Disordered Chloride Fluoride Phases

2 Disordered Chloride Fluoride Phases 11.7.2.1 Samples Prepared by Precipitation 

X-ray powder diffraction pattern of (a) unsubstituted BaCiF and (b) precipitated Bai fSn fCii+yFi y soiid soiution. The arrows show the iine shift relative to unsubstituted BaCIF (Reproduced from Ref. 51.) 

Elemental analysis showed that the precipitate has the formula Ba5.66SnCI7.52F6.03- Therefore, it is not BaCIF. It contains tin 15% of barium has been substituted by tin. In addition, in contrast with BaCIF, the CI/(CI -F F) ratio is not 0.5 the precipitate is enriched in chlorine, and it is poorer in fluorine. Other precipitates obtained at different Ba/(Ba-F Sn) ratios larger than 0.85 showed some significant variations in their chemical composition. The presence of tin and excess chlorine in the precipitated samples can be held responsible for the change of unit cell parameters. 

Since the same set of diffraction lines is present in both compounds, and particularly there are no lines at smaller angles and there is also no line splitting, there is no superstructure and no further lattice distortion. It is, therefore, clear that Sn and Ba are disordered on the same Wychoff site, and the excess Cl is disordered with F on the F site, while the Cl and F sites remain ordered (alternating layers parallel to (a, b)) (Fig. 11.3c), the latter being necessary in order for the structure to have the PbCIF type [73,74]. Because of its variable composition, the compound is a solid solution, and its chemical composition should be written Ba xSnxCI +yF y in order to show it is a substituted BaCIF. Since there is substitution on both the metal site and the F site, it is a doubly substituted solid solution. No such double substitution in the PbCIF structural type had been reported before. The decrease of 0 can be attributed to the Sn/Ba substitution since Sn  

TABLE 11.5 Unit Cell Parameters of BaCIFand Bao.85Sno.15CI1.nFo.89  

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