Oxyhalides Oxybromides

Many lanthanide oxyhalides, MOCl (M=La—Ho), MoBr (M= lanthanides and Y) and MOI (M = La, Sm, Eu, Tm, Yb) crystallize in tetragonal PbFCl structure type [98, 137—141). However, the heavier lanthanide oxychlorides (Tm, Yb, Lu) do not crystallize in the PbFCl structure [137) probably because of the short chlorine-chlorine contacts. In the case of the oxybromides, this perturbing effect does not exist and tbe PbFCl structure is preserved through the lanthanide series and including YOBr [139). 

In these oxyhalide complexes, the inter-halide distances between layers are smaller than the sum of the ionic radii of the halides (Table 11). In the oxybromides and ox5dodide series the M—Br (next layer) and M—I (next layer) distances increase (Table 11) with increase in parameter c (Table 10) as these distances depend solely on the dimensions of the c-axis. In the oxychloride series the M—Cl (next layer) distance, however, show a decrease along the lanthanide series. So does the parameter c (Table 10). This is the reason why the oxychlorides of Tm, Yb and Lu cannot maintain the PbFCl type structure. 

Other oxyhalides, mostly oxychlorides and oxybromides, result from the controlled hydrolysis of the trihalides, and are of interest for two main reasons. First, they are quite unrelated to the oxyhalides of bismuth. Although both antimony and bismuth form compounds MOX the structures of the antimony compounds are quite different from those of the compounds BiOX, which have been described on p. 408. The more complex oxyhalides of Sb have no analogues among Bi compounds. Second, a feature of the published structures of the antimony oxyhalides is the coordination of Sb by either three or four O atoms. It should perhaps be remarked here that the investigation of the structures of these complex compounds is difficult, and the precise positions of the O atoms are by no means certain. However, it appears that a feature of these compounds is the formation of extended Sb—O systems, generally layers, interleaved with halogen... 

Several studies of the effect of pressure on the radial Bkq crystal field parameters have been reported. Lanthanide oxyhalides (REOX RE = La, Gd, Y X = Cl, Br) doped with Eu + have been extensively studied. The REOX lattices are iso-structural and incorporate Eu + in bonding sites with 4 symmetry. In the oxybromides, Eu " is coordinated to four oxygens and four bromides. The RE-0 bond lengths ( 2.3 - 2.4 A) are significantly shorter than the RE-Br bond lengths ( 3.2-3.3 A). A fifth, more distant bromide ( 3.5-3.9 A) is located on the C4 axis [191]. Eu + in the oxychlorides is coordinated similarly (RE-0 2.3-2.4 A, RE-Cl 3.0-3.2 A), but differs with respect to the position of the fifth chloride. The smaller size of the chlorides allows the fifth, axial chloride to approach more closely Eu " and enter the coordination sphere at a distance of 3.0 - 3.2 A [ 192]. 

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