Showing papers in "Acta Crystallographica in 1950"

The crystal structure of hollandite, the related manganese oxide minerals, and α-MnO2

Abstract: Hollandite cryptomelane, coronadite and alpha-MnO 2 form an isostructural series of the general formula A 2-y B 8-z X 16 , A being large ions such as Ba 2+ , Pb 2+ , and K + , B small and medium-sized ions such as Mn 4+ , Fe 3+ , Mn 2+ , and X in the cases investigated O 2- and OH - ions. The unit cell, which contains one formula unit, is tetragonal or pseudotetragonal, in the latter case monoclinic. Approximate dimensions of the tetragonal unit cell are a=9.8 and c=2. 86 A. When deformed the short axis will be the b axis of the monoclinic cell. The deviation of beta from 90° is 0.5-1.5°, and the difference a-c is 0. 1-0.2 A. The space group of the tetragonal cell is C 4h 5 -I4/m and the atomic positions are (0, 0, 0; 1/2, 1/2, 1/2) + (2-y A in 2 (b): 0, 0, 1/2); (8-z B in 8 (h): x 1 , y 1 , 0; y 1 , x 1 , 0; y 2 , x 2 , 0); (8 X in 8 (h): x 2 , y 2 , 0; etc.); (8 X in 8 (h): x 3 , y 3 , 0, etc.). For a hollandite specimen the parameters were calculated to be x 1 =0.348, x 2 =0.153, x 3 =0.542, y 1 =0.167, y 2 =0.180, y 3 =0.167. The A ion is surrounded by eight oxygen ions at a distance of 2. 74 A forming a cube and at a greater distance (3.31 A) by four oxygen ions forming a square at the same z level as the A ion. The B ion is surrounded by six oxygen ions forming a octahedron, and with a mean distance of B-O=1.98 A. In the cases investigated, the variable y in the general formula varies from 0.8 and 1.3 and z from 0.1 to 0.5.

The probability distribution of X‐ray intensities. II. Experimental investigation and the X‐ray detection of centres of symmetry

Abstract: T h e p r o b a b i l i t y d i s t r i b u t i o n s o f r e f l e c t e d X r a y i n t e n s i t i e s h a v e b e e n i n v e s t i g a t e d e x p e r i m e n t a l l y . T h e r e s u l t s a r e i n a g r e e m e n t w i t h t h e t h e o r y , a n d t h e p o s s i b i l i t y is d e m o n s t r a t e d o f d i s t i n g u i s h i n g b e t w e e n c e n t r o s y m m e t r i c a n d n o n c e n t r o s y m m e t r i c s t r u c t u r e s a n d p r o j e c t i o n s w i t h i n t h e l l m l t a t i o n s o f t h e t h e o r y . I ) e t a f l s o f a c o n v e n i e n t d i s t i n g u i s h i n g t e s t a r e p r e s e n t e d a n d t h e e f f e c t s u p o n i t o f a b s o r p t i o n , e x t i n c t i o n a n d e x p e r i m e n t a l e r r o r s i n i n t e n s i t i e s a r e d i s c u s s e d .

X-ray diffraction study of cerous phosphate and related crystals. I. Hexagonal modification

Abstract: The phosphates of trivMent lanthanum, cerium, and neodymium have been found to be dimorphic. One phase is monoclinie, isomorphous with the mineral monazite; the other is a hexagonal structure of a new type which easily converts to the monoclinic form at moderately high temperatures. The hexagonal modification crystallizes with the symmetry of D~-C6~2. There are three molecules of XPOa in the unit hexagonal cell, where X may be La, Ce or Nd. The cell dimensions of the three crystals have been determined, and the complete structure has been deduced. In this structure X is co-ordinated to eight oxygen atoms--four at 2.34A. and four at 2-66A.--in such a manner as to leave open, oxygen-lined channels along the hexagonal axis. The presence of zeolitic water in these channels [XPO4(0-0.5H~O)] is probably necessary to stabilize the structure.

Precision measurements of lattice parameters of non‐cubic crystals

Abstract: It is shown that lattice parameters of non-cubic crystals can be accurately determined by linear extrapolation against the function 1⁄2(cos e 0/sin 0-b cos ~ 0/0) by selecting lines with suitable indices. Use can be made of the slope of the extrapolation curve of one of the parameters to derive the slope of the extrapolation curve of a second parameter. By this means, a true extrapolated value of the second parameter may be obtained when only a single appropriate reflexion is available. I t is also shown that the observed sin e 0 values can be accurately corrected for drift by means of an expression of the form D sin e 0(cos ~ 0/sin 0-b cos e 0/0). Examples of lattice-parameter determinations and drift corrections are given for cobalt and for NiaTi.

Raies de Debye–Scherrer et repartition des dimensions des domaines de Bragg dans les poudres polycristallines

Abstract: Introduction et plan Pour route raie Debye--Scherrer l'on peut d6finir un domaine angulaire moyen de diffusion, soit par la ' largeur moyenne' de la raie---distance angulaire entre les points oh l'intensit6 tombe ~ la moiti~ de sa valeur au maximum---soit par la largeur int~grale (1) (Laue, 1926)--quotient de l'intensit~ int~gr~e et de l'intensit~ au maximum. En se basant sur l'analogie des r6seaux optiques, Scherrer (Zsigmondy, 1920, p. 387) a vu, le premier, la possibilit6 de relier la largeur de raies Debye--Scherrer aux dimensions des eristallites, r6seaux ~ trois dimensions. Par la suite sa c61~bre formule (2) n 'a gubre subi de modification. Souvent elle se trouve affect6e d 'un coefficient num6rique, voisin de l'unit6, qui varie avec la d6finition ehoisie de ]a largeur de raie et avec les auteurs.

The crystal structure of decaborane, B10H14

Abstract: The crystal structure of decaborane has been determined from X-ray diffraction data. Crystals prepared by sublimation at room temperature, or above, show a high degree of polysynthetic twhming, giving rise to diffuseness of reflections for which h and k are odd. In the untwinned condition, the crystal is monoclinie, but pseudo-orthorhombie, and it is convenient to choose the twofold axis in the c direction. The space group is then Ull2/a-C~a. With a0=14-45, b0=20-88 , c 0 -5-68 A.,/? = 90"0 °, the cell contains 8 molecules of B10H14 and the calculated density is 0.96 g.cm. -3. The individual crystals that make up the actual, highly twinned crystalline edifice are so short in the b direction (a few unit translations) that it is convenient ~o consider the system as a partially ordered crystal, the disordered state of which is described by a unit cell one-quarter the size of the

Compounds of uranium with the transition metals of the first long period

Abstract: In the alloy systems of uranium with M_u, Fe, Co and l~i, the following compounds occur: U6Mn, U6Fe, UsCo and U61~i , all tetragonal and isostructural, and of a new structural type; UCo, a bodycentered cubic compoLmd, Z--8, with a unique structure; UNi, structure undetermined; UM_u~, UF% and UCo~, with the C-15 structure; UNi2, with the C-14 structure; UNi 5 with the PdB% structure, closely related to the G-15 structure. In addition, UCu 5 has a similar structure. Lattice constants and full structure determinations are reported for all compounds except UNi. For the latter, only powder data have been available, and the structure is complex.

The crystal and molecular structure of 1:4‐dimethoxybenzene

Abstract: l:4-Dimethoxybenzene crystallizes in the space group Pbca with four centro-syrmnetrical molecules per unit cell. A quantitative X-ray investigation, based on visual intensity measurements of about 170 reflexions from crystallographically different planes in the axial zones, has led to a complete determination of the structure, and the results have been refined by double Fourier projections on the planes (100) and (010). The co-ordinates of all the atoms are given. The methyl groups lie in the plane of the ring, and the following bond lengths have been found: C(1)-C(2) = 1.36 A., C(2)-C(3) = 1.37 A., C(3)-C(1') = 1.44 A. (all in the benzene ring) ; C(1)-O = 1.36 A.; O-C(CHa)= 1.35 A., all _+ 0.02 A. The valency angle of oxygen is 121 ___ 2 °. Some of these results are rather unexpected, and though some brief comments are made, detailed consideration is withheld pending their confirmation by the study of other similar compounds.

Unit‐cell dimensions of graphite

Abstract: GELLER, S. & HOARD, J. L. (1950). Acta Cryst. 3, 121. HENDE~SHOT, O. P. (1937). t~ev. Sci. Instrum. 8, 436. HE~ZBE~G, G., PATAT, F. & VE~LEGE~, H. (1937). J. Phys. Chem. 41, 123. LAUBE~GAYE~, A. W. & SEA2S, D. S. (1945). J. Amer. Chem. Soc. 57, 164. LEVY, H. & B/tOCKWA¥, L. O. (1937). J. Amer. Chem. Soc. 59, 2085. PAULIlVG, L. (1945). Nature of the Chemical Bond. Ithaca: Cornell University Press. PAVLINO, L. & HUCGINS, M. L. (1934). Z. Krystallogr. 487, 2O5. PAULINO, L., SPRINOATZ,, H. D. & Pn.LMER, K. J. (1939). J. Amer. Chem. Soc. 61, 927. SCEOMAXER, V. & STEVENSON, D. P. (1941). J. Amer. Chem. Soc. 63, 37.