## 1 Introduction  
Since the discovery of a high-\(T_{\rm C}\) superconducting cuprate by [PERSON] and [PERSON] [1], there has been considerable progress in the field of solid-state chemistry and physics. In particular, we have deepened the understanding of metal oxides, while making improvements and development of experimental techniques and theories. It is well known that a perovskite structure formulated as _AB_O\({}_{3}\) (\(\lambda\) = large \(s\)-, \(d\)-, or \(f\)-block cation; \(B\) = smaller transition metal cation) has rich variety in structural, electronic and magnetic properties, ranging from superconductivity, through ferroelectricity, to photocatalytic activity. Figure 1(a) shows the ideal perovskite structure in which \(A\) cation occupies an interstitial site of an eight corner-sharing _B_O\({}_{6}\) octahedra. This type of structure can be extended to the layered perovskite intergrowth system termed the Ruddlesden-Popper (RP) phase, \(A\)2_A_'\({}_{n-1}\)_B_\({}_{n}\)O\({}_{3n+1}\) (\(n\) = the number of perovskite block), the structures with \(n\) = 1 and 2 which are depicted in Figure 1(b) and (c). Each perovskite block is intervened with double rock-salt _A_O layers. Thanks to the ability of the \(A\) and \(B\) sites to adopt various metal cations, we are able to finely control the chemical compositions as well as the physical properties, as exemplified by magnetoresistive manganite [2; 3; 4] and superconducting cuprate [5; 6]. While the majority of studies have concentrated on the influence of cation substitution, little effort has been made to control the structural and physical properties by manipulating the anion lattices. Considering that the anion strongly affects the crystal field and electronic state of the metal center, we can expect that substitution of anions with different bonding nature, valence state or ionic radius from oxygen in a metal oxide can enhance the original physical properties or induce new exotic phenomena. In fact, LaTiO\({}_{2}\)N [7] and Sr\({}_{2}\)CuO\({}_{2}\)F\({}_{2+\delta}\)[8] exhibit visible-light photocatalytic activity and superconductivity, respectively.  
In general, it is very difficult to stabilize two kinds of hetero anions in one structure using conventional solid-state reactions, in comparison with compounds with a single anion, such as an oxide, sulfide or nitride. As for the oxyfluoride compounds, several synthetic approaches are employed to overcome the problem, namely a low-temperature reaction using fluor  
Figure 1: Structure of the ideal perovskite structure, _A_\({}_{n-1}\)_B_\({}_{n}\)O\({}_{3n+1}\) (\(n\) = 1, 2, \(\infty\)).  
hydrothermal reaction [12] or high-pressure synthesis [13, 14]. Interestingly, O/F site order and/or F contents inserted, which are closely correlated with the structural and physical properties, depend on the reaction method used. In particular, layered perovskite structures exhibit three types of anion distribution patterns; (i) regular or random anion occupation pattern in the perovskite blocks, (ii) fluorine insertion into only interstitial sites between the perovskite blocks, or (iii) fluorine occupation of both the terminal apical sites and the interstitial sites. In this paper, we first review recent progress in the oxyfluoride chemistry of perovskite-based compounds, then present a synthesis strategy to further extend layered perovskite systems.