Showing papers in "Zeitschrift Fur Kristallographie in 1986"

Four monazite type structures: comparison of SrCrO4, SrSeO4, PbCrO4 (crocoite), and PbSeO4

Abstract: For a crystal chemical comparison within the monazite type structure the crystal structures of SrCr04' SrSe04' PbCr04 (crocoite), and PbSe04 were refined [space group P21/n, Z == 4; a == 7.065(4),7.101(2), 7.127(2), 7.154(4) A, b == 7.375(4), 7.340(2), 7.438(2), 7.407(4) A; c == 6.741(4),6.874(2),6.799(2),6.954(4) A; [3 == 103.08(4), 103.48(2), 102.43(2), 103.14(4) 0]. As expected, in the MeOl0 polyhedra the average Sr 0 bonds are slightly shorter than the Pb-O bonds; further the average Me-O bonds are shorter in the chromates than in the two selenates. In SrSe04 and in PbSe04 the average Se0 bonds are equal within limits of error (1.640 and 1.642 A). On the contrary in SrCr04 the average Cr-O bond is 1.626 A, in PbCr04 however 1.663 A. The largest bond length distortions within the X04 tetrahedra were found in SrCr04 and PbSe04; the largest bond angle distortion occurs in PbCr04.

Refinement of the crystal structure of tetraphosphorus decaoxide

Abstract: Redetermination de la structure cristalline et des parametres de maille de P 4 O 10 . Groupe d'espace R3c. La geometrie de P 4 O 10 a l'etat cristallin est en accord avec une symetrie tetraedrique T d . Base amelioree de comparaison entre oxydes (III/V) du phosphore

The crystal structure of saleeite, Mg[UO2PO4]2 · 10H2O

Abstract: Crystal structure / MgIU02P04h . lOH20 Abstract. Saleeite, Mg (U02P04h . 10H20, is monoclinic, space group P2ifc, with a = 6.951(3), b = 19.947(8), e = 9.896(4) A, f3 = 135.17(2)°, Z = 2, U = 967.4A3 and De = 3.21 g/cm3, Fooo = 844, U = 161 cm-l, A = 0.7107 A (MoKcx),at 298 K. Least-squares refinement ofthe structure with 1324 single reflexions gave R = 0.053. The structure consists of U02P04 sheets, similar to those in threadgoldite, between which are located Mg2+ ions and water molecules.The Mg2+ ions are octahedrally coordinated to six of the 10 interlayer waters, but the last four water molecules are not bound to Mg. Saleeite is pseudotetragonal, the last few water molecules distorting it from true tetragonal symmetry. Previous assignments of saleeite as tetragonal appear to be incorrect; the other fully hydrated members of the autunite series may also have symmetry lower than the tetragonal symmetry assumed in the literature.

Enstatite, Mg2Si2O6: A neutron diffraction refinement of the crystal structure and a rigid-body analysis of the thermal vibration

Abstract: Synthetic enstatite, MgzSiz06, is orthorhombic, space group Pbca, with eight formula units per cell and lattice parameters a = 18.235(3), b = 8.818(1), c = 5.179(1) A at 230 C. A least-squares structure refinement based on 1790neutron intensity data convergedwith an agreement factor R(FZ) = 0.032, yielding Mg-O and Si-O bond lengths with standard deviationsof 0.0007 and 0.0008 A, respectively. The variations observed in theSi-O bond lengths within the silicate tetrahedra A and B are caused bythe differences in primary coordination of the oxygen atoms and the proximityof the magnesium ions to the silicon atoms. The latter effect is mostpronounced for the bridging bonds of tetrahedron A. The smallest 0- Si- 0 angle is the result of edge-sharing by the Mg(2) octahedron and theA tetrahedron. An analysis of rigid-body thermal vibrations of the two crystallographically independent (Si04J tetrahedra indicates considerable librationalmotion, leading to a thermal correction of apparent Si - 0 bond lengths as large as + 0.002 A at room temperature.

Copper(II) silicates and germanates with chain structures

Abstract: Investigations in the system CuO—CaO—MgO — Si02—Ge02 revealed the compounds CaCuGe206 and CuMgSi206. Both have clinopyroxene structure, but different cation distributions with M2 = Ca, Ml = Cu and M2 = Cu, Ml = Mg, respectively. CaCuGe206 has a polymorphic transition into a different pyroxene variety at 275 °C. No further Cu pyroxenes could be synthesized with other cations. In CuGe03, which is unique in having an einer single chain, up to 40% Ge could be substituted by Si, but the pure silicate was not obtained.

High-order thermal-motion tensor analyses of tridymite

Abstract: Two least-squares methods based on the Gram-Charlier expansion and the Edgeworth expansion were applied to a test structure to examine their usefulness in analysing multimodal density distribution of atoms. The Gram-Charlier expansion was more successful to reproduce the test structure than the Edgeworth expansion. The least-squares method based on the Gram-Charlier expansion was applied to the high temperature forms of tridymite to analyse density distribution of Ο atoms. The obtained density distribution of Ο was concave and nearly concentric at both 693 and 733 Κ (hexagonal), but clearly bimodal, at 493 Κ (orthorhombic), with the mean positions of low densities.

Die Kristallstrukturen von drei Modifikationen des Cu(SeO3)

Abstract: In the system CuO—Se02—H2O three new copper(II)-selemtes were synthesized under hydrothermal conditions. The crystal structures were determined by Single crystal X-ray techniques: Cu(Sc03)-II [a = 5.962(1) Ä, b = 7.010(1) Ä, c = 12.240(1) Ä, Space group Pcab, Z = 8, K = 0.020], Cu(Se03)-III [a = 7.725(1) Ä, b = 8.241(1) Ä, c = 8.502(1) k,ß = 99.16(2)°, space group P2Jn, Z = i, K = 0.046] and Cu(Se03)-IV [a = 4.736(2) Ä, b = 8.579(4) Ä, c = 10.618(5) Ä, a = 66.69(2)°, ß = 88.53(2)°, y = 88.40(2)°, space group PT, Z = 6, Ä» = 0.044]. All the Cu atoms are "square planar" four coordinated by oxygen atoms with Cu—O < 2.10 Ä. In addition four of the seven crystallographically diflerent Cu atoms have one distant O atom and one Cu atom has two distant O atoms. For two Cui'^^" atoms the fifth O atom has C u O < 2.22 Ä and the "square planar" coordination figure is strongly bent. In Cu(Se03)-IV one CuO^ square shares a common O—O edge with a SeOs group (O—O = 2.411 A). The previously known high temperature — high pressure Compound Cu(Se03)-I has an approximately 12% closer packed atomic arrangement.

Neutron diffraction study of the zeolite yugawaralite at 13 K

Abstract: The crystal structure ofyugawaralite, CaAl2Si6016 . 4H20 (Z = 2; Pc, a = 6.700(1), b = 13.972(2), c = 10.039(5) A, f3 = 111.07(2)°) at 13 K was refined from neutron diffraction data. The mean T - 0 distances in each tetrahedron (Si-O = 1.609-1.618 A; AI-O = 1.731-1.736 A) and the refinement of the neutron scattering lengths confirm complete Si, Al order in the framework. The Ca atom is coordinated to four frame- work oxygen atoms (Ca - 0 = 2.480 - 2.558 A) and to four water oxygens (Ca-Ow = 2.315-2.541 A). Three of the four water molecules show disorder with either multiple alternative oxygen or hydrogen sites, or both. Disorder of this type appears to be related to water molecules weakly bonded to framework oxygens, and is not necessarily related to Si/Al disorder in a zeolite framework. A water molecule in a partially occupied site (12% occupancy) was found to interact only via hydrogen bonding to the surrounding oxygen atoms and is not coordinated to the Ca atom. H. . . 0 contacts from 1.87 to 2.71 A indicate a wide range of hydrogen oxygen interactions.

The crystal structure of β-Mg2PO4OH, a synthetic hydroxyl analogue of wagnerite

Abstract: Affinement de la structure cristalline de β-Mh 2 PO 4 OH. Groupe d'espace P2 1 /c. Construction d'un reseau tridimensionnel complexe: incluant des tetraedres PO 4 reguliers, des octaedres cis-MgO 4 (OH) 2 deformes et des bipyramides trigonales MgO 4 (OH) deformees. Representation graphique des variations des parametres de maille dans la serie Mg 2 PO 4 F-Mg 2 PO 4 OH. Discussion des spectres IR

Polytypes and polytypism

Abstract: The definition of polytypism recommended by the IMA/IUCr joint committee on nomenclature is reviewed in the light of the structural variation observed in mineral and alloy systems. As a result of this review it is concluded that previous definitions of polytypism are too restrictive -to be practical, and a definition which emphasises the structural basis of polytypism is proposed: "Polytypism arises from the different modes of stacking of one or more structurally compatible modules." The various types of structural variation included under this definition are described using as examples minerals, ceramics, and alloys.

The crystal structures of Na2TiF6 and Na2SiF6

Abstract: Redetermination de la structure cristalline de Na 2 TiF 6 et de Na 2 SiF 6 . Groupe d'espace C2 a la place d'une symetrie quadratique. Mise en evidence de la coexistence de diverses phases pseudotrigonales a partir de mesures effectuees sur des cristaux mâcles et sur poudre

New family of silicate phases with the pollucite structure

Abstract: A new family of silicate phases, CszMSi5012: M = Be, Mg, Fe, Co, Ni, Zn, Cd; RbzMSi5012: M = Mg, Fe, (Co, Zn) has been prepared with the cubic pollucite structure, a in the range 13.3 to 13.8 A, space group Ia3d, Z = 8. The structure of one, RbzMgSi5012' has been refined to R = 10.8% using X-ray powder data. The structures are built of a 3D, (MSi50u)z- framework containing large, 12-coordinated cavities for the alkali cations. These structures are unusual in that a wide range of divalent cations have partially replaced Si4+ in the silicate anion framework.

High-temperature transformation of tridymite single crystals to cristobalite*

Abstract: Etude de la transformation reconstructive, a haute temperature, de la tridymite en cristobalite, en utilisant des cristaux de haute qualite et non macles. Mise en evidence de relations d'orientation entre la tridymite et la cristobalite formee. Preservation des unites structurales par la transformation, due aux relations structurales etroites existant entre tridymite et cristobalite

Differential geometry of a model membrane consisting of a lipid bilayer with a regular array of protein units

Abstract: Assuming a regulär packing of protein units in a lipid bilayer, corresponding to a model of the structure of biomembranes, the possibUity of formation of two-dimensional minimal surfaces is analysed. Such intrinsically curved lipid bilayers are able to accomodate varying amounts of lipid bilayers per unit front of membrane. This property is particularly interesting in order to explain the large variations of Upid materials involved in endoand exocytosis. A theory for transformations between such minimal surfaces, which can account for continuous variations of lipid bilayers per unit front, is presented. Intröduction One of the most intriguing problems of cell membranes is their ability of controlled transport process across the continuous lipid bilayer structure. Coordinated transports with an enormom capacity can be achieved by socalled coated vesicles, involving fusion of vesicles or budding ofT to form vesicles from lipid bilayer units. These processes mean that the amoimt of lipid bilayer per unit front of membrane must be able to vary within wide limits without disturbing various other membrane functions. * Permanent address: E>epartment of Applied Mathematics, Research Schoo] of Physical Sciences, Australian National University, Box 4, Canberra, 2601, Australia 238 S. T. Hydc, S. Andersson and K. Larsson In connection with work on lipid-water phases, which consist of lipid bilayers forming infinite periodic minimal surfaces, we have realized that a two-dimensional minimal surface structure of a lipid bilayer with a regulär array of protein units can exist. Such a structure possesses numerous interesting properties which merit consideration as a model for the structure of cell membranes. The present paper concems such a minimal surface structure and a theory of transformations is presented, which can account for variations in bilayer amounts during membrane transport. Differential geometry of bilayers Minimal surfaces may be described in various ways, but the most precise geometric description is that the siuface's average curvature is everywhere zero. Since the average curvature is the sum of the principal curvature at each point the surface is equally concave and convex. Thus the surface is (within the neighbourhood of each point) a Symmetrie saddle. (These surfaces derive their name from the fact that they are local minimal of the surface area for a fixed boundary.) By a judicious choice of the surface boundaries, we may continuously extend a minimal surface throughout Space to form an infinite periodic minimal surface (IPMS). Such surfaces have been found to be useful in describing liquid crystals [1] and many inorganic crystal structures [2], One such IMS is the P-surface [3], which is the minimal surface dividing Space into two equivalent interpenetrating cubic labyrinths. Similarly, the F-surface bisects space continuously into two equivalent diamond tunnel networks. Lipid bilayers forming two-dimensional minimal surfaces Cross-sections of these surfaces perpendicular to one of the tunnel directions, yield the quasi 2-dimensional (2 D) minimal surfaces, of \"cubic\" and \"hexagonal symmetry\" resp. we are most interested in here. Figures 1 and 2 are wire nets of the 2D minimal surfaces corresponding to the IPMS of Pand D-types. The 'holes' in the surface are assumed to be filled by protein units and the surface is the center of a lipid bilayer. We wish to investigate the effects of a differential geometric transformation in these 2-dimensional IPMS. Some knowledge of differential geometry is useful in describing this transformation. Periodic minimal surfaces cannot be analytically parametrised in euclidean 3-dimensional space, however they are simply described in terms of a polynomial, Rio) , embedded in a complex plane with coordinates (a, ix) (/• is the complex constant, where to = '= Wc f/(l + (ü2)/?(cü)

The shortest known interpolyhedral O - O distance in a silicate

Abstract: Presentation de distances O-O interpolyedres dans des structures cristallines inorganiques ayant des valeurs inferieures a 2,90 A. Mise en evidence de la plus courte distance O-O dans un silicate, la phenolite Be 2 SiO 4 qui mesure 2,75 A. Discussion de cette valeur d'un point de vue stereochimique

Splitpositionen für Alkalimetallkationen in den Thioindaten MIn5S8 (M = K, Rb, Cs)?

Abstract: The intensive yellow and transparent single crystals of the ternary thioindates MInsS8 (M = K, Rb, Cs) crystallize isotypically in the monoclinic space group C2/m (Z = 2). They are characterized by a three-dimensional network of InS6-octahedra forming parallel channels filled completely with Μ-ions. The thermal parameters of the latter ones indicate a peculiar pseudo-anisotropy which can be interpreted in terms of split-positions. Due to the decreasing size of the alkali metal cation the positional splitting of the Μ'''-ions however is increasing in the sequence CsInsS8, RbIn5S8, KIn5S8, thus making unsuccessful attempts to prepare NaIn5S8 understandable.

Ancient oriental pattern and related zeolite type networks

Abstract: Several zeolite structure types show the typical projection of ferrierite down the c-direction, a pattern that is also present in some examples of early-oriental art. Unbranched zweier, dreier and vierer single chains as well as selected complex silicate chains are used as building units in order to generate structure models of this kind. For the single chains the construction principle is based on an appropriate interpretation of the edges of the projection net as projected chains. A subset of higher symmetric models is analysed in more detail with the distance least-squares method. Introduction A number of tilings in ancient oriental art can be associated with projections of several zeolite structures. A typical example is the ferrierite projection down c, shown in Figure 1, which can be found in Indian art of the 17th century. There are also tilings of this kind in an 18th century decoration of the Masjid-i-Jami at Isfahan, Iran (El-Said and Parman, 1976), where beautiful rosettes decorate the 10-membered rings (Fig. 2). The zeolite structure types shown in Table 1 can be characterized by their common ferrierite projection. The aim of this paper is to give an idea of the diversity of possible structures with this projection. The construction principle of the model structures is based on the interpretation of the edges of the projection net as projected single chains and their Τ—Ο—Τ linkages to other chains, see Figure 3. The combination of chains with nets has been discussed extensively by Smith (1979, 1983). In order to limit the number of the various models we restrict ourselves to structures with lattice constants a and b as depicted in Figure 3 and mainly 238 R. Gramlich-Meier Fig. 1. Schematic picture of an Indian pattern from the Mausoleum of Γ timäd al-Dawla, Agra (El-Said and Parman, 1976) compared to the ferrierite projection down c. Fig. 2. Drawing of the ferrierite-like tiles from the Masjid-i-Jami, Isfahan, Iran (El-Said and Parman, 1976). to the simplest chain types in the direction perpendicular to the projection as shown in Figure 4. A similar selection has been used by Smith (1979). As mentioned by Liebau (1985) the description of silicate frameworks with chains as building units is only one of several different kinds of classification. However, in this case it is the simplest method of structure generation. Structure models with single chains If the structures are considered to be built up from identical chains perpendicular to the ferrierite projection, linked by oxygen atoms, each node of the projection can be associated with only one chain (cf. Fig. 3). ConsiderZeolite type networks 239 Table 1. Known zeolite structures with \"ferrierite projection\".