Tetragonal PbFCl type

The FeOCl structure is met with in cations that prefer octahedral coordination. It can be approximated to some distortion of the PbFCl type. In the tetragonal PbFCl-type compounds, the corrugated M—O layers are replaced by planar M—F layers. The PbFCl structure is also found for oxyhalides (for example, in the LnOCl series from manthanum to erbium). 

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). 

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. 

Fig. 17, Tetragonal symmetry projective view (above) and cut A-B (below) of a [LSli-layer formed by a sequence of [L4S] tetrahedrons (ideally PbFCl-type distorted CeSI-, FeOCl-, and NdSBr-types). (Redrawn from C. Dagron and F. Thevet, Ann. Chim. 6, 67 (1971), Fig. 6, p. 77.)...

LaMnSi is tetragonal with the PbFCl-type P4/nmm, a = 4.177, c = 7.372 (Knigenko et al., 1977 X-ray powder analysis). For sample preparation, see Y-Mn-Si. Johnson (1974/76) confirmed the structure (a = 4.185, c = 7.433) and the ferromagnetic ordering temperatureof = 305 K. Sample preparation as for YMnSi. LaMnSi can also be obtained by solid state reactions from LaSi and Mn in sealed silica tubes lined with graphite (800 °C, 3d). 

According to X-ray powder diffraction data by Bodak et al. (1970) YbFeSi is tetragonal with the PbFCl-type of structure [P4/nmm, a = 3.972(3), c = 6.784(5)]. For sample preparation, see CeFeSi. 

The hydrotellurides (RH ,)Te, from La to Pr. These phases are hydrogen deficient. They are tetragonal, probably of the PbFCl-type (Carter, 1961). 

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