PbFCl-type compounds

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. 

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). 

Marusin et al. (1985a, b) reported the existence of the scandium compounds ScFeC2, ScCoCj and ScNiC2, which belong to PbFCl-type or UCoC2-type (Li and Hoffmann 1989). The lattice parameters were determined to be a = 3.3344(7) A, c = 7.292(2) A and Z = 2 for SCC0C2. 

RXH. The hydride halides RXH of the divalent rare earth metals have been known for a long time. All of them, EuXH, YbXH with X = Cl, Br, 1, and SmBrH (Beck and Limmer 1982) crystallize in the PbFCl-type structure, which is also adopted by the hydride halides of the alkaline earth metals MXH (Ehrlich et al. 1956), by the mixed halides RXX of divalent lanthanides, and many oxyhalides ROX of the trivalent metals. The colorless compounds RXH of R = Sm, Eu, Yb therefore have to be addressed as normal salts. The hydrogen content of these compounds is strictly stoichiometric. 

According to X-ray powder data by Knigenko et al. (1977) the compound CeMnSi crystallizes with the PbFCl-type of structure with space group P4/nmm and lattice parameters a = 4.130, c = 7.279 for sample preparation, see YMnSi. 

NdFeSi was claimed to be single-phase with an orthorhombic TiNiSi-type of structure. The lattice parameters (a = 11.18, b = 6.89 and c = 5.32), however, do not correspond to a TiNiSi-type unit cell. The existence of a compound NdFeSi was confirmed by Bodak et al. (1970) but at variance with Mayer and Felner (1973) a PbFCl-type of structure [P4/nmm, a = 4.057(3), c = 6.893(5)] was obtained from arc-melted alloys heat treated at 800 ° C for 3 months (low-temperature phase ). Due to the high temperature of preparation and homogenization the TiNiSi-type phase as reported by Mayer and Felner (1973) is likely to represent a high-temperature modification. 

The crystal structure of the LaRuGe compound was investigated by Welter et al. (1993b). It was found to adopt the PbFCl-type structure, a = 0.4294, c = 0.7019 (X-ray powder diffraction). The compound was prepared from commercially available high purity elements. A compact of a stoichiometric mixture was heated in sealed under argon silica tube at 1173 K for preliminary homogenization and then melted in an induction furnace. 

If we compare a series of compounds with the same tetrahedron layer, such as ThNCl, ThNBr and ThNI, we notice that the lattice constant a remains practically constant, while the value of c increases and the layer character becomes more pronounced. It follows from Table 139 that the layer character becomes more pronounced in rare-earth series, LnOCl or LnOBr, on going from La to a heavier rare-earth atom. For MFX phases Beck [983] derived from electrostatic calculations that with smaller cations M as well as with larger anions X c/a will increase. The limiting phase with r(X) r(F) has c/a — 1.5 which roughly corresponds to the FeSi2 type. Symmetry reduction transforms the FeSi2 type to the PbFCl type with c/a = n/2. 

MSeF compounds are known for Y, La to Lu except Pm, Eu, and Tb. The compounds with large rare earth ions adopt crystal structures with cation coordination numbers of (8 + 1) or 9 (PbFCl type), and 9 (a-LaSeF). The smaller cations have coordination numbers from 6 to 8 (with a mean value 7), depending on whether the coordination by the anions is pure or mixed. 

Isothermal bulk modulus (GPa) and its pressure derivative K for ambient pressure phases of lanthanide and actinide compounds of the Fe2As and PbFCl structure types. 

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