Defect structure of anion-excess fluorite-related Ca1−xYxF2+x solid solutions

On the basis of the transformation of a cube within a fluorite-type matrix into an archimedean antiprism of the kind found in numerous ordered anion-excess fluorite-related superstructures, a new polyhedral cluster, labeled 4:4:3, [according to B. T. M. Willis, Proc. Br. Ceram. Soc.1, 9 (1964) and A. K. Cheetham, B. E. F. Fender, and M. J. Cooper, J. Phys. C4, 3107 (1971)], is proposed to explain the defect structure and short-range order in Ca_1?xY_xF_2+x solid solution. In agreement with the spectroscopic, dielectric, and electric experiments, this new structural model fits perfectly well the measured occupation numbers for normal F and interstitial F′ and F″ fluorine atoms for the whole range of compositions without requiring the too short F′F′ distances generated by the previously proposed 2:2:2 or 3:4:2 clusters. Such 4:4:3 clusters and nearly identical 4:4:4 or 4:4:5 ones, are probably present in the other highly and moderately doped Ca_1?xLn_xF_2+x solid solutions; they could be precursors for the largest clusters, i.e., ordered microdomains, observed for samples annealed for a long time.     

Préparation et étude d'un oxysilicate de fer de valence mixte Fe7(SiO4)O6 (iscorite)

Un cristal de Fe₇SiO₁₀ a été isolé par J. Smuts, J. Steyn, et J. Boeyens (Acta Crystallogr. B25, 1251 (1969)), dans un réfractaire sidérurgique usagé, et appelé iscorite. Selon ces auteurs, la maille est monoclinique et la structure basée sur un empilement de blocs FeO et Fe₃SiO₆; mais le facteur d'accord est médiocre (R = 0,165) et les distances interatomiques très incorrectes. Nous avons donc préparé, pour la première fois, cette phase, et montré en combinant l'analyse chimique, la densité, l'étude aux rayons X du système FeSiO, et l'analyse d'un cristal à la microsonde, que la composition la plus correcte est Fe₇(Si_0.94Fe_0.06)O₁₀. L'étude d'un cristal confirme et précise la maille de Smuts et al.: monoclinique, avec a = 21,336(2), b = 3,0679(3), c = 5,8744(8)Å, β = 98°,06(1), $\text{P2}\text{m}$ ou $\text{P2}{}_1\text{m}$. Cependant les diagrammes de Weissenberg présentent des lignes continues de diffusion pour certaines strates qui indiquent un désordre microstructural intense. Il en va de même pour la diffraction des électrons. Il n'est donc pas étonnant que l'affinement, sur la base du modèle de Smuts et al., ait donnéR = 0,09 dans l'un ou l'autre groupe d'espace. Les mesures magnétiques conduisent à un paramagnétisme avec forte contribution orbitale pour Fe²⁺, et mise en ordre vers 250 K avec, en-dessous, un comportement de ferromagnétique faible. Contrairement à la wüstite, l'iscorite ne se dismute pas facilement; elle donne avec Fe₉PO₁₂ des solutions solides limitées.A crystal of Fe₇SiO₁₀ has been isolated by J. Smuts, J. Steyn, and J. Boeyens (Acta Crystallogr. B25, 1251 (1969)), from a furnace refractory brick, and has been called iscorite. According to these authors, the cell is monoclinic and the structure based upon a stacking of FeO and Fe₃SiO₆ blocks; however the reliability factor is poor (R = 0.165) and the interatomic calculated distances are not correct. Therefore, we have prepared this phase for the first time; combining chemical analysis, density measurements, X-ray investigation of the FeSiO system, and electron probe microanalysis of a crystal, we have shown that the correct composition is Fe₇(Si_0.94Fe_0.06)O₁₀. The crystal study confirms, with a higher accuracy, the parameters for the cell proposed by Smuts et al.: a = 21.336(2), b = 3.0679(3), c = 5.8744(8)Å, β = 98°.06(1), $\text{P2}\text{m}$ or $\text{P2}{}_1\text{m}$. However, the Weissenberg patterns display continuous diffusion lines, indicating an intense microstructural disorder, in agreement with the electron diffraction patterns. Hence it is not surprising to obtain, on the basis of the Smuts et al. model, a value R = 0.09 with both space groups. The magnetic measurements indicate a paramagnetism with strong orbital Fe²⁺ contribution and a transition at ～250 K to an ordered magnetic state, with a weak ferromagnetism below 250 K. In contrast to the case of wustite, iscorite does not easily disporportionate; it forms limited solid solutions with Fe₉PO₁₂.     

The structure of dehydrated Na zeolite A (SiAl = 1.09) by neutron profile refinement

Contrary to the recent work of J. M. Thomas et al. (J. Chem. Soc. Chem. Commun., 678, 1981), it was found that a high-resolution neutron powder diffraction pattern of a sample of dehydrated Na zeolite A (with $\text{Si}\text{Al}$ ratio 1.09) can be indexed and refined in the cubic groups $\text{Pm}\text{3}\text{m (R}{}_{\mathrm{<mtext>pw</mtext>}}\text{= 11.0\%)}$ (A. K. Cheetham and J. C. Taylor, J. Solid State Chem.21, 253, 1977) and $\text{Fm}\text{3}\text{c (R}{}_{\mathrm{<mtext>pw</mtext>}}\text{= 10.3\%)}$, whereas in the rhombohedral space group $\text{R}\text{3}$ refinement was unsuccessful. For two other samples having $\text{Si}\text{Al}$ ratios of 1.03 and 1.12 the diffraction patterns again showed no evidence of rhombohedral distortion and can be indexed using a cubic unit cell. It appears that the Si, Al ordering of zeolite A, and hence the crystal symmetry, must depend upon the conditions of preparation of the zeolite.     

The structure and oscillational motion of 57Fe atoms in interstitial sites in Al as determined from interference of Mössbauer γ radiation

The first excited site of the ⁵⁷Fe atom entrapped in an interstitial site in aluminum, as reported by W. Petry, G. Vogl, and W. Mansel (Phys. Rev. Lett.45, 1862 (1980)). from a Mössbauer spectroscopic study of a single crystal, is analyzed by consideration of the value of the Hooke's law constant of the FeAl bonds obtained from the values for elemental Fe and Al. The eight wavefunctions for the eightfold nearly degenerate excited state are described as 2s1p1d1f hybrids of three-dimensional harmonic oscillator wavefunctions relative to the center of the undistorted Al₆ octahedron or as localized 1s functions relative to the center of the distorted octahedron. These considerations provide a qualitative understanding of the observations on this system.     

Constituents of the root of Dictamnus albus L.

In addition to several known constituents and their artefacts, the root of Dictamnus albus L. has yielded the new natural products, isomaculosidine (9) and preskimmianine (15, R== Me).²¹     

The dependence of the lattice parameter and density of Zn1−xMnxTe on composition

The lattice parameter of the diluted magnetic semiconductor Zn_1?xMn_xTe has been determined as a function of composition. This material crystallizes in the zinc-blende structure for values of x below about 0.75. The results show that for this range of composition the lattice parameter satisfies Vegard's law and is given by a(x) = (6.103 + 0.237x) Å. This result corrects earlier published values of a(x) for this material (Juza et al., Z. anorg. allg. Chem.285, 61 (1956)), which are in considerable error. Because of its rather pronounced dependence on x, the lattice parameter provides an excellent method for determining sample composition. By extrapolating the expression of a(x) to x = 1, the results also provide a value of 6.340 ± 0.005 Å for the lattice parameter of the hypothetical zinc-blende phase of pure MnTe. The strong dependence of the lattice parameter of Zn_1?xMn_xTe on x is responsible for most of the variation of its density with composition.     

An independent investigation of the crystal structure of 1,8-dinitronaphthalene (orthorhombic form) at 22 and 97°C

The crystal and molecular structure of the orthorhombic form of 1,8-dinitronaphthalene, C₁₀H₆N₂O₄, has been reinvestigated at 22°C and examined at 97°C using single-crystal diffractometer data in order to throw some light on the structural aspects of the polymorphism of the title compound. The space group of the crystal is P2₁2₁2₁ with a = 11.375 (1), b = 14.974 (5), c = 5.388 (1) Å at 22°C and a = 11.475 (1), b = 15.002 (1), c = 5.425 (6) Å at 97°C. Z = 4 at both temperatures. The data, collected with CuK_α radiation, were refined by full-matrix least squares to R = 0.049 for 1087 reflexions (22°C) and R = 0.061 for 1078 reflexions (97°C). The structure was found to be in agreement with that determined by Z. A. Akopian, A. I. Kitaigorodskii, T. J. Struchkov (Zh. Strukt. Khim.6, 729 (1965)). Changes occurring on heating to a temperature of 97°C, which is close to the transition temperature (of c → pc type) 100–105°C, suggest that the possible mechanism of the phase transition is that of the displacive transformation of secondary coordination. Anistotropic temperature factors of the majority of atoms increased by ca. 50% with no particular direction of atom vibrations being distinguished.     

The equilibrium shapes of crystals and of cavities in crystals

Surface free energies are assumed to be the sum of the excess free energies of bonding of molecules in or near the surface, and the stable form of a crystal or cavity is assumed to be the form that makes the sum of these excess free energies a minimum. When only plane surfaces are allowed, this model predicts the same shapes for crystals as an equation of Wulff (2. Kristallogr. 34, 449 (1901)), which is based on the macroscopic thermodynamic relation of Gibbs (“The Collected Works, Vol. 1.: Thermodynamics,” Longmans, Green, New York (1931)). The model predicts rounding of edges and corners of kinds which are not allowed by the Wulff relation and predicts that spherical forms of particles and cavities can be stable despite anisotropic surface free energies. The model provides a useful framework for analysis of whether unstable crystal or cavity shapes will evolve into stable or metastable forms. Some crystals and cavities that have been assumed to have equilibrium shapes instead have metastable shapes.     

The structure of PR toxin,a mycotoxin from Penicillium roqueforti

An earlier report from our laboratories described the isolation, partial characterization, and biological activity of a new mycotoxin from cultures of Penicillium roqueforti, and the name “PR toxin” was tentatively assigned to the compound. On the basis of further chemical and spectral evidence we propose that PR toxin has the structure shown in formula 1.¹