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Monoclinic Distortion (monoclinic + distortion)
Selected AbstractsX-ray diffraction study of the phase transition of K2Mn2(BeF4)3: a new type of low-temperature structure for langbeinitesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001A. Guelylah The potassium manganese tetrafluoroberyllate langbeinite compound has been studied in the temperature range 100,300,K. Using DSC measurements, a phase transition has been detected at 213,K. The space group of the low-temperature phase was determined to be P1121 using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled with respect to the prototypic P213 cubic phase. Lattice parameters were determined by powder diffraction data [a = 10.0690,(8), b = 20.136,(2), c = 10.0329,(4),Å, , = 90.01,(1)°]. A precise analysis of the BeF4 tetrahedra in the low-temperature phase shows that two independent tetrahedra rotate in opposite directions along the doubled crystallographic axis. A symmetry mode analysis of the monoclinic distortion is also reported. This is the first report of the existence of such a phase transition in K2Mn2(BeF4)3 and also of a new type of low-temperature structure for langbeinite compounds. [source] X-ray structure determination of the monoclinic (121,K) and orthorhombic (85,K) phases of langbeinite-type dithallium dicadmium sulfateACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2000A. Guelylah The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl2Cd2(SO4)3 have been determined at 121 and 85,K, respectively, by X-ray diffraction. A precise analysis of these structures shows the existence of some differences compared to langbeinites of type II. The monoclinic structure differs very little from the high-temperature cubic structure and the distortion relating the monoclinic structure to the cubic one is very small. SO4 tetrahedra seem to rotate under orthorhombic symmetry in the monoclinic phase. A symmetry distortion analysis of the ferroelectric monoclinic distortion discloses the importance of the secondary modes with orthorhombic symmetry, especially for the O atoms of the SO4 groups. [source] GeTe2O6, a germanium tellurate(IV) with an open frameworkACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009Nefla Jennene Boukharrata The structure of an already evidenced but still uncharacterized GeTe2O6 phase consists of isolated GeO6 octahedra connected via isolated TeO3 units. The germanium cations occupy a site with symmetry. The Te and O atoms are in general positions of the P21/n space group. This structure corresponds to a new type of tetravalent tellurate and is different from other AB2X6 structures in which the B cation presents a stereochemically active electronic lone pair. It derives from the pseudo-hexagonal MI2O6 (M = Mg, Mn, Co and Fe) type by a strong monoclinic distortion caused by the much smaller size of Ge4+ compared with the divalent M cations. [source] CdBiO2Cl: synthesis and powder structure solutionACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2001Sergei D. Kirik The title compound, cadmium bismuth dioxide chloride, CdBiO2Cl, was obtained as a white powder by reaction of solid BiOCl with CdO at 973,K. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques, including direct methods for atom location and Rietveld fitting for the final refinement. Being monoclinic, the crystal structure can be related to tetragonal Sillen layered phases. The main structural elements present are CdBiO2+ pleated metal,oxygen layers alternating with Cl layers along the c axis, whereas along the b axis, all atoms are on mirror planes. The formation of a strong Cd,Cl bond draws the layers together, causing layer deformation and a monoclinic distortion in the layer arrangement. [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] Sildenafil citrate (Viagra) complexes with bivalent ionsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 1 2006Petr Melnikov Abstract The interaction of Ca2+ -ions with sildenafil citrate (Viagra) leads to the precipitation of a new polymorph variety of sildenafil base. Under the same conditions, Mg2+, Zn2+, and Cd2+ ions form structurally related crystalline complexes of the composition Me2+C28H34N6O11S. Lattice parameters have been determined showing that magnesium compound belongs to an orthorhombic system, while the zinc and cadmium compounds are its monoclinic distortions. All three compounds are thermally stable, undergoing decomposition above 175°C with the consequent formation of carbonates Me2+CO3 and oxides. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:225,227, 2006 [source] | ||||||||||