Showing papers in "Zeitschrift Fur Kristallographie in 2002"
TL;DR: In this article, the authors proposed a new sphere of influence algorithm combined with a fuzzy solvent boundary to provide a simple, fast and robust route from substructure sites found by the program SHELXE to an initial electron density map, with an indication as to which heavy-atom enantiomorph is correct.
Abstract: SHELXE was designed to provide a simple, fast and robust route from substructure sites found by the program SHELXD to an initial electron density map, if possible with an indication as to which heavy-atom enantiomorph is correct. This should be understood as a small contribution to high-throughput structural genomics. The new sphere of influence algorithm combined with a fuzzy solvent boundary enables some chemical knowledge to be incorporated into the density modification in a general and effective manner. In the special cases of high solvent content (greater than 0.6) or very high resolution data (better than 1.5 A) high quality maps can be produced. This raises the possibility of a new paradigm for atomic resolution structure refinement: instead of alternating atom parameter refinement with weighted electron density maps calculated with the phases of the current model, which inevitably leads to some model bias, all model building should be based on the model free experimental density map.
396 citations
TL;DR: In this paper, a tentative method of the analysis of powder diffraction patterns of nanocrystals is introduced which is based on the examination of the variation of lattice parameters calculated from individual Bragg lines (named the "apparent lattice parameter", alp).
Abstract: Fundamental limitations, with respect to nanocrystalline materials, of the traditional elaboration of powder diffraction data like the Rietveld method are discussed. A tentative method of the analysis of powder diffraction patterns of nanocrystals is introduced which is based on the examination of the variation of lattice parameters calculated from individual Bragg lines (named the "apparent lattice parameter", alp). We examine the application of our methodology using theoretical diffraction patterns computed for models of nanocrystals with a perfect crystal lattice and for grains with a two-phase, core-shell structure. We use the method for the analysis of X-ray and neutron experimental diffraction data of nanocrystalline diamond powders of 4, 6 and 12 nm in diameter. The effects of an internal pressure and strain at the grain surface is discussed. This is based on the dependence of the alp values oil the diffraction vector Q and on the PDF analysis. It is shown, that the experimental results support well the concept of the two-phase structure of nanocrystalline diamond.
90 citations
TL;DR: In this article, the crystal structures of the title compounds and related normal tetrahedral structures are discussed with respect to the preference of either hexagonal or cubic packing of the anions.
Abstract: The crystal structures of yellow Cu 3 PS 4 and of black Cu 3 SbS 4 were refined from single crystal X-ray diffraction data. Cu 3 PS 4 crystallizes orthorhombic in an ordered wurtzite superstructure type with the space group Pmn2 1 (no. 31), a = 7.282(1) A, b = 6.339(1) A, c = 6.075(1) A, V = 280.38(8) A 3 , and Z = 2. The refinement converged to R = 0.0276, and wR2 = 0.0710 for 737 unique reflections and 44 parameters. Cu 3 SbS 4 crystallizes tetragonal in an ordered sphalerite superstructure type with the space group I42m (no. 121), a= 5.391(1) A, c = 10.764(1) A, V = 312.83(9) A 3 , and Z = 2. The refinement converged to R = 0.0213, and wR2 = 0.0532 for 492 unique reflections and 14 parameters. The crystal structures of the title compounds and related normal tetrahedral structures are discussed with respect to the preference of either hexagonal or cubic packing of the anions.
68 citations
TL;DR: The crystal structure of krohnkite [Na 2 Cu(SO 4 ) 2.2H 2 O], which contains infinite chains composed of CuO 6 octahedra corner-linked with SO 4 tetrahedra, was originally determined by Dahlman as discussed by the authors.
Abstract: The crystal structure of krohnkite [Na 2 Cu(SO 4 ) 2 .2H 2 O], which contains infinite chains composed of CuO 6 octahedra corner-linked with SO 4 tetrahedra, was originally determined 1952 by Dahlman. Since then, a large number of both minerals (e.g., collinsite, fairfieldite, etc.) and synthetic compounds with closely related structures have been investigated, but the structural relation was not always recognised. In the present review we compare the structures of A n M(XO 4 ) 2 . 2H 2 O (A = mono- or divalent cation, n = 1, 2; M = di- or trivalent cation; X = penta- or hexavalent cation) compounds with krohnkite-type infinite chains, and propose a structural classification into six types. The rod group symmetries of the chains are compared. Furthermore, the linkage of these chains to double chains (in krausite-type compounds) and sheets (in yavapaiite-, merwinite-, bafertisite-type and similar structures) is discussed, and the occurrence of octahedral-tetrahedral chains topologically identical to those in krohnkite (referred to as krohnkite-type) and similar chains (referred to as krohnkite-like) in several related compounds is pointed out.
63 citations
TL;DR: The direct-methods program SnB as discussed by the authors provides an efficient means for solving protein substructures containing many heavy-atom sites (current record: 160) and can facilitate a high degreeofautomationandenablerapidstructuredetermina-tion bybothexperiencedandnovicecrystallographers.
Abstract: . The direct-methods program SnB provides anefficient means for solving protein substructures contain-ing many heavy-atom sites (current record: 160). In orderto meet the high-throughput requirements of structuralgenomics projects, substructure determination needs to betightlyintegratedwithotheraspectsoftheprotein-phasingprocess. This has been accomplished through the designofacommonJavainterface,BnP,forSnB andcomponentsofPHASES,apopularandprovenprogramsuitethatpro-vides all the tools necessary to proceed from substructurerefinement to the computation of an unambiguous proteinelectron-densitymap.Therefore, BnP will facilitate a highdegreeofautomationandenablerapidstructuredetermina-tionbybothexperiencedandnovicecrystallographers. Introduction In the absence of a suitable model for molecular replace-ment, the determination ofanew protein structure is typi-callyatwo-stepprocess.Iftwoormoreintensitymeasure-ments are available for eachreflection withdifferencesarising from some property of a small substructure, thenthe positions of the substructure atoms can be found firstandusedasabootstraptoinitiatethephasingofthecom-plete structure. Suitable substructures may consist of hea-vyatomssoakedintoacrystal,and theintensitymeasure-ments can be made from bothunsubstituted (native) andsubstituted(derivative)crystals.Alternatively,thesubstruc-turemayconsistofanomalousscatterers,suchasseleniumin the form of selenomethionine, that have been incorpo-ratedintothecrystal,andmeasurementsofanomalousdis-persioncanbemadeatoneormorewavelengths.Heavy-atom substructures can be solved using compu-tational procedures that are based on either Patterson ordirect methods. In either case, the positions of the sub-structure atoms are determined from isomorphous oranomalous difference coefficients.Althougheither compu-tationalmethodcanbeusedeffectivelyforsmallsubstruc-tures (e.g., less than 20 sites), direct methods tend to befaster, and their relative efficiency increases as the size ofthe substructure increases. So far, the largest substructurethat has been shown to be solvable by Patterson-basedmethods contained 66 Se sites (T.Terwilliger, P.Adams,personalcommunications) whereasthelargestsubstructurethat has been solved by direct methods contained 160 Sesites(vonDelft,Blundell,2002).
60 citations
TL;DR: The SIR2002-N program as mentioned in this paper has two important new features: a) a figure of merit which is able to recognize the trials potentially able toprovide the correct solution; b) a procedure capable of exploiting electron density distributions misplaced with re-spect to the correct origin.
Abstract: . The program SIR2002, the heir of SIR2000-N,shows two important new features: a) a figure of meritwhich is able to recognize the trials potentially able toprovide the correct solution; b) a procedure capable of ex-ploiting electron density distributions misplaced with re-spect to the correct origin. The two features dramaticallyincrease the power of direct methods for macromolecularcrystallography. Symbolsandnotation F h : structure factor with vectorial index hj h : phase of Ff j ðhÞ: scattering factor of the j-th atomC s ð R s ;T s Þ: s-th symmetry operator, R s is the rota-tional part, T s the translational partF hk ¼ j h þ j k j þ I i ðxÞ: modified Bessel function of order iD i ðxÞ¼I i ðxÞ=I 0 ðxÞE h ¼ R h expðij Þ: normalized structure factor withvectorial index hA hk ¼ 2s 3 s 3=22 jE h E k E hþk j;R W : Wilson’s normalized structure factor moduleR M : Main’s normalized structure factor moduleDR¼ R W R M m: order of the point group of the space groupN: number of atoms in the unit cellN
56 citations
TL;DR: In this article, the authors measured the optical rotation in RbTiOAsO 4 (RTA) with the tilter method and found an optical rotation of? 12 = +17(3)°/mm at a wavelength of 670 nm, when a (100) sample was tilted about [001].
Abstract: Measurement of optical rotation in RbTiOAsO 4 (RTA) with the tilter method resulted in an optical rotation of? 12 = +17(3)°/mm at a wavelength of 670 nm, when a (100) sample was tilted about [001]. A tilt about [010] showed no rotation, as expected from the directional dependence of optical rotation calculated from the tensor in point group mm2. The absolute Miller-indices of the samples were found using X-ray anomalous scattering. The calculations with a dipole-dipole model show that the As 5+ -ions in RTA correlate with the main structural contribution of the optical rotation. However, there seems to be an even larger intrinsic contribution due to the Ti 4+ -ions as a result of the distorted octahedral co-ordination with oxygen.
45 citations
TL;DR: In this article, the authors presented the extraction of chemical short range order parameters from the x-ray PDF of a quenched Cu3Au sample using the atomic pair distribution function (PDF) obtained from powder diffraction data.
Abstract: Many crystalline materials show chemical short range order and relaxation of neighboring atoms. Lo- cal structural information can be obtained by analyzing the atomic pair distribution function (PDF) obtained from powder diffraction data. In this paper, we present the suc- cessful extraction of chemical short range order parameters from the x-ray PDF of a quenched Cu3Au sample.
44 citations
TL;DR: The crystal structure of Bi 4 O 5 I 2 has been solved from a 3:1 twinned crystal; accurate lattice parameters have been determined by a Rietveld refinement.
Abstract: The crystal structure of Bi 4 O 5 I 2 has been solved from a 3:1 twinned crystal; accurate lattice parameters have been determined by a Rietveld refinement. Bi 4 O 5 I 2 crystallizes with space group P2 1 (no. 4), Z = 4, a = 14.944(1), b = 5.6983(3), c = 11.263(1) A, β = 99.81(1)°. Its structure is isotypic to the structure of the homologous Bi 4 O 5 Br 2 for which more reliable lattice parameters have been determined by a Rietveld refinement. The effects of replacement of Br by I on the structure are discussed and compared with corresponding effects in the BiOX family (X = Cl, Br, I). A structural reason for the deviation of the twin supercells of the two compounds from orthogonality is discussed.
38 citations
TL;DR: In this article, the energy level diagram obtained for Mn 3+ with the above pseudo-symmetry was used to obtain the crystal field parameter 10 Dq = 13540 cm -1 for x Mn3+(M3) = 0.931.
Abstract: Six new structure refinements and eleven sets of polarised, single-crystal electronic absorption spectra, E∥X, Y and Z, in the energy range 35000-5000 cm -1 were obtained on natural and synthetic orthozoisite-type thulites and clinozoisite-type piemontites: Ca 2 (Al 3-p M p 3+) [OH |O| SiO 4 | Si 2 O 7 ] where M 3+ = Mn 3+ or (Mn 1-n 3+Fe n 3+) for the synthetic or natural minerals, respectively. Electron microprobe analyses of the single crystals studied revealed substitutional degrees p M3+ = 0.13 or 0.51 in natural and synthetic thulite, respectively, and 0.57 ≤ p M3+ Y) and v III at 24000-22000 cm -1 (E ∥ X) is best interpreted by assuming a C 2v (C 2 ) pseudo-symmetry of the M(3) sites, a super-group of their site symmetry C s . Evaluation of the energies of v I , v II and v III on the basis of the energy level diagram obtained for Mn 3+ with the above pseudo-symmetry yields the crystal field parameter 10 Dq = 13540 cm -1 for x Mn3+(M3) = 0.931. 10 Dq increases slightly by 30cm -1 per -0.1x Mn3+(M3) . Such values and the Jahn-Teller splitting of the octahedral crystal-field ground-state of Mn 3+ , 6 = v I , yield a crystal field stabilisation energy of Mn 3+ (M3) of 14080 cm -1 for x Mn3+(M3) = 0.931. CFSE Mn3+ increases slightly by 28 cm -1 per -0.1x Mn3+(M3) . Such values are appreciably smaller than those typical of Mn 3+ substituting for Al in the axially elongated [M(1)O 6 ] octahedra in the andalusite structure type. This different behaviour of Mn 3+ in the two structure types is likely due to the smaller deviation of (c/a) oct in piemontite M(3) compared to andalusite M(1) for the same site fractions of Mn 3+ . In addition, the axial inversion effect -compressed [M(3)O 6 ] in the clinozoisite-type or elongated [M(1)O 6 ] in the andalusite-type, involving the electron hole of 3d 4 in d z 2 or d (x2-y2) , respectively - may play a role.
35 citations
TL;DR: The chemistry of nitrides and nitridometalates is a rapidly growing field in solid state chemistry as mentioned in this paper, and a short review is intended to give a brief but comprehensive overview on the compounds and phases formed with manganese, which cover an especially broad range of oxidation states.
Abstract: The chemistry of nitrides and nitridometalates is a rapidly growing field in solid state chemistry. This short review is intended to give a brief but comprehensive overview on the compounds and phases formed with manganese, which cover an especially broad range of oxidation states. Furthermore, the present paper tries to put the observed structures and properties of the compounds into a broader context.
TL;DR: In this article, the Pb, Tl, Ag, As, Sb sulfosalt rathite is derived from a topological parent structure with orthorhombic symmetry, space group Pmnb.
Abstract: Members of the 4,4 sartorite homologues can be deduced from a topological parent structure with orthorhombic symmetry, space group Pmnb, a = 4.233, b = 7.918, c = 24.692 A. The observed modulations in 4,4 sartorite homologues are associated with symmetry lowering of the parent structure and superstructure formation. The Pb, Tl, Ag, As, Sb sulfosalt rathite is a 4,4 member of the sartorite homologous series with space group symmetry P2 1 /c and is isostructural to synthetic Ba 12 Sb 18.64 S 40 and closely related to Ba 10.48 Pb 5.52 Sb 16 S 40 of reduced symmetry P2 1 . In these structures the two-fold screw axes run parallel to b of the parent structure. Isotopological is also dufrenoysite Pb 16 As 16 S 40 , another 4,4 homologue of the same series, of space group symmetry P2 1 . In dufrenoysite, however, the two-fold screw axes run parallel to c of the parent structure. To elucidate the modulation of the parent structure, rathite has been structurally and chemically reinvestigated emphasising cation order/ disorder relations. Single crystal X-ray structure refinement (space group P2 1 /c; a = 8.496(1), b = 7.969(1), c = 25.122(3) A, β = 100.704(2)°; R 1 = 3.67% for 2718 reflections with 1 > 2σ[I]) yielded three split cation positions indicating heterovalent substitutions. Considering that isoelectronic Pb 2+ and Tl + cannot be distinguished in our diffraction experiment the chemical composition (Pb 9.25 Tl 1.37 ) Σ = 10.62 Ag 2.01 (As 19.09 Sb 0.83 ) Σ = 19.92 S 40 derived from electron microprobe analyses matches the formula Pb 10.96 Ag 1.90 (As 18.09 Sb 1.06 ) Σ = 19.15 S 40 obtained from the structure refinement. The complex composition is caused by two independent heterovalent substitutions Ag + + As 3+ → 2 Pb 2+ and Tl + + As 3+ → 2 Pb 2+ leading to the simplified rathite formula Pb 8 Pb 4-x (Tl 2 As 2 ) x (Ag 2 As 2 )As 16 S 40 . The substitution involving Ag seems to be essential for the formation of rathite and distinguishes rathite from dufrenoysite Pb 16 As 16 S 40 . The Tl substitution in rathite appears to be optional. In addition, minor Sb 3+ may replace As 3+ .
TL;DR: In this paper, a bonding model for alimited groups of borates is proposed, which allows the ratio of triangularly to tetrahedrally coordinated B atoms to be calculated from the chemical formula.
Abstract: A bonding model, valid for alimited group of borates, is formulated which allows the ratio of triangularly to tetrahedrally coordinated B atoms to be calculated from the chemical formula. The anion complex, which is formed by covalently bound atoms, has non-directed ionic bonds with the surrounding cations. To complete its electron octet, each O atoms of the complex forms exactly two covalent bonds either with 2 B and/or with 1 B and 1 H atom, depending on the number of OH groups. The B atoms have no preference for either triangular of tetrahedral O coordination. However, a change of a B-O base triangle to an iso-electronic base tetrahedron decreases the number the O atoms which can be used for B-O-H bonds and increases the number of shared O atoms engaged in B-O-B bonds - and vice versa. The ratio of B-O triangles to tetrahedra is adjusted such that O atoms can form the proper number of B-O-B and/or B-O-H bonds. Using the model one can also prove that a borate with OH groups and all its dehydration products (independent whether they are crystallized or a glass) should have the same ratio of three-to four-coordinated B atoms. Under the presupposition that the bonding model is applicable, one can specify a simple mixture of base triangles and/or base tetrahedra with which it should be possible to construct an anion complex. The observed complex is either built up with this simple mixture or with a mixture of base polyhedra derived from the sinple one by cross-substitution which does not change the triangle to tetrahedron ratio. The realization of the bonding model is in principle possible only in a restricted composition range in which lie one quarter of the known borates. The equation to calculate the triangle to tetrahedron ratio has been tested on all borates within this range. An agreement was found for 85 percent of the 222 investigated crystal structures.
TL;DR: In this article, the crystal structures of two K 1-x Cs x BSi 2 O 6 solid solutions have been refined at room temperature by the Rietveld method.
Abstract: The crystal structures of two K 1-x Cs x BSi 2 O 6 solid solutions have been refined at room temperature by the Rietveld method: x = 0.12, a = 12.6858(4) A, R wp = 7.66%, R F = 5.56% and x = 0.50, a = 12.8480(2) A, R wp = 7.64%, R F = 3.10%. They are isostructural to cubic KBSi 2 O 6 with the space group I43d. The structure is built up from (Si,B)O 4 tetrahedra linked in four-, six- and eight-fold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically. Thermal behaviour of KBSi 2 O 6 and K 0.5 Cs 0.5 BSi 2 O 6 has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi 2 O 6 has been found in situ under heating. The new polymorphic I43d (cubic) - laid (cubic) transition and the new la3d cubic polymorphic phase has been proposed for K 1-x Cs x BSi 2 O 6 from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.
TL;DR: In this article, the authors studied the properties of the R 2 Co 2 Al (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y, Er et al., Tb 2 CoB 2 Al) and showed that the electrical resistivity, measured in the 13-300 K temperature range for all phases, always shows a transition point at temperatures ranging from 14 K to 94 K for Tm 2 CO 2 Al to 94 k for Tb CO 2 AL, indicating the influence of both the crystalline
Abstract: The new R 2 Co 2 Al compounds (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y), studied by single crystal and powder X-ray diffraction, crystallize with the W 2 CoB 2 structure type. Pr 2 Co 2 Al is dimorphic, showing also its own monoclinic structure (a = 9.595 A, b = 5.602 A, c = 7.753 A, β = 103.89°), which is closely related to the W 2 CoB 2 type. Both structures are based on different packing of chains of cobalt-centred double trigonal prisms formed by rare earth and aluminum atoms. The electrical resistivity, measured in the 13-300 K temperature range for all phases (save Y 2 Co 2 Al), always shows a transition point at temperatures going from 14 K for Tm 2 Co 2 Al to 94 K for Tb 2 Co 2 Al. As already found for Gd 2 Co 2 Al, these points should correspond to an order-disorder magnetic transition, and the deviations from de Gennes scaling indicate the influence of both the crystalline electric field and the strong coupling between 4f and conduction electrons.
TL;DR: The structural variations within the aegirine (NaFeSi 2 O 6 ) - Li-aegirines (LiFeSi2 O 6 ), solid-solution series have been determined from structure refinements of single-crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K as discussed by the authors.
Abstract: The structural variations within the aegirine (NaFeSi 2 O 6 ) - Li-aegirine (LiFeSi 2 O 6 ) solid-solution series have been determined from structure refinements of single-crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K. At 298 K, all compounds have space group C2/c. Replacing Na + by Li + in aegirine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle β by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2-fold in aegirine to 6-fold in pure LiFeSi 2 O 6 , the bond length decreases by 7.1% with increasing replacement of Na + by Li + . The decrease of the M2-oxygen coordination number correlates with a straightening of the tetrahedral chains which are slightly kinked in aegirine and fully extended in Li-aegirine (O3-O3-O3 bridging angle = 174.2(1)° and 180.8(1)° respectively). Small variations of individual and mean Si-O bond lengths and distinct changes in O-Si-O bond angles were also found. Within the Fe 3+ O 6 - M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na 1-x Li x FeSi 2 O 6 with Li + concentrations x > 0.75 crystallize in the space group P2 1 /c. For the compounds with x < 0.75, no phase transition from C2/c → P2 1 /c was observed down to low temperatures (30 K). A x - T phase diagram for the stability of the low temperature P2 1 /c structure is provided. The characteristica of the P2 1 /c low temperature form of the (Na,Li)FeSi 2 O 6 compounds agree with those of pure LiFeSi 2 O 6 , recently described by Redhammer et al. (2001).
TL;DR: A new indium yttrium germanate with symmetry described by the space group C2/m (No. 12) has been prepared by high temperature solid state reaction as polycrystalline powder material.
Abstract: A new indium yttrium germanate presenting the thortveitite structure with symmetry described by the space group C2/m (No. 12) has been prepared by high temperature solid state reaction as polycrystalline powder material. This crystallizes in the monoclinic system, with cell parameters a = 6.8286(1) A, b = 8.8836(2) A, c = 4.9045(1) A, β = 101.8340(7)°, V = 291.195(9) A 3 and Z = 2. The structure was characterized by X-ray powder diffraction and Rietveld refinement of the diffraction pattern. The In 3+ and Y 3+ cations occupy the same octahedral site forming a hexagonal arrangement on the ab planes. In their turn, the hexagonal arrangements of InO 6 /YO 6 octahedral layers are held together by sheets of isolated diorthogroups constituted by a double tetrahedra sharing a common vertex. In this compound, the Ge 2 O 7 diorthogroup shows the C 2h symmetry implying a Ge-O-Ge angle of 180°, being an important feature of the thortveitite structure, which has been controversial in some reported papers. A remarkable photo-luminescence effect (in comparison with glass) was observed when the sample was irradiated with α-particles beam during the RBS experiments employed to analyze the chemical stoichiometry.
TL;DR: Topological analysis and comparing of the crystal structures of anhydrous inorganic salts containing triangular oxo-anions (borates, carbonates and nitrates) was performed using the program package for multipurpose crystallochemical analysis TOPOS as discussed by the authors.
Abstract: Topological analysis and comparing of the crystal structures of anhydrous inorganic salts containing triangular oxo-anions (borates, carbonates and nitrates) was performed using the program package for multipurpose crystallochemical analysis TOPOS. The examples of topological similarities between these classes of substances and 2920 framework binary compounds A y X z were revealed and discussed. It is shown that in most cases the ionic arrays constructed according to one of close packings or to a b.c.c. motif can be selected in the crystal structures of anhydrous inorganic salts. The conclusions were made about the structure-forming role of atoms and atomic groups in these substances. The crystal structures of anhydrous salts are substantiated to be correctly considered as packings of metal cations and quasi-spherical oxo-anions.
TL;DR: In this paper, the crystal structure of the α-Na 2 B 8 O 13 stable modification has been refined by single crystal X-ray diffraction at 293, 573 and 773 K.
Abstract: The crystal structure of the α-Na 2 B 8 O 13 stable modification has been refined by single crystal X-ray diffraction at 293, 573 and 773 K. The compound is monoclinic, P2 1 /a space group, unit-cell parameters at 273 K are: a = 6.507(3), b = 17.796(8), c = 8.377(5) A, β = 96.60(4)°, Z = 4. The structure contains two interlocking boron-oxygen frameworks, each of them consisting of alternating single and double rings composed of two triangles and a tetrahedron so called triborate and pentaborate groups. Two non-equivalent sodium atoms linking two frameworks are coordinated by 8 oxygens. They are connected into finite chains consisting of four NaO 8 polyhedra by sharing oxygen edges. The α-Na 2 B 8 O 13 thermal expansion investigated by high temperature powder X-ray diffraction has sharply anisotropic character including negative thermal expansion: the coefficients of a thermal expansion tensor in temperature region 273-1000 K are α 11 =55-0.042 . (273 + t), α 22 = 11, α 33 = -15 + 0.032 . (273 + t) (× 10 -6 K -1 ), μ = (c α 33 ) = 42°. That high anisotropy is caused by anisotropic thermal vibrations of the Na atoms. Under heating a high stability of the B-O rigid groups and a weak contraction of some B-O bond lengths were revealed. In response to anisotropic thermal behaviour of Na atoms rigid B-O groups of the anion framework rotate relative to each other as a three-dimensional hinge.
TL;DR: In this article, a detailed model of the H-RUB-18 (chemical composition: Si 4 O 7 (OH) 2 ) was obtained by a step-by-step process using a combination of complementary methods: Crystal chemical considerations and solid state NMR spectroscopy.
Abstract: The layer silicate H-RUB-18 (chemical composition: Si 4 O 7 (OH) 2 ) was prepared by ion exchange from the parent material Na-RUB-18. The high degree of structural disorder typical for this material precluded a classical structure analysis based on diffraction data only. Nevertheless, a detailed picture of the structure was obtained by a step by step process using a combination of complementary methods: Crystal chemical considerations and solid state NMR spectroscopy allowed to establish a rough model of the structure which was optimized by forcefield molecular dynamics (MD) simulations. A subsequent Rietveld refinement in space group I4 1 /amd confirmed the model consisting of a sequence of pseudo tetragonal silicate layers with an intra-layer repeat unit a = 7.38 A and inter-layer distances of c = 7.44 A. The main type and degree of disorder was analyzed by the simulation of various stacking sequences of rigid silicate layers which served as the building blocks. The random displacement of consecutive layers is accompanied by a slight distortion of the layers as indicated by an additional density functional theory (DFT) energy minimization calculation in triclinic symmetry. The given displacement and distortion allow for the formation of hydrogen bonds between the terminal OH groups necessary for the stabilization of the structure. The concept of a structure analysis presented in this paper may be suitable as a guide line to be applied to many other disordered materials.
TL;DR: In this article, Bismuth germanate single crystals containing low angle and very low angle boundaries were selected as specimen for annealing experiments at 1000 °C for varying lengths of time in the range: 8 h to 80 h.
Abstract: Bismuth germanate single crystals containing low angle and very low angle boundaries were selected as specimen for annealing experiments at 1000 °C for varying lengths of time in the range: 8 h to 80 h. Specimen with larger surface along (111) and (100) planes were investigated. The tilt angles of the boundaries were found to be in the range: 16 arc sec to 88 arc sec. Half widths of the diffraction curves of different subgrains were in the range: 15 arc sec to 39 arc sec. High resolution X-ray diffractometry and topography were employed for structural characterization and secondary ion mass spectrometry (SIMS) was used for study of impurity distribution. Annealing led to significant improvement in the crystalline perfection of specimen and all but one of the boundaries could be removed. The very low angle boundaries, which could not be annealed out, had a tilt angle of 41 arc sec. The half widths of diffraction curves of the two remaining subgrains were: 10 arc sec and 17 arc sec. The growth of subgrains as a result of annealing was investigated. SIMS investigation of annealed specimen, in which a very low angle boundary persisted, showed the boundary was decorated with silicon impurity, which presumably acted as a pining center. Detailed diffractometric and topographic experiments helped in delineating the two subgrains around the very low angle boundary. The morphological features of the very low angle boundary were studied in detail.
TL;DR: In this paper, a new sodium calcium silicate with composition Na 2 Ca 2 Si 2 O 7 has been synthesized, which is monoclinic, space group I12/a1, a = 10.4906(5) A, b =10.3762(6) A and c = 24.5349(12) A.
Abstract: Single crystals of a new sodium calcium silicate with composition Na 2 Ca 2 Si 2 O 7 have been synthesized. The title compound is monoclinic, space group I12/a1, a = 10.4906(5) A, b = 10.3762(6) A, c = 24.5349(12) A, β = 91.217(6)°, V = 2670.1(2) A 3 , Z = 16. The crystal used for the data collection showed twinning by pseudomerohedry, a feature we took account of by separating the reflections from the different twin individuals. Structure determination was based on direct methods followed by difference Fourier calculations and least squares refinements (R(|F|) = 0.056 for 2414 independent observed reflections (I > 2σ(I)) and 245 parameters). Na 2 Ca 2 Si 2 O 7 belongs to the group of mixed anion silicates comprising insular [SiO 4 ]-groups and [Si 3 O 10 ]-trimers. Both anion groups are located in layers perpendicular to [001]. Ten symmetrically independent M-sites occupied by Na and/or Ca crosslink between the tetrahedral anion groups. Another very interesting understanding of the structure can be obtained from the comparison with the ABO 3 perovskite structure type. The compound can be regarded as a superstructure of perovskite (a ≃ 2√2a p , b ≃ 2√2a p , c ≃ 6ap) with 22.2% oxygen vacancies. The vacancies are ordered in such a way that the silicon atoms residing on the B-sites of the perovskite structure are coordinated to four oxygens. The relationship with perovskite can be expressed in the following crystal chemical formula: [(Na,Ca)][(Na,Ca) 1/4 Si 3/4 ][O 7/9 □ 2/9 ] 3 .
TL;DR: In this paper, the authors characterized the crystallization and annealing behavior of two caesium borosilicate glass samples with approximate boroleucite composition by means of differential scanning calorimetry measurements as well as X-ray powder diffraction investigations with subsequent profile fitting of the observed patterns according to the Rietveld and/or Pawley method.
Abstract: Crystallisation and annealing behaviour of two caesium borosilicate glass samples with approximate boroleucite composition were characterised by means of differential scanning calorimetry measurements as well as X-ray powder diffraction investigations with subsequent profile fitting of the observed patterns according to the Rietveld and/or Pawley method. While one sample crystallised primarily in an orthorhombic phase (a = 6.592(3) A, b = 11.825(5) A, c = 12.620(6) A; possible space groups: Pmc2 1 ((26), setting: P2 1 am), Pma2 (28) and Pmma ((51), setting: Pmam)) which has been unknown until now and transformed irreversibly into the well-known cubic boroleucite phase at T 850°C, cubic caesium boroleucite (space group: la3d (230)) was crystallised directly from the other sample. In this case, the orthorhombic phase was metastable. At the onset temperature of glass crystallisation T x,on = 770 °C, it existed only for about 75 min. The change of the lattice parameter a of the cubic unit cell of caesium boroleucite in the course of the thermal treatments was explained by the alteration of the size of the TO 4 tetrahedra (T = Si, B).
TL;DR: In this article, the primary geometry about the Te IV atom is a pseudo-trigonal-bipyramidal arrangement, with two halide atoms in apical positions, and the lone pair of electrons and C atoms in the equatorial plane.
Abstract: C 16 H 12 Cl 4 Te, (1), M r = 473.66, P2 1 /c, a = 8.3919(9), b = 14.474(1), c = 14.585(1) A, β = 94.057(9)°, Z = 4, R 1 = 0.0378. C 16 H 12 Br 4 Te, (2), M r = 651.50, Pnma, a = 19.159(2), b = 16.787(1), c = 5.781(1) A, Z = 4, R 1 = 0.0437. In both compounds the primary geometry about the Te IV atom is a pseudo-trigonal-bipyramidal arrangement, with two halide atoms in apical positions, and the lone pair of electrons and C atoms in the equatorial plane. The Te IV atoms are involved in secondary interactions resulting in the self-assembly of a chain-like supramolecular array in (1) and into a three dimensional supramolecular array in (2). Compound (1) has a Z configuration with the chlorine atom and the tellurium moiety bonded at the same side in adjacent atoms. In (2) the bromine atom and the dibromotellurium moiety are bonded at adjacent carbons in a cis fashion.
TL;DR: The crystal structures of the four synthetic double salt compounds K 2 Mn(SO 4 ) 2. 2 H 2 O (space group P1, no. 2, Z = 2, a = 6.574(1), b = 7.542(1), c = 11.21(2)°, β = 93.86(2), β = 99.77(1).
Abstract: The crystal structures of the four synthetic double salt compounds K 2 Mn(SO 4 ) 2 . 2 H 2 O (space group P1, no. 2, Z = 2, a = 6.574(1), b = 7.332(1), c = 10.700(1) A, α = 72.89(1), β = 73.91(1), γ = 69.77(1)°, V = 453.6(1) A 3 ) and K 2 Cd(SeO 4 ) 2 . 2 H 2 O (space group P1, no. 2, Z = 2, a = 6.625(1), b = 7.542(1), c = 11.349(1) A, α = 72.17(1), β = 74.55(1), γ = 70.13(1)°, V = 499.3(1) A 3 ) as well as (NH 4 ) 2 Cu(SeO 4 ) 2 . 2 H 2 O (space group P1, no. 2, Z = 1, a = 5.784(1) A, b = 7.107(1) A, c = 7.371(1) A, α = 104.21(2)°, β = 93.04(2)°, γ = 112.27(2)°, V = 268.19(7) A 3 ), and KFeH(SO 4 ) 2 . 2 H 2 O (space group P1, no. 2, Z = 1, a = 4.626(1), b = 5.820(1), c = 8.265(1) A, α = 103.86(1), β = 99.77(1), y = 95.66(1)°, V= 210.64(2) A 3 ) have been determined from single-crystal X-ray diffraction data collected with a CCD area detector at 293 K, and were refined to R(F) = 0.0231, 0.0251, 0.0230 and 0.0194, respectively. The first two compounds are isostructural. The atomic arrangements are described, with a focus on the hydrogen bonding. All compounds contain krohnkite [Na 2 Cu(SO 4 ) 2 . 2 H 2 O]-type tetrahedral-octahedral chains but show different topological arrangements of these chains. A classification is proposed in an accompanying paper. A particular feature of KFeH(SO 4 ) 2 . 2 H 2 O is a disordered H atom involved in a short hydrogen bond, which shows no ordering down to 110 K.
TL;DR: The crystal structure of Cu 4 Bi 4 Se 9, synthesized at 400 °C, was determined from single crystal X-ray diffraction data and refined to the R 1 value of 0.05 as discussed by the authors.
Abstract: The crystal structure of Cu 4 Bi 4 Se 9 , synthesized at 400 °C, was determined from single crystal X-ray diffraction data and refined to the R 1 value of 0.05. The compound is orthorhombic, with a = 32.692 A, b = 4.120 A, and c = 12.202 A, space group Pnma. The structure contains three square pyramidal Bi sites, an octahedrally coordinated Bi site as well as two tetrahedrally and two irregularly coordinated Cu sites. The structure is an intergrowth of PbS-like slabs with irregularly configured slabs of Bi pyramids and Cu tetrahedra. It contains covalently bonded Se 2 groups. Isotypy with Cu 4 Bi 4 S 9 is connected with substantial changes in coordination details for two out of five distinct Cu sites. Modular relationships to the structures of the cuprobismutite series of Cu-Bi sulfosalts can be expressed as different ways of recombination of the same large structural fragment in the structures of Cu 4 Bi 4 Se 9 and of the first cuprobismutite homologue, Cu 4 Bi 5 S 10 .
TL;DR: In this article, the authors reported the pressure dependence of the lattice parameters at high hydrostatic pressures up to 10.2 GPa and showed that the compression of langasite crystals is anisotropic, the c/a-ratio appears to be a linear function of the pressure applied according to c/α = 0.62460(6) + 0.00055(1). p/GPa.
Abstract: Langasite (La 3 Ga 5 SiO 14 ) crystallizes in the polar space group P321. Here we report the pressure dependence of the lattice parameters at high hydrostatic pressures up to 10.2 GPa. The results presented in terms of compressibility agree well with results from measurements of the elastic constants. According to a Birch-Murnaghan equation of state, the bulk modulus and its pressure derivative are 134.6(2) and 2.8(1) GPa, respectively. The compression of langasite crystals is anisotropic, the c/a-ratio appears to be a linear function of the pressure applied according to c/a = 0.62460(6) + 0.00055(1) . p/GPa. Altogether four single crystals have been investigated. Two of them showed a splitting of reflections (hhl). This splitting reaches a maximum at approximately 9 GPa and proved to be reversible.
TL;DR: In this article, a general formula M 2+ (C 2 H 4 O 5 P - ) 2 (H 2 O) 2, (MAP) was proposed, which is an octahedrally coordinated mixture of two monoanions and two water molecules.
Abstract: All crystals of the title compounds, with general formula M 2+ (C 2 H 4 O 5 P - ) 2 (H 2 O) 2 , (MAP), are isomorphous and crystallize in the space group P1 with Z = 2. The two crystallographically independent M 2+ cations are situated on a center of symmetry and are octahedrally coordinated by two phosphonate monoanions and two water molecules. In the case of Cu 2+ cation, Jahn-Teller effect causes strong extension of two Cu-O bonds. The monoanions coordinate the metal ions by one carboxylic and one phosphonate oxygen atom, which is facilitated by their specific overall conformation. In each monoanion, one of the phosphonate O atoms is nearly antiperiplanar ap to the carboxylic C atom, and the remaining phosphonate O atoms are nearly gauche (+sc and -sc) with respect to the mentioned C atom. The carboxylic O atoms are in positions close to +sc and -ac in relation to the phosphonate P atom. The complex molecules form well separated layers in all MAP compounds. The phosphonate and carboxylic groups act as donors in medium strong and weak intermolecular O-H... O hydrogen bonding and, as acceptors, in C-H... O hydrogen interactions in all structures. Additionally both crystallographically independent water molecules are donors of weak hydrogen bonding to phosphonate oxygen atoms from adjacent complex molecules.
TL;DR: In this article, a single set of structure factor magnitudes complete at low resolution plus information of a general type are sufficient to get initial phases for macromolecular crystals which allow one to see the molecular packing and an approximate envelope.
Abstract: A single set of structure factor magnitudes complete at low resolution plus information of a general type are sufficient to get initial phases for macromolecular crystals which allow one to see the molecular packing and an approximate envelope. Followed by a more careful analysis based on the same information, these ab initio phases can be extended so that the corresponding maps show secondary structure elements.
TL;DR: In this article, the crystal structures of the compounds BaMGaO 4 (M = Al, Fe) have been investigated using conventional single-crystal X-ray diffraction data for M = Fe and with synchrotron radiation data for Al.
Abstract: The crystal structures of the compounds BaMGaO 4 (M = Al, Fe) have been investigated using conventional single-crystal X-ray diffraction data for M = Fe and with synchrotron radiation data for M = Al. BaAlGaO 4 (space group P6 3 22, a = 5.2788(4) A and c = 8.7835(7) A, V = 211.97(4) A 3 , Z = 2, D calc. = 4.67 g cm -3 , wR2 = 0.089 for 242 independent reflections with I > 2σ(I)) is isostructural with BaAl 2 O 4 -I. Main building units are layers of six-membered rings of (Al,Ga)O 4 -tetrahedra perpendicular to [001]. The sequence of directedness of the tetrahedral apices in all single six membered rings (S6R) is UDUDUD (U: up, D: down). Stacking of layers parallel to c results in a three-dimensional framework containing tunnels, where the Ba atoms are located. They are coordinated by nine oxygen ligands in form of distorted tricapped trigonal antiprisms. BaFeGaO 4 also belongs to the hexagonal crystal system (space group P6 3 , a = 10.8377(6) A and c = 8.6865(6) A, V = 883.59(9) A 3 , Z = 8, D calc. = 4.92 g cm -3 , wR2 = 0.099 for 1042 independent observed reflections). The crystal showed twinning by merohedry which was accounted for in the refinement calculations. Within a single tetrahedral layer of the framework structure two different types of ditrigonal-shaped rings are distinguishable: one fourth of the rings have an UDUDUD topology, whereas the sequence of the remaining rings is UUUDDD. The Fe 3+ - and Ga 3+ -cations show an ordered distribution among the four symmetrically independent T-sites: two positions contain either Fe or Ga, whereas on the remaining two sites there is a definite preference for one of the two cation species. The Ba atoms in the cavities of the framework are coordinated by eight to nine oxygen neighbors. The observed twinning in BaFeGaO 4 can be attributed to the existence of pseudo twofold axis within the tetrahedral layers, running parallel [100] and the corresponding symmetrically equivalent directions.