other fluorinating agents, such as NH\({}_{4}\)F and XeF\({}_{2}\), were shown to be useful in later work [9; 10]. This compound was initially obtained by reaction of a 1-D structure Sr\({}_{2}\)CuO\({}_{3}\) with F\({}_{2}\) gas. As observed in Ba\({}_{2-x}\)Sr\({}_{x}\)PdO\({}_{2}\)F\({}_{2}\)[31], the network of corner-sharing CuO\({}_{4}\) units drastically changes from a 1-D chain to a 2-D layer in the fluorination process (see Figure 8). However, fluorine anions partially occupy not only interstitial sites between the perovskite blocks but also the apical sites. The superconducting temperature takes a maximum value of \(T_{\rm C}\) = 46 K at \(\delta\)\(\sim\) 0.3. Interestingly, reduction of Sr\({}_{2}\)CuO\({}_{2}\)F\({}_{2+\delta}\) in a flowing H\({}_{2}\)/N\({}_{2}\) gas atmosphere yields an insulating \(T^{*}\)-structure Sr\({}_{2}\)CuO\({}_{2}\)F\({}_{2}\)[36]; fluorine removal at the apical sites and rearrangement of the remaining F\({}^{-}\) anions form a fluorite Sr\({}_{2}\)F\({}_{2}\) layer and square planar coordinated CuO\({}_{4}\). Sr\({}_{2}\)TiO\({}_{3}\)F\({}_{2}\) or Ba\({}_{2}\)ZrO\({}_{3}\)F\({}_{2}\)\(x\)H\({}_{2}\)O can be prepared by the reaction of \(n\) = 1 RP Sr\({}_{2}\)TiO\({}_{4}\) or Ba\({}_{2}\)ZrO\({}_{4}\) with NH\({}_{4}\)F, CuF\({}_{2}\), ZnF\({}_{2}\) or PVDF [37; 38]. In both cases, fluorination occurs by substitution of two F\({}^{-}\) anions for one O\({}^{2-}\) anion, but the fluorine insertion manner and coordination environment around the metal center are different between these two compounds (see Figure 9). Fluoride anions in Sr\({}_{2}\)TiO\({}_{3}\)F\({}_{2}\) occupy both the apical sites and interstitial sites in alternate rock-salt layers. Additionally, the TiO\({}_{5}\)F octahedron is highly distorted, probably due to O/F site order at the apical sites. In contrast, Ba\({}_{2}\)ZrO\({}_{3}\)F\({}_{2}\)\(x\)H\({}_{2}\)O  
Figure 8: **(a)**, **(b)**, **(c)** Fluorination of \(A_{2}\)BO\({}_{3}\) to \(A_{2}\)BO\({}_{2}\)F\({}_{2-\delta}\) (\(A\) = Sr, Ba; \(B\) = Cu, Pd). **(d)** Rearrangement of oxide and fluoride anions during fluorination reaction. possesses fluorine, located at the apical sites and in each rock-salt layer. The Zr metal center takes an octahedral coordination, with O/F anions being disordered at the apical sites. It is important to note that a RP-type layered manganese nitride Ce\({}_{2}\)MnN\({}_{3}\), which is isostructural with Sr\({}_{2}\)CuO\({}_{3}\), accommodates fluorine in a different way [39]. Unlike Sr\({}_{2}\)CuO\({}_{2}\)F\({}_{2+\delta}\) and Ba\({}_{2-x}\)Sr\({}_{x}\)PdO\({}_{2}\)F\({}_{2}\), fluorination involves local migration of the original apical oxygen to the equatorial anion vacant sites, but anion substitution or oxygen removal does not take place. Moreover, additional fluorine atoms are incorporated in alternate rock-salt layers. The resultant structure resembles that of Sr\({}_{2}\)TiO\({}_{3}\)F\({}_{2}\) and the N/F site order results in deformation of the MnN\({}_{4}\)F octahedron. Upon fluorination, the magnetic properties changes the Pauli paramagnetic behavior to a paramagnetic one.