## 3 Recent Results on New Layered Iron and Cobalt Oxylfluoride Compounds
### Highly Fluorinated Iron Oxides  
Various fluorinating agents have been reported so far, for example, F\({}_{2}\) gas, NH\({}_{4}\)F, XeF\({}_{2}\), CuF\({}_{2}\) 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 XeF\({}_{2}\) in YBa\({}_{2}\)Cu\({}_{3}\)O\({}_{7-\delta}\) increase with decreasing the oxygen deficient contents [45]. The fluorination of perovskite SrFeO\({}_{3}\) using PVDF yields SrFeO\({}_{2}\)F while the reaction of brownmillerite Sr\({}_{2}\)Fe\({}_{2}\)O\({}_{5}\) with F\({}_{2}\) gas results in the formation of two cubic phases different from SrFeO\({}_{2}\)F [17; 46].  
On the other hand, as shown above, the \(n=2\) RP-type layered iron oxide Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{6}\) was fluorinated with F\({}_{2}\) gas to give the oxyfluoride Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{6}\)O\({}_{0.87}\)[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 F\({}_{2}\) gas, or a fluorinating agent with higher reducing power is employed. According to such perspectives, we successfully synthesized a more highly fluorinated iron oxide, Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{5.44}\)F\({}_{1.56}\), by reaction of Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{7-\delta}\) (\(\delta\)-0.25) [47]. As is the case in Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{6}\)O\({}_{0.87}\), fluoride anions preferentially occupy the terminal apical anion sites with oxide anions, but the O/F sites are displaced from the ideal 4\(e\) (0, 0, \(z\)) to more general sites 16\(m\) (\(x,\,x,\,z\)). Moreover, fluorination with PTFE results in more significant expansion of the \(c\)-axis (21.406(2) A) and the deviation of the O-Fe-O bond angle in the plane from 180\({}^{\circ}\) is 15.28\({}^{\circ}\) nearly close to that in Ba\({}_{3}\)In\({}_{2}\)O\({}_{5}\)F\({}_{2}\), reflecting the increased fluorine content in Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{5.44}\)F\({}_{1.56}\). 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. [PERSON] _et al._ have successfully synthesized three more fluorinated phases, Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{5.28}\)F\({}_{1.72}\), Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{4}\)F\({}_{4}\), and Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{3}\)F\({}_{6}\), by changing the molar ratios of Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{7-\delta}\) to PVDF [48]. While Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{5.28}\)F\({}_{1.72}\) has structural features similar to Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{5.44}\)F\({}_{1.56}\), fluorine atoms in Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{4}\)F\({}_{4}\) 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, Sr\({}_{3}\)Fe\({}_{2}\)O\({}_{3}\)F\({}_{6}\), which is assumed to correspond to complete filling of both apical sites and interstitial sites by fluorine, magnetically orders below r.t.