PbFCl

Lead II) chloride, PbCl2. Insoluble cold water, sparingly soluble hot. Matlockite is PbFCl. PbCL forms many complex chlorides and also basic chlorides. Cassel yellow (approx. PbCl2 7PbO) is prepared by fusion of the constituents and is used as a pigment. 

Fig. 17. Tetragonal symmetry projective view (above) and cut A-B (below) of a [LS],-layer formed by a sequence of [L,S] tetrahedrons (ideally PbFCl-t3q)e distorted CeSI-, FeOCl-, and NdSBr-types). (Redrawn from C. Dagron and F. Thevet, Ann. Ckim. 6, 67 (1971), Fig. 6, p. 77.)...

When three different kinds of spherical ions are present, their relative sizes are also an important factor that controls the stability of a structure. The PbFCl type is an example having anions packed with different densities according to their sizes. As shown in Fig. 7.5, the Cl- ions form a layer with a square pattern. On top of that there is a layer of F ions, also with a square pattern, but rotated through 45°. The F ions are situated above the edges of the squares of the Cl- layer (dotted line in Fig. 7.5). With this arrangement the F -F distances are smaller by a factor of 0.707 (= /2) than the CP-CP distances this matches the ionic radius ratio of rF-/rcl- = 0.73. An F layer contains twice as many ions as a CP layer. Every Pb2+ ion is located in an antiprism having as vertices four F and four... 

Numerous compounds adopt the PbFCl structure. These include, apart from fluoride chlorides, oxide halides MOX (M = Bi, lanthanoids, actinoids X = Cl, Br, I), hydride halides like CaHCl and many compounds with metallic properties like ZrSiS or NbSiAs. 

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). 

Table 10. Comparison of the lattice constants (A) for the PbFCl type lanthanide oxyhalides (137-141)...

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. 

Formally the phases HfSb2(h), ThAs2(h),. . . UP2,. . . belong to the Cu2Sb type too, since they crystallize in the antistructure. Chemically, however, they are binary PbFCl-type compounds. This is reflected also by the axial ratio which is near 2. Therefore, we have discussed these phases together with their ternary analogues. 

BkOCl (154), BkOBr (155), and BkOI (153,155) have been synthesized and found to exhibit the PbFCl-type tetragonal structure. Although presently unreported, BkOF certainly can be prepared and probably exhibits polymorphism. 

G. Neue Vertreter des ThCr2Si2-Typs und dessen Verwandtschaft zum Anti-PbFCl-Gitter. Z. Naturforsch. B 25,1350-52 (1970). 

One ubiquitous ternary structure is that of PbFCl (ZrSiS, BiOCl, Co2Sb, Fe2As).16,71 We ll call it MAB here because in the phases of interest to us the first element is often a transition metal and the other components, A and B, are often main group elements. Diagram 91 shows one view of this structure, 92 another. 

The transition metal oxyhalides occur with four main structure types (FeOCl, AlOCl, SmSI, and PbFCl) but only the FeOCl structure undergoes topotactic redox reactions. A layer of the orthorhombic FeOCl structure is shown in Figure 21. The structure is characterized by double sheets of distorted octahedra that share edges. Each iron atom is coordinated to four oxide anions and two chloride ions, with the two chloride ions in cis octahedral positions on the outside of the layer. The arrangement of octahedra within a layer is similar to that found in y-FeOOH and y-AlOOH. 

---