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Showing papers in "Acta Crystallographica in 1966"




Journal ArticleDOI
TL;DR: In this article, a Neutronenleiter aus zwei entgegengesetzt gekrijmmten Teilstficken 'einfacher direkter Sichtweite' zusammengesett werden.
Abstract: Wie in Fig. 2 gezeigt, kann der Neutronenleiter aus zwei entgegengesetzt gekriJmmten Teilstficken 'einfacher direkter Sichtweite' zusammengesetzt werden. Durch die Richtungsanderung treffen Neutronen, die im ersten Abschnitt noch unter sehr kleinen Winkeln nur an der gekrfimmten Aussenfl~iche reflektiert wurden (girlandenreflektiert), im zweiten Abschnitt mit zu grossem Winkel auf und gelangen nicht mehr durch den Leiter. Die fiber den Querschnitt gemittelte Durchl~issigkeit far den doppelt gekrfimmten Neutronenleiter, zeigt ebenfalls Fig. 1. Fiir k,_> l/2k,, m ist die Durchl~issigkeit Null. W~ihlt man k~=0,8 k . . . . dann ist der Verlust gegenfiber einem geraden Neutronenleiter nur 30 Vo. Die in z-Richtung austretenden Neutronen haben am linken und rechten Rand die maximale Wellenzahl k, = k .... in der Mitte die maximale Wellenzahl k, = I/2k ..... Die Impulsraumbreite in der k=-Richtung ist ebenfalls in der Mitre des Strahls am gr6ssten. Der Neutronenstrahl ist beziiglich seiner Impulsraumverteilung symmetrisch zur Mitte des Strahls, im Gegensatz zum einfach gekrfimmten Neutronenleiter (siehe auch Maier-Liebnitz, 1965). Wie man aus Fig. 2 ersieht, gelangen schnelle Neutronen und ~,-Strahlen bestenfalls nach zweimaliger Streuung aus dem Neutronenleiter. Die Anordnung k6nnte symmetrisch verktirzt werden bis zu (1⁄2 + 1/1⁄4)Lu, jedoch ist dieser Gewinn nicht wesentlich und aus Abschirmungsgrfinden ist es auf jeden Fall zweckm~issig die Baul/inge gleich Lu zu belassen, auch gilt die Kurve in Fig. 1 exakt nur ftir L >_ LII. Ein Dimensionierungsbeispiel ffir 4/~ Neutronen m6ge den Sachverhalt noch veranschaulichen: Gewtinscht sei eine Strahlbreite yon 3 cm. Mit k~ = 0,8 k .... berechnet sich nach voriger Gleichung der Krtimmungsradius und damit nach Fig. 2 die L~inge zu 45 Meter. Der Divergenzwinkel am Ausgang des Neutronenleiters ist ffir beide Richtungen senkrecht zum Strahl 50 Winkelminuten bei 4 A Neutronen.

367 citations


Journal ArticleDOI
TL;DR: The structures of WTe2 and a high-temperature monoclinic polymorph of MoTe2 have been solved by Patterson methods as mentioned in this paper, where single crystals were grown by vapor transport methods.
Abstract: The structures of WTe2 and a high-temperature monoclinic polymorph of MoTe2 have been solved by Patterson methods Single crystals were grown by vapor transport methods Cell dimensions, as measured on precession photographs, are for WTe2, a=6282/~, b= 3496/~, c= 14"073 A Similarly for MoTe2, a=633 A, b=3469/~, c=1386/~, B=93°55 Intensities were measured for both crystals from zeroand first-level Weissenberg photographs WTe2 and MoTe2 are given the space groups Pnm21 and P21/m, respectively Minimum function maps prepared by a superposition method gave approximate trial structures for both compounds which were refined by least-squares methods to R values of 125 % and 13-9 % for WTe2 and MoTe2 respectively Both compounds are layer structures with double sheets of tellurium atoms bound together by interleaving metal atoms An off-center positioning of metal atoms in the tellurium octahedra buckles the tellurium sheets and allows metal atoms in adjacent octahedra to approach each other Each metal atom, therefore, has eight neighbors, six tellurium atoms and two metal atoms, and a significant amount of metal-metal bonding is introduced

326 citations


Journal ArticleDOI
TL;DR: In this article, a method involving inspection has been devised for deriving the angular values about an axis [hkI] in the cubic system which will lead to a coincidence-site lattice relationship.
Abstract: A method involving inspection has been devised for deriving the angular values about an axis [hkI] in the cubic system which will lead to a coincidence-site lattice relationship. The idea of a generating function is used here in an extension of the procedures evolved by Frank for [100] and [111] rotations and by Dunn for [110] rotations.

307 citations



Journal ArticleDOI
TL;DR: The crystal structure of L-alanine, C3H702N, has been determined and refined by analysis of complete three-dimensional diffraction data from copper X-radiation.
Abstract: The crystal structure of L-alanine, C3H702N, has been determined and refined by analysis of complete three-dimensional diffraction data from copper X-radiation. The crystals are orthorhombic with space group P212~21 ; the unit-cell dimensions are a = 6.032, b = 12.343, c = 5.784 A. Least-squares refinement of the positional parameters of all 13 atoms and of the individual anisotropic temperature factors of the six heavy atoms, using 581 observed reflections of non-zero weight, yielded a final R index of 0.049. The structure bears a striking resemblance to that Of DL-alanine, and involves the use of all available protons in N-H • • • O hydrogen bonds, with lengths of 2.83, 2.85, and 2.81 A,.

281 citations





Journal ArticleDOI
TL;DR: In this article, the existence of the tautomeric form (I) of pyridaz-3-thione not only in solution but in the solid state as well.
Abstract: to be 0.112 A shorter than the sum of the Pauling covalent radii for carbon and sulphur. This bond must therefore possess considerable double-bond character. Cucka (1963) also puts forward an argument for the existence of a similar ring system in that his calculations support the evidence for hydrogen atoms attached to C(2), C(3), C(4) and N(2). Thus it appears, from these two X-ray investigations and Hedgley's observations, that there is now sufficient evidence to confirm the existence of the tautomeric form (I) of pyridaz-3-thione not only in solution but in the solid state as well.




Journal ArticleDOI
TL;DR: In this article, the phase problem in X-ray crystal analysis has been studied, and the results show that it is NP-hard to find the optimal solution to the problem.
Abstract: BLAKE, D., CALVIN, G. & COATES, G. E. (1959). Proc. Chem. Soc. p. 396. CARTER, F. L. t~ HUGHES, E. W. (1957). Acta Cryst. 10, 801. COATES, G. E. & PARKIN, C. (1961). J. Inorg. Nucl. Chem. 22, 59. CHou, K. T. (1963). K'o-hsueh t'ung-pao, 8, 47. CRUICKSHANK, D. W. J., PILLING, D. E., BuJOSA, A., LOYELL, F. M. & TRUTER, M. R. (1961). Computing Methods and the Phase Problem in X-ray Crystal Analysis, p. 32. Oxford: Pergamon Press. HOARD, J. L. (1933). Z. Kristallogr. 84, 231. International Tables for X-ray Crystallography (1962). Vol. III, pp. 202, 211. Birmingham: Kynoch Press. KIMBALL, G. E. (1940). J. Chem. Phys. 8, 188. LIDE, D. R. & MANN, D. E. (1958). J. Chem. Phys. 29, 914. MATHEWS, F. S. & LIPSCOMB, W. N. (1959). J. Phys. Chem. 63, 845. NYBURG, S. C. & HILTON, J. (1959). Acta Cryst. 12, 116. PAULING, L. (1960). The Nature of the Chemical Bond. Ithaca: Cornell Univ. Press. ROLLETT, J. S. & SPARKS, R. A. (1960). Acta Cryst. 13, 273.


Journal ArticleDOI
TL;DR: In this paper, a rigorous treatment of the multiple scattering of x-rays in amorphous samples has been developed in connection with the x-ray studies of the structure of simple glasses.
Abstract: A rigorous treatment of the multiple scattering of x-rays in amorphous samples has been developed in connection with the x-ray studies of the structure of simple glasses. A paper concerning this research has been submitted for publication to Acta Crystallographica.








Journal ArticleDOI
TL;DR: In this article, the problem of phase determination with respect to the number of atoms in the asymmetric unit, the relative amount of negative scattering matter and the variance of the individual terms was investigated.
Abstract: accepted down to a cut-off value of 4.0, and below this value the le'lc would be eliminated if lel < 1.0 and accepted down to le '1c>2-8 if Icl >3.0. The numbers used for criteria (a), (b) and (c) with myoinositol are larger than those for napthalene because the number of atoms in the asymmetric unit is larger and the le'lc values are larger for myoinositol. The numbers used here in the application of the criteria are evidently somewhat arbitrary but should serve as a guide. It is not clear at this point how useful the introduction of additional statistical considerations would be for the present, and therefore the investigation of such matters is deferred. It might be pointed out however that further study along such lines would include considerations such as the number of atoms in the asymmetric unit, the relative amount of negative scattering matter, the number of terms contributing to a particular calculation and the variance of the individual terms. Procedures for phase determination are particularly dependent in their initial stages on relationships among the largest le'l values. Also, it is desirable to have at least ten phases among the larger le'l values per atom in the asymmetric unit for the computation of a Fourier series. Thus the success of a procedure for structure determination, based upon the calculations presented here, depends on how well the computed larger [e'lc are correlated with the larger le'lo. Aside from phase considerations, it is apparent that useful information should derive from comparing Patterson functions computed from the coefficients I~12 and I~'12 when both positive and negative scattering matter are present. Mr Stephen Brenner wrote the computing programs and carried out all the calculations. I am indebted to him for his very fine cooperation. The thought to investigate this problem arose from a conversation with Dr Carroll K.Johnson of the Oak Ridge National Laboratory.