Showing papers in "Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry in 1981"
363 citations
TL;DR: The structure of 2C10H12SeX/2+.pF~, a solid which exhibits superconductivity at 1.2 GPa below 0.9 K, has been determined at ambient pressure and temperature (295 K).
Abstract: The structure of 2C10H12SeX/2+.pF~ -, M r = 1041.06, a solid which exhibits superconductivity at 1.2 GPa below 0.9 K, has been determined at ambient pressure and temperature (295 K). (TMTSF)2PF 6 crystallizes in the triclinic space group P1 with a = 7.297 (1), b = 7.711(1), c = 13.522 (2)A, a = 83.39(1), fl = 86.27(1), ), = 71.01(1) °, Z = 1, V = 714.3/k 3, D e = 2.420, D m (flotation in CH2CI2CH2C12/ CH2Br2CH2Br2) = 2.43 Mg m -a, ~t(Mo Ka) = 10.267 mm-k Least-squares refinement of the structure gave R = 0.038 and R~ = 0.043 for 1466 reflections. The nearly planar TMTSF units are almost perpendicular to a (deviation 1.1 °) and form stacks along a. Within a stack the units repeat by an inversion leading to overlap displacements alternating in the direction of the long molecular axis and two independent interplanar distances of 3.63 and 3.66/k. The slight dimerization is clearly reflected in the intermolecular Se-Se distances. PF~ is centrosymmetric and shows considerable thermal motion.
180 citations
TL;DR: In this paper, the crystal structure of the martensite phase of the intermetallic compound TiNi has been determined based upon single-crystal electron and powder X-ray diffraction data.
Abstract: The crystal structure of the martensite (low-temperature) phase of the intermetallic compound TiNi has been determined based upon single-crystal electron and powder X-ray diffraction data. The structure is monoclinic, P1121/m, with dimensions a = 2.885 (4), b=4.622 (5) ,c=4.120 (5) A and 7= 96.8 (1.0) °. The atomic positions were determined by considering the collapse of the high-temperature structure occurring by a shuffle of its {110} planes, taking into account the constraints imposed by the difference in size of the Ti and Ni atoms. The final structure yielded an R of 0.17 for the first 22 reflections and contains a set of interatomic bond lengths commensurate with those observed in other structures in the Ti-Ni system.
175 citations
TL;DR: In this paper, atomic net charges and electron density distributions of the olivine structures forsterite, fayalite, and tephroite were determined from accurate single-crystal X-ray data on the respective synthetic crystals.
Abstract: Atomic net charges and electron density distributions of the olivine structures forsterite (a-Mg2SiO4), fayalite (a-Fe2SiO4) and tephroite (a-Mn2SiO 4) have been determined from accurate single-crystal X-ray data on the respective synthetic crystals. The atomic net charges were obtained from the direct integration of the electron density within the sphere of a newly defined radius, the effective distribution radius, around each cation. Such net charges revealed that the tetrahedral Si atom is less ionic than the octahedral cations and the transition-metal Fe and Mn are less ionic than Mg. These charges also showed that the M(1)-site cation is less ionic than the M(2)-site cation when the cations of the same kind occupy both M(1) and M(2) sites. * Present address: Department of Earth Sciences, Ehime University, Bunkyo-cho, Matsuyama 790, Japan. 0567-7408/81/030513-06501.00 Radial distributions of valence electrons outside the Ar cores of the transition metals show that the 3d electrons have their maximum densities about 0.4 ~ 0.5 A from the centre of the atom. In three-dimensional difference Fourier maps of forsterite the positive peaks appeared 0.96 A, on average, from the centre of the Si atom along each of the four S i -O directions. [Crystal data: forsterite: a = 4.7534(6), b = 10.1902(15), e = 5.9783 (7)A, space group Pbnm, final R = 0.0209; fayalite: a = 4.8195 (6), b = 10.4788(17), c = 6.0873 (8)A, space group Pbnm, final R = 0.0255; tephroite: a = 4.9023 (6), b = 10,5964 (16), c = 6.2567 (9) A, space group Pbnm, final R = 0.0310.]
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TL;DR: In this article, the structure of the high-temperature phase of (NH4LiSO 4) has been investigated from a structural point of view, and the structure was refined to R = 0.055 for 1060 observed reflexions.
Abstract: NH4LiSO 4, T = 478 K, orthorhombic, Pmcn, a 5.299(2), b = 9.199(2), c = 8.741 (3)A, v = 426.1 A 3, z = 4, D c = 1.933 Mg m -3. Residual maps displayed double-peaked contours for O atoms. The structure was refined to R = 0.055 for 1060 observed reflexions on the basis of the split-atom method. SO4 and LiO4 tetrahedra lie, respectively, with equal probability in two equilibrium arrangements which have mirror symmetry with respect to each other. Introduction. NH4LiSO 4 undergoes two phase transitions at about 283 K (Pepinsky, Vedam, Okaya & Hoshino, 1958) and 459.7 K (Mitsui, Oka, Shiroishi, Takashige, Iio & Sawada, 1975). The intermediate phase is ferroelectric with spontaneous polarization along the a axis (Mitsui et aL, 1975). The roomtemperature structure was determined by Dollase (1969). We now report the crystal structure of the high-temperature phase, as part of a continuing study of the mechanism of successive phase transitions from the structural point of view. Colourless single crystals were grown by evaporation of an aqueous solution containing equimolar amounts of (NH4)2SO 4 and Li2SO4.H20 at room temperature. An untwinned spherical specimen, 0.8 mm in diameter, was mounted on a Rigaku AFC-5 0567-7408/81/030664-03501.00 © 1981 International Union of Crystallography A M M O N I U M L I T H I U M S U L P H A T E 665 automat ic four-circle diffractometer. The temperature was regulated at 478 + 1.0 K with a heated air flow. The measurements were made in the 0 2 0 scanning mode with a scan speed of 8 ° min -~ in 20 and corrected for background and Lp factors but not for absorpt ion (#r = 0.26). Reflexions with 20 < 80 ° were measured with Mo Ka radiat ion monochromat ized by a graphite plate. Three s tandard reflexions were moni tored every 200 measurements and no significant variat ion was observed. 1154 independent reflexions were obtained; of these, 32 were rejected because of the appreciable extinction effect. Final ly, 1060 reflexions with values of I Fol greater than a ( I F o I) were used for least-squares calculations. The space group is either P21cn or Pmcn f rom the systematic extinctions but it has been uniquely determined to be Pmcn because spontaneous polarization vanishes in the high-temperature phase. The structure was refined by the use of the averaged positions of Dollase and their mirrorimage ones with respect to x = 1⁄4 and ] as initial parameters . In the calculat ions equal weights were used throughout, the quanti ty minimized being ~ (I F o I -I Fc l )2 /~ IF o 12. Ref inement using neutral scattering factors (International Tables for X-ray Crystallography, 1968) converged with R = ~ [ I F o l IF~I [ I f IFol = 0 . 0 9 3 , but large thermal parameters were obtained for O Table 1. Positional parameters and thermal parameters [Ueq (x l02 ) for S, N H 4 and O; Uis o ( x l 0 2 ) for Li] with e.s.d.'s in parentheses The equivalent isotropic thermal parameters have been calculated from U~q = (Un + U22 + U33)/3.
TL;DR: In this article, the crystal structure of [Zn(bpy) 2(ONO)]NO 3 (2) has also been determined by X-ray analysis, and the two complexes are related by a linear combination of the components of the E mode of vibration of the parent trigonaloctahedral trischelate metal(II) structure.
Abstract: host lattice, the crystal structure of [Zn(bpy) 2(ONO)]NO 3 (2) has also been determined. The structures of bis(2,2'-bipyridyl)mononitritocopper(II) tetrafluoroborate [Cu(C10HaN2)2(ONO)]BF 4, C20H16CuNsO+.BF4 (1), and bis(2,2'-bipyridyl)mononitritozinc(II) nitrate [Zn(C~0HsN2)2(ONO)]NO 3, C20H16NsO2Zn+.NO3 (2), have been determined by X-ray analysis. Both complexes crystallize in the monoclinic space group P2~/n with, for (1), a = 10.839 (3), b = 12.367 (3), c = 16.219 (3)A, fl = 105.25 (5) °, U = 2097.5 :k 3, Z = 4, R = 0.0468 (R w = 0.0566) for 2769 unique reflections and, for (2), a = 11.27 (5), b = 11.93 (5), c = 15.49 (6) A, fl = 101.2 (2) ° , U = 2042.7A 3, Z = 4, R = 0.0953 (R w = 0.1033) for 2083 unique reflections. Both complexes involve an MN40 2 chromophore but while (1) has a (4+ 1 + l*)-type coordination involving a trigonal-bipyramidal stereochemistry with a squarepyramidal distortion with an elongation along one of the in-plane Cu-N directions and an off-the-axis sixth O ligand, (2) involves a cis-distorted ZnN402 chromophore stereochemistry. The two structures may be related by a linear combination of the components of the E mode of vibration of the parent trigonaloctahedral tris(chelate)metal(II) structure.
TL;DR: The cyclopropane ring (A) has many properties which differ from higher cycloalkanes and are more analogous to those of the vinyl group (C=C), while a conformationdependent conjugative ability is exhibited with zr-acceptor substituents as discussed by the authors.
Abstract: The cyclopropane ring (A) has many properties which differ from higher cycloalkanes and are more (a) analogous to those of the vinyl group (C=C). In particular, the C(A) hybrid orbitals used in substituentbond formation are closer to sp 2 than sp 3, while a conformation-dependent conjugative ability is exhibited with zr-acceptor substituents. The Cambridge Crystallographic Database (CCD) has been used to obtain the (b) mean X-ray-determined geometries of A-A fragments (A = C-C, C=C, C=O, C-N), which are compared with the geometries of analogous fragments where/x is replaced by C-C and C=C. Changes in the D-A bond length (D = C-C, A, C=C) have been analysed in terms of hybridization and conjugative effects. The X-ray data indicate that the C(A) hybrid involved in the substituent bond is approximately sp 2"2, while the (c) relative conjugative ability A:vinyl is ca 71%. Problems involved in precise retrieval and geometric analysis of high-incidence organic fragments from CCD are discussed in terms of (i) restrictions on substitution and environment, (ii) acceptance criteria for coordinate sets, (iii) geometric screening of derived geometry to obtain unbiased mean values.
TL;DR: In this article, the crystal structure refinement of a natrolite from Gulacs Hill (Hungary) was studied and the relationship between cell parameters and the degree of disorder was discussed in relation to the present findings.
Abstract: The crystal structure refinement of a natrolite from Gulacs Hill (Hungary) [Naa6A116Si24Os0 . 16H20 , cell parameters a = 18.354 (10), b = 18.587 (10), c = 6.608 (4)AI shows a partial disorder in the (Si,A1) distribution of about 30% in all the tetrahedral sites (R = 5.4, R w = 5.2% for 463 observed reflections). The relationships between cell parameters and the degree of disorder are discussed in relation to the present findings and to those available in the literature. There is an increase of 0.027 A in the c parameter in going from the ordered to the disordered plqase; the b a difference, which is zero in the disordered phase, increases to 0.35 A in the ordered one. The rotation and twisting of the tetrahedral chain, under the influence of Na, are responsible for the b a difference.