PbFCl-type

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... 

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. 

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.)...

Nitrides and Phosphides. The passage of a high-frequency discharge through the mixture VCl4(g)-H2-N2 affords cubic VN0.95. The valence-band spectrum of VN has been analysed. Single crystals of VP2 have been obtained by chemical transport it has the NbAs type structure, whereas the high-pressure phase VP, 75 has a PbFCl type structure. ... 

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. 

Fig. 6. An-angement of R +, S- and (Sj) layers perpendicular to the c-direction of the PbFCl-type structure NdSj (2Pi/c), SmSi go (P42/n),...

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. 

GdMnSi crystallizes with the PbFCl-type of structure (P4/nmm, a = 4.02, c = 7.16) and orders ferromagnetically at — 295 K (Johnson, 1974/76). For sample preparation see YMnSi. GdMnSi can also be obtained by solid state reactions from mixtures of GdSi and Mn within an AI2O3 crucible and sealed in silica tubes (800°C, 2d). 

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. 

PrCoSi is isostructural with the PbFCl-type structure P4/nmm, a = 4.035(3), c = 6.934(5) (X-ray powder analysis by Bodak et al., 1970). For sample preparation and atom parameters, see CeFeSi. 

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