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Twin Components (twin + component)
Selected AbstractsOn the tetragonality of the room-temperature ferroelectric phase of barium titanate, BaTiO3JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2009Dean S. Keeble The room-temperature phase of the important ferroelectric material barium titanate, BaTiO3, was re-investigated by single-crystal X-ray diffraction on a sample grown by the top-seeded solution growth method, with the intention of demonstrating once again that the structure has tetragonal symmetry consistent with the space-group assignment P4mm and thus resolving recent controversy in the scientific community and literature [Yoshimura, Kojima, Tokunaga, Tozaki & Koganezawa (2006). Phys. Lett. A, 353, 250,254; Yoshimura, Morioka, Kojima, Tokunaga, Koganezawa & Tozaki (2007). Phys. Lett. A, 367, 394,401]. To this end, the X-ray diffraction pattern of a small (341,µm3) sample of top-seeded solution-grown BaTiO3 was measured using an Oxford Diffraction Gemini CCD diffractometer employing Mo,K, radiation and an extended 120,mm sample-to-detector distance. More than 104 individual diffraction maxima observed out to a maximum resolution of 0.4,Å were indexed on two tetragonal lattices. These were identical to within the standard deviations on the lattice parameters and were related to each other by a single rotation of 119.7° about the [11] direction of the first tetragonal lattice (the major twin component), although the actual twinning operation that explains the observed diffraction pattern both qualitatively and quantitatively is shown to be conventional 90° twinning by the m[101] operation. Importantly, it is not necessary to invoke either monoclinic symmetry or a coexistence of tetragonal and monoclinic phases to explain the observed diffraction data. [source] Structural comparison of three N -(4-halogenophenyl)- N,-[1-(2-pyridyl)ethylidene]hydrazine hydrochloridesACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2010Julia Heilmann-Brohl 2-{1-[(4-Chloroanilino)methylidene]ethyl}pyridinium chloride methanol solvate, C13H13ClN3+·Cl,·CH3OH, (I), crystallizes as discrete cations and anions, with one molecule of methanol as solvent in the asymmetric unit. The N,C,C,N torsion angle in the cation indicates a cis conformation. The cations are located parallel to the (02) plane and are connected through hydrogen bonds by a methanol solvent molecule and a chloride anion, forming zigzag chains in the direction of the b axis. The crystal structure of 2-{1-[(4-fluoroanilino)methylidene]ethyl}pyridinium chloride, C13H13FN3+·Cl,, (II), contains just one anion and one cation in the asymmetric unit but no solvent. In contrast with (I), the N,C,C,N torsion angle in the cation corresponds with a trans conformation. The cations are located parallel to the (100) plane and are connected by hydrogen bonds to the chloride anions, forming zigzag chains in the direction of the b axis. In addition, the crystal packing is stabilized by weak ,,, interactions between the pyridinium and benzene rings. The crystal of (II) is a nonmerohedral monoclinic twin which emulates an orthorhombic diffraction pattern. Twinning occurs via a twofold rotation about the c axis and the fractional contribution of the minor twin component refined to 0.324,(3). 2-{1-[(4-Fluoroanilino)methylidene]ethyl}pyridinium chloride methanol disolvate, C13H13FN3+·Cl,·2CH3OH, (III), is a pseudopolymorph of (II). It crystallizes with two anions, two cations and four molecules of methanol in the asymmetric unit. Two symmetry-equivalent cations are connected by hydrogen bonds to a chloride anion and a methanol solvent molecule, forming a centrosymmetric dimer. A further methanol molecule is hydrogen bonded to each chloride anion. These aggregates are connected by C,H...O contacts to form infinite chains. It is remarkable that the geometric structures of two compounds having two different formula units in their asymmetric units are essentially the same. [source] Pseudomerohedrally twinned monoclinic structure of unfolded `free' nonactin: comparative analysis of its large conformational change upon encapsulation of alkali metal ionsACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2009Ilia A. Guzei The title compound, C40H64O12, crystallizes in a pseudomerohedrally twinned primitive monoclinic cell with similar contributions of the two twin components. There are two symmetry-independent half-molecules of nonactin in the asymmetric unit. Each molecule has a pseudo- S4 symmetry and resides on a crystallographic twofold axis; the axes pass through the molecular center of mass and are perpendicular to the plane of the macrocycle. The literature description of the room-temperature structure of nonactin as an order,disorder structure in an orthorhombic unit cell is corrected. We report a low-temperature high-precision ordered structure of `free' nonactin that allowed for the first time precise determination of its bond distances and angles. It possesses an unfolded and more planar geometry than its complexes with encapsulated Na+, K+, Cs+, Ca2+ or NH4+ cations that exhibit more isometric overall conformations. [source] Enantiotropic phase transition and twinning in 2,2,3,3,4,4-hexafluoropentane-1,5-diolACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2009Jeong-Myeong Ha Four crystal structure determinations of 2,2,3,3,4,4-hexafluoropentane-1,5-diol (HFPD), C5H6F6O2, were conducted on a single specimen by varying the temperature. Two polymorphs of HFPD were found to be enantiotropically related as phases (I) and (II), both in the space group P1. These structures contain closely related R44(20) sheets. A structure determination was completed on form (Ia) at 283,K. Form (Ia) was then supercooled below the phase transition temperature at 279 to 173,K to give form (Ib) for a second structure determination. Metastable form (Ib) was transformed by momentary warming and recooling to give form (II) for a third structure determination at 173,K. Form (II) transformed to form (Ic) upon warming to 283,K. Enantiotropic phase transitions between phases (I) and (II) were confirmed with X-ray powder diffraction and differential scanning calorimetry. Form (Ia) was found as a twin by nonmerohedry by a reflection in (011). This twinning persists in all phases described. Additional twinning was found after the phase (I) to phase (II) transformation. These two additional twin components are related to the first pair by a 180° rotation about the (012) plane. This latter pair of twins persisted as the specimen was warmed back to form (Ic) at 283,K. [source] Twinning and structure of Eu0.6Sr0.4MnO3ACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2006Nicola Rotiroti The crystal structure of europium strontium manganese trioxide, Eu0.6Sr0.4MnO3, has been refined using a multiply twinned single crystal containing six twin components. The MnO6 octahedra show Jahn,Teller distortions with nearly fourfold symmetry, but the octahedral tilting scheme reduces the crystal symmetry to orthorhombic (space group Pbnm). The refinement of site occupancies and the analysis of difference Fourier maps show that the Eu3+ and Sr2+ cations occupy different crystallographic positions with eightfold and twelvefold coordination, respectively. [source] | ||||||||||