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Orthorhombic Cell (orthorhombic + cell)
Selected AbstractsIncommensurate structure of Ca2Al2O5 at high temperatures , structure investigation and Raman spectroscopyACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2008Biljana Lazic A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca2Al2O5 transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca2Al2O5 were synthesized in an end-loaded piston cylinder press at 2.5,GPa and 1273,K. The diffraction pattern observed at 1090,(10),K by in situ single-crystal diffraction experiments can be indexed by an I -centred orthorhombic cell and a modulation wavevector of q = 0.595,(1)c*. A (3,+,1)-dimensional model in superspace group Imma(00,)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO6] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO4] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca2Al2O5 is isotypic to the modulated high-temperature modification of Ca2Fe2O5. A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075,(10),K; on cooling, satellite reflections can be observed down to 975,(10),K. The characterization of Ca2Al2O5 is completed by Raman spectroscopy, including a partial interpretation of the spectra. [source] Structure of the defect perovskite Ce1/3NbO3: a redetermination by electron and neutron powder diffractionACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2000C. Bridges The crystal structure of the defect perovskite Ce1/3NbO3, cerium niobium oxide, has been re-examined by neutron powder and electron diffraction. The results of a powder neutron Rietveld refinement indicate that the structure is monoclinic: space group P2/m with Z = 4, a = 5.5267,(3), b = 7.8824,(2), c = 5.5245,(3),Å, , = 90.294,(1)°, V = 240.67,(2),Å3 at 298,K with ,2 = 2.570. Previous reports have described the Ce1/3NbO3 structure in a smaller (V/2) orthorhombic cell based solely upon X-ray powder diffraction data. The presence of weak reflections in the electron diffraction pattern provides conclusive evidence for a monoclinic superstructure of the orthorhombic cell. While these superlattice reflections are barely detectable with X-ray radiation, they are clearly visible in the neutron diffraction experiments. The superlattice reflections are shown to arise from a tilting of the NbO6 octahedra which results in the reduction of symmetry from orthorhombic to monoclinic. It is also found that the Ce3+ and Nb5+ cations are displaced from the centres of their respective polyhedra to accommodate the bond-valence requirements of the crystal structure. It is likely that distortions of this type are present in other Ln1/3NbO3 and Ln1/3TaO3 defect perovskites. [source] Pseudomerohedral twinning, pseudosymmetry and complex hydrogen-bonded sheets in 2-methoxy-4-(pyrrolo[1,2- a]quinoxalin-4-yl)phenolACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2010Juan C. Castillo The title compound, C18H14N2O2, crystallizes as a nonmerohedral twin, but the structure can be satisfactorily refined as a merohedral twin, in which a monoclinic unit cell with a cell angle , close to 90° emulates a metrically orthorhombic cell. The two molecules in the asymmetric unit are very similar in structure and they are related by an approximate pseudo-screw axis. The molecules are linked into complex sheets by a combination of two O,H...N hydrogen bonds and four C,H...O hydrogen bonds. [source] Isolation, crystallization and preliminary X-ray analysis of a methanol-induced corrinoid protein from Moorella thermoaceticaACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 5 2005Weihong Zhou A corrinoid protein was induced and overexpressed in methanol-grown cells of the thermophilic anaerobic bacterium Moorella thermoacetica. The protein was purified from cytosolic extracts. After screening for crystallization conditions and optimization, crystals were obtained that diffracted strongly on a rotating-anode X-ray source. A diffraction data set was collected and processed including reflections to 1.9,Å resolution. Reflections were indexed in a primitive orthorhombic cell with unit-cell parameters a = 55.69, b = 62.74, c = 34.54,Å. N-terminal amino-acid sequencing indicates that the crystals contain a C-terminal fragment of the protein. [source] Intermetallics as Zintl Phases: Yb2Ga4Ge6 and RE3Ga4Ge6 (RE=Yb, Eu): Structural Response of a [Ga4Ge6]4, Framework to Reduction by Two ElectronsCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2004Marina A. Zhuravleva Abstract Two new intermetallic compounds, Yb2Ga4Ge6 and Yb3Ga4Ge6, were obtained from reactions in molten Ga. A third compound, Eu3Ga4Ge6, was produced by direct combination of the elements. The crystal structures of these compounds were studied by single-crystal X-ray diffraction. Yb2Ga4Ge6 crystallizes in an orthorhombic cell with a=4.1698(7), b=23.254(4), c=10.7299(18) Å in the polar space group Cmc21. The structure of RE3Ga4Ge6 is monoclinic, space group C2/m, with cell parameters a=23.941(6), b=4.1928(11), c=10.918(3) Å, ,=91.426(4)° for RE=Yb, and a=24.136(2), b=4.3118(4), c=11.017(1) Å, ,=91.683(2)° for RE=Eu. The refinement [I>2,,(I)] converged to the final residuals R1/wR2=0.0229/0.0589, 0.0411/0.1114, and 0.0342/0.0786 for Yb2Ga4Ge6, Yb3Ga4Ge6, and Eu3Ga4Ge6, respectively. The structures of these two families of compounds can be described by a Zintl concept of bonding, in which the three-dimensional [Ga4Ge6]n, framework serves as a host and electron sink for the electropositive RE atoms. The structural relation of RE3Ga4Ge6 to of Yb2Ga4Ge6 lies in a monoclinic distortion of the orthorhombic cell of Yb2Ga4Ge6 and reduction of the [Ga4Ge6] network by two electrons per formula unit. The results of theoretical calculations of the electronic structure, electrical transport data, and thermochemical and magnetic measurements are also reported. [source] | ||||||||||