Early in the study on the layered oxyfluoride perovskite, high temperature solid-state reaction, one of the easiest synthetic methods, was commonly employed. However, the variety of transition metals in the reported oxyfluoride compounds is quite limited, mainly due to the high chemical stability of the simple starting fluoride materials. To the best of our knowledge, the first example of the RP-type layered perovskite compound is K₂NbO₃F, which was reported in 1962 by Galasso and Darby [15]. As shown in Figure 2(a), the niobium oxyfluoride adopts the tetragonal structure in the space group of I4/mmm with octahedral coordination around Nb atom and O/F site disorder at the apical sites. The preferential occupation of the fluoride anion at the apical sites elongated the Nb-O/F bonds (2.0642 Å) along the c axis compared with those (1.9780 Å) in the ab plane. Subsequently, the same authors reported an isostructural iron oxyfluoride, Sr₂FeO₃F [16] and determined the crystal structure to be I4/mmm. However, Weller and his collaborators demonstrated that the exact crystal symmetry could be described as P4/nmm [17]. In contrast to K₂NbO₃F, the Fe counterpart possesses the O/F site order at the apical sites leading to a strong distortion of the FeO₅F octahedron (see Figure 2(b)). In fact, the bond length between Fe and O or F is 1.904 or 2.730 Å. Therefore, the iron metal center effectively takes a square pyramidal coordination against five O²⁻ anions. Weller et al. also extended the layered oxyfluoride system to Ba₂BO₃F (B = Sc and In) [18]. Both compounds exhibit preferential occupation by F⁻ anions at the apical sites, but the anion-site order/disorder in B = In/Sc.

In addition to the RP-type layered perovskite, Dion-Jacobson (DJ) and Aurivillius (AV) -type layered oxyfluoride phases are reported. The formulas of DJ and AV phases are expressed as AA’_n−1B_nO_3n+1 and (Bi₂O₂)(A’_n−1B_nO_3n+1), respectively. ASrNb₂O₆F (A = Li, Na, Rb) [19] (Figure 3) and RbLnTiNbO₆F (Ln = La, Pr, Nd) [20] were synthesized by conventional solid-state reaction. In contrast to the above RP phases, the F atoms prefer to occupy the equatorial and central apical anion sites in the double-layered perovskite block, not the terminal apical ones, because the covalency of the chemical bond between Nb and the terminal apical oxygen is incompatible with the ionicity of the Nb-F bond. On the other hand, Kobayashi et al. reported the reductive fluorination of DJ-RbLaNb₂O₇ and RP-NaYTiO₄ into RbLaNb₂O_7−xF_x and NaYTiO_4−xF_x using poly(vinylidene)fluoride (PVDF) [21]. PVDF or poly(tetrafluoroethylene) (PTFE) are effective fluorinating agents, utilized by Slater for the first time [11]. This fluorination proceeds in a topotactic manner; the framework of the precursor is maintained through the reaction. While ASrNb₂O₆F and RbLnTiNbO₆F with the non-magnetic B cations are insulating, RbLaNb₂O_7−xF_x possesses mixed valence states between Nb⁴⁺ and Nb⁵⁺ cations, which makes it electrically conductive.

For n = 2 RP-type layered perovskite, Ba₃In₂O₅F₂ [22] and Sr₃Fe₂O₆F_0.68 [17] are known. Common to both compounds, fluorine atoms occupy the terminal apical sites. In comparison with Ba₃In₂O₅F₂, synthesized by conventional high-temperature reaction, Sr₃Fe₂O₆F_0.68 is obtained by a low-temperature fluorination using F₂ gas. As shown in Figure 4, the precursor Sr₃Fe₂O₆ has an oxygen vacancy at the central apical site, but its oxidative fluorination causes local migration of the terminal apical site to the original oxygen vacant site. Because the intercalated F atoms occupy the terminal apical sites with O, the deviation of the O-Fe-O bond angle in the plane from the ideal 180° is only 7.6°, much smaller than the corresponding value of 15.28° in Ba₃In₂O₅F₂ with full fluorine occupation of the terminal apical sites.

Three kinds of AV-type layered oxyfluoride perovskites, Bi₂BO₅F (B = Nb, Ta) and Bi₂TiO₄F₂ (Figure 5), were synthesized by Aurivillius [23]. Later work presented the ferroelectric phase transitions at T_C = 303, 283, and 284 K, respectively, but the relationship between the crystal structure and the ferroelectricity is still controversial. Hydrothermal synthesis yields better sample quality than conventional solid-state reaction [24]. Bi₂TiO₄F₂ adopts the simple body-centered tetragonal structure in the space group I4/mmm [24] while Bi₂NbO₅F is proposed to adopt Pbca symmetry [25]. These centrosymmetric crystal structures contradict the requirement for the ferroelectricity. In fact, the reexamination of electrical properties in Bi₂NbO₅F demonstrated neither second-harmonic generation nor a permittivity anomaly associated with ferroelectric phase transition reported previously. It is believed from bond-valence-sum calculations that F atoms tend to occupy the equatorial sites for Bi₂TiO₄F₂ or the apical sites for Bi₂NbO₅F.

We would like to show a new class of anion ordered perovskite materials, KNaNbOF₅ and KNaMO₂F₄ (M = Mo⁶⁺, W⁶⁺) expressed as the general formula ABB’(O,F)₆ [12]. Poeppelmeier et al. successfully synthesized these three compounds by hydrothermal reaction. Layers of K⁺ cations and cation vacancies are alternately located in the A site along the c axis, and Na⁺ and B (Nb⁵⁺, Mo⁶⁺, W⁶⁺) cations are ordered in a rock salt configuration. More interestingly, fluoride anions are located in K⁺ cation layers while apical oxide anions are located in the adjacent A-site layer containing the A-site vacancies. Similar to DJ-ASrNb₂O₆F, the Nb⁵⁺ cation form short Nb=O bonds and one long Nb-F bond opposite the O²⁻ anion, leading to strong distortion of the Nb⁵⁺-centered octahedron. This type of O/F anion order has never been seen in any other oxyfluorides. It is apparent that the cation order in both A and B sites influences the O/F anion order.

There are a few examples that involve fluorine insertion into only interstitial sites between the perovskite block layers. The RP-type layered manganese oxides, LaSrMnO₄ (n = 1) and Ln_1.2Sr_1.8Mn₂O₇ (Ln = Pr, Nd, Sm, Eu, and Gd) (n = 2), accommodate fluorine in the (La/Sr)O rock-salt layers between the Mn-O perovskite blocks, followed by large expansion of the c-axis of 1 ~ 3 Å (see Figure 6). Utilization of F₂ gas, NH₄F, CuF₂ or PVDF as a fluorination agent at low temperatures yields LaSrMnO₄F₂ [26,27] and Ln_1.2Sr_1.8Mn₂O₇F₂ [26,28] where fluorine is inserted in each perovskite block. In addition, heating these fluorinated compounds with the corresponding precursors in appropriate ratios results in a staged intercalation structure, namely LaSrMnO₄F [27,29] and La_1.2Sr_1.8Mn₂O₇F [27] where F is inserted between alternate rock-salt layers. It should be noted that the F sites in the rock-salt layers are different between LaSrMnO₄F and LaSrMnO₄F₂, and La_1.2Sr_1.8Mn₂O₇F and Ln_1.2Sr_1.8Mn₂O₇F₂; the F atoms in LaSrMnO₄F and La_1.2Sr_1.8Mn₂O₇F are located in the interlayer space so as to bridge between La/Sr and the apical oxygen, while the F atoms in LaSrMnO₄F₂ and Ln_1.2Sr_1.8Mn₂O₇F₂ occupy tetrahedral sites of (La/Sr)₄ in the rock-salt layers. In spite of the valences of Mn cations close to 4+, no long-range magnetic order is observed down to 5 K.

The n = 2 RP phase Sr₃Ru₂O₇ is also fluorinated using CuF₂ to give the oxyfluoride Sr₃Ru₂O₇F₂ [30]. In the fluorinated phase, fluorine is inserted in the tetrahedral sites of Sr₄ in rock-salt layers between perovskite blocks (see Figure 7). The precursor crystallizes in the tetragonal structure with I4/mmm, but the fluorine insertion lowers the crystal symmetry to orthorhombic symmetry (Pbam), which is associated with rotation and tilting of the RuO₆ octahedra. The magnetic properties also change after the fluorination reaction, from the ferromagnetic state with T_C = 105 K to weak ferromagnetic state with T_N = 185 K.

Fluorination of the RP-Ba_2−xSr_xPdO₃ (0 ≤ x ≤ 1.5) by PVDF involves unusual oxygen displacement through anion exchange [31,32], which is different from the case of the fluorination of Sr₃Fe₂O₆ described above [17]. As shown in Figure 8, the structure of Ba_2−xSr_xPdO₃ comprises corner-linked chains of PdO₄ squares along the a-axis, similar to the one-dimensional structure in Sr₂CuO₃. Fluorination to Ba_2−xSr_xPdO₂F₂ involves structural conversion to T’-structure (isostructural with Nd₂CuO₄), namely rearrangement of the PdO₄ network from a 1-D chain to a 2-D plane. The remaining O²⁻ anions in the apical site move to the original vacant site and the two inserted F⁻ anions build fluorite block layer with Ba/Sr cations. Because the Pd²⁺ cation exhibits a strong preference for square lattice geometry, no additional fluorine insertion at the apical sites is allowed.

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 Greaves et al. and Wiley et al., independently [33,34,35], so we will present the following four examples in this paper, namely Sr₂CuO₂F_2+δ [8,36], Ba₂ZrO₃F₂∙xH₂O [37], Sr₂TiO₃F₂ [38], and nitride-fluoride Ce₂MnN₃F_2−δ [39].

The discovery of a superconducting copper oxyfluoride Sr₂CuO₂F_2+δ by Greaves et al. triggered the search for new oxyfluoride perovskite materials by a low-temperature route [8]. Other fluorinating agents, such as NH₄F and XeF₂, were shown to be useful in later work [9,10]. This compound was initially obtained by reaction of a 1-D structure Sr₂CuO₃ with F₂ gas. As observed in Ba_2−xSr_xPdO₂F₂ [31], the network of corner-sharing CuO₄ 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_C = 46 K at δ ~ 0.3. Interestingly, reduction of Sr₂CuO₂F_2+δ in a flowing H₂/N₂ gas atmosphere yields an insulating T’-structure Sr₂CuO₂F₂ [36]; fluorine removal at the apical sites and rearrangement of the remaining F⁻ anions form a fluorite Sr₂F₂ layer and square planar coordinated CuO₄.

Sr₂TiO₃F₂ or Ba₂ZrO₃F₂∙xH₂O can be prepared by the reaction of n = 1RP Sr₂TiO₄ or Ba₂ZrO₄ with NH₄F, CuF₂, ZnF₂ or PVDF [37,38]. In both cases, fluorination occurs by substitution of two F⁻ anions for one O²⁻ 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 Sr₂TiO₃F₂ occupy both the apical sites and interstitial sites in alternate rock-salt layers. Additionally, the TiO₅F octahedron is highly distorted, probably due to O/F site order at the apical sites. In contrast, Ba₂ZrO₃F₂∙xH₂O 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 Ce₂MnN₃, which is isostructural with Sr₂CuO₃, accommodates fluorine in a different way [39]. Unlike Sr₂CuO₂F_2+δ and Ba_2−xSr_xPdO₂F₂, 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 Sr₂TiO₃F₂ and the N/F site order results in deformation of the MnN₄F octahedron. Upon fluorination, the magnetic properties changes the Pauli paramagnetic behavior to a paramagnetic one.

As reviewed above, a variety of fluorinating agents greatly contributes to oxyfluoride chemistry. Considering the limited variety of transition metals, however, a further search for new oxyfluoride phase is necessary in order to better understand oxyfluoride. Co is among the 3d transition metals studied to a lesser extent. In fact, only one Co-based perovskite compound has been reported: LaSrCoFeO₅F [40], where O/F sites are randomly distributed as well as Fe/Co sites. High-pressure synthesis is an effective alternative approach to low-temperature fluorination, although expensive apparatus required for the reaction method is necessary. Recently, we have achieved the synthesis of the first example of RP-type layered cobalt oxyfluoride Sr₂CoO₃F, under a pressure of 6 GPa at 1,700 °C [41]. This compound adopts a simple body-centered tetragonal structure with the space group I4/mmm (Figure 2(c)). Magnetic susceptibility measurements revealed an antiferromagnetic phase transition at around 320 K, which is different from the ferromagnetic behavior in the corresponding oxide Sr₂CoO₄ [42]. Furthermore, neutron powder diffraction study characterized the S = 2 high-spin state at Co cations. The structural features in the cobalt oxyfluoride are also worthy of attention. The O/F anions are disordered at the apical sites, and the cobalt cation shifted from the basal plane takes a square pyramidal coordination. We noticed that the coordination environment around Co center is unusual when compared with related oxyfluoride compounds (see Figure 2). As described above, a similar O/F site disorder is observed in Ba₂ScO₃F and K₂NbO₃F, but each B cation with d⁰ electronic configuration takes octahedral coordination without shifting from the basal plane. In comparison, Sr₂FeO₃F and Ba₂InO₃F have a square-pyramidal coordinated metal center, but O/F site occupation occurs in an ordered manner. Thus, coexistence of anion disorder and square-pyramidal coordination, which has never seen in related oxyfluoride, is realized in the new cobalt oxyfluoride. It was initially assumed that square-pyramidal coordination stabilizes the O/F site ordered state, but this is not the case in Sr₂CoO₃F. Square-pyramidal coordination of Co cations is commonly seen in cobalt-based materials with d⁶ high spin configuration, such as BiCoO₃ [43] and Sr₂CoO₃Cl [44]. The observed O/F disorder, however, is non-trivial. The role played in the unusual coordination environment around Co is probably related to the reaction condition employed: high-pressure and high-temperature synthesis. A denser environment at high pressure or entropic effects associated with high temperature are likely to stabilize the anion-disordered phase.

Various fluorinating agents have been reported so far, for example, F₂ gas, NH₄F, XeF₂, CuF₂ and PVDF and PTFE [8,9,10,11,21]. Each fluorinating agent exhibits a distinct fluorinating power and reaction pathway. Thus, the fluorine contents in the resultant material depend on the choice, not only of the fluorinating agent, but also the anion lattice of the precursors. For example, the fluorine contents incorporated by XeF₂ in YBa₂Cu₃O_7−δ increase with decreasing the oxygen deficient contents [45]. The fluorination of perovskite SrFeO₃ using PVDF yields SrFeO₂F while the reaction of brownmillerite Sr₂Fe₂O₅ with F₂ gas results in the formation of two cubic phases different from SrFeO₂F [17,46].

On the other hand, as shown above, the n = 2 RP-type layered iron oxide Sr₃Fe₂O₆ was fluorinated with F₂ gas to give the oxyfluoride Sr₃Fe₂O₆O_0.87 [17]. This fluorination did not involve anion substitution. There are two approaches to extending the fluorine contents; a precursor with smaller oxygen concentrations is reacted with F₂ gas, or a fluorinating agent with higher reducing power is employed. According to such perspectives, we successfully synthesized a more highly fluorinated iron oxide, Sr₃Fe₂O_5.44F_1.56, by reaction of Sr₃Fe₂O_7−δ (δ~0.25) [47]. As is the case in Sr₃Fe₂O₆O_0.87, fluoride anions preferentially occupy the terminal apical anion sites with oxide anions, but the O/F sites are displaced from the ideal 4e (0, 0, z) to more general sites 16m (x, x, z). Moreover, fluorination with PTFE results in more significant expansion of the c-axis (21.406(2) Å) and the deviation of the O-Fe-O bond angle in the plane from 180° is 15.28° nearly close to that in Ba₃In₂O₅F₂, reflecting the increased fluorine content in Sr₃Fe₂O_5.44F_1.56. And, the antiferromagnetic phase transition temperature greatly differs from below r.t. to 390 K.

Interestingly, work that is more recent has demonstrated further extended fluorination. Slater et al. have successfully synthesized three more fluorinated phases, Sr₃Fe₂O_5.28F_1.72, Sr₃Fe₂O₄F₄, and Sr₃Fe₂O₃F₆, by changing the molar ratios of Sr₃Fe₂O_7−δ to PVDF [48]. While Sr₃Fe₂O_5.28F_1.72 has structural features similar to Sr₃Fe₂O_5.44F_1.56, fluorine atoms in Sr₃Fe₂O₄F₄ occupy half interstitial sites between rock-salt layers as well as terminal and central anion sites (see Figure 10). These two compounds exhibit antiferromagnetic order at r.t. In contrast, Sr₃Fe₂O₃F₆, which is assumed to correspond to complete filling of both apical sites and interstitial sites by fluorine, magnetically orders below r.t.