PbFCl structure

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). 

PbFCl structure, 97-100 Pcierls distortion, 4, 92-102 Perturbation theory, 41-42, 108 Phonon modes, 94... 

The alkaline-earth salts MHX, where M = Ca, Sr, or Ba, and X = Cl, Br, or I, have been prepared by melting the hydride MH2 with the dihalide MX2 or by heating M + MX2 in a hydrogen atmosphere at 900°C. The compound CaHCl was formerly thought to be the sub-halide CaCl. All these compounds have the PbFCl structure (p. 408), but the H atoms were not directly located. Compounds described as MgHX (X = Cl, Br, I) have been shown to be physical mixtures of MgH2 with MgX2.( )... 

Compounds which crystallize with the BiOCl (PbFCl) structure include ... 

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). 

The most likely impurity is LaOI which has the PbFCl structure as do many of the other rare earth metal oxyiodides.3 The stronger 25% of the powder diffraction lines for LaOI (A), with intensities in parentheses, are 3.05(10), 2.92(8), 2.06(4), 1.72(5), 1.71(5). The powder pattern of Lal3 is not especially useful for establishing its purity unless all the lines from a high-resolution (Guinier) pattern are compared. 

The yield of trichloride is largely limited by recovery losses after sublimation, being >95% after one sublimation and 80% after four. The powder diffraction pattern of the trichloride as such is not a particularly useful measure of purity unless a high-resolution (Guinier) technique is used and all lines are measured. Examination of the X-ray diffraction patterns for impurities is better, especially for the oxychloride. Most of the MOC1 phases have the PbFCl structure, and their patterns are listed in the ASTM file.10 The strongest 25% of the lines for LaOCl (in A, intensities in parentheses) are 3.54(9), 2.914(8), 2.642(10), 2.060(4), 1.624(4). 

This compound is obtained by reacting thorium oxide with H2S or sulphur vapour above 1273 K. It was reported by Zachariasen [1949ZAC] to be tetragonal (PbFCl structure, - P4/nmm) with a = (3.962 + 0.002) and c = (6.746 + 0.004) A, isomorphous with heavier actinide oxysulphides. From single crystal data, Amoretti et al. [1979AMO/GIO] confirmed the space group attributed by [1949ZAC] and reported the lattice parameters as o = (3.973 + 0.004) and c = (6.773 + 0.008) A. 

The heat capacities of the non-magnetic oxychalcogenides ThOS(cr) and ThOSe(cr) and of the isomorphous antiferromagnetic UOS(cr) and UOSe(cr) were measured from ca. 5 to ca. 300 K. The samples were prepared by mixing stoichiometric amounts of the dioxides and disulphides and annealed in vacuo in silica tubes at 1273 K for three days. X-ray diffraction analysis of the thorium compounds showed the expected PbFCl structure. For ThOS, a weak extra line due to Th02 was detected, and for ThOSe, several non-identified very weak extra lines were foimd. For the adiabatic heat capacity... 

NbSiAs, NbGeAs, NbSiSb, NbGeSb, and TaSiAs have been prepared and shown to possess the tetragonal PbFCl structure. In the last-named of these compounds, each Ta atom is surrounded by a square antiprism of four Si atoms (Ta—Si = 267 pm) and four As atoms (Ta— As = 269 pm), with an additional As atom located above the AS4 face (Ta—As = 277 pm). ... 

The idea that the symmetry of the polysulfides is controlled by mutual geometrical conjugation of the separate R, 8 and (82) layers was presented by Podberezskaya et al. (1997). From this point of view, the structure is described by an alternation along the period, cotaSA in the parent PbFCl structure, layers of -(82) -R -8 -8 -R -(82) -, where each layer has tetragonal symmetry 4mm. The... 

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. 

Beck and Strobel (1985) found that the sulfide fluorides ErSF, YbSF, and LuSF transform from their usual j8-YSF structure to the PbFCl structure around 1.5, 2, and 3 GPa, respectively, at temperatures T 970)K, and with a volume decrease of the order of 16%. The pressure-induced structures were determined on the quenched samples. 

In the PbFCl structure the smallest and more electronegative anions form a planar square layer framed by the cations which themselves span a planar layer of tetrahedra around the anions. The PbO-type (PbF) layer is neutralized by a layer of the second anion. Thus, sheets ClnPb F2nPb Cl are superposed in such a way that each Cl is opposed to a cation of the adjacent sheet. Each cation thus acquires a neighborhood Pb—4F + 4C1+1C1. 

Fig. 128. The tetragonal PbFCl structure Transition from a three-dimensional lattice to a layer structure due to a variation of the anion Cl Br -> I and of the cation La Yb, respectively. Largest spheres Halogen atoms. The cations are stippled. The tetrahedral [OLn4/4] skeleton, common to the LaOX compounds, is emphasized below. For regular tetrahedra and spherical ions a = V8/3(rM3+ + ro2-), Zu = al2 2c and a 2r ...

It is remarkable that in the LnOCl series the PbFCl structure becomes unstable at ErOCl with respect to the SmSI structure. The O—O distances are equal to a/V2 and Cl—Cl = a within the same layer, but these contacts are not decisive since the LnOBr series exists down to LuOBr with distinctly smaller a values. 

The rhombohedral SmSI structure represents a trigonal analog of the tetragonal PbFCl structure. With respect to the latter structure, the cation coordination number is reduced from 8-9 to 7 ... 

The square antiprism of the PbFCl structure is replaced in the SmSI structure by a trigonal antiprism. Here, however, the smaller anions form a strongly corrugated layer or a double layer and the additional neighbor above one antiprism base belongs to the inner anion double layer. Thus it does not represent a possible bond that may or may not exist depending upon the ratio of the atomic radii. On... 

The orthorhombic FeOCl structure is a distorted version of the PbFCl structure. It contains a central sheet (FeO) analogous to the (PbF) sheet in PbFCl but as in the SmSI structure the central anion layer is puckered and, moreover, the Cl... 

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