J.F. Berar

New low‐temperature crystalline phase of ferrocene: Isomorphous to orthorhombic ruthenocene

A new low‐temperature phase of ferrocene Fe (C5H5)2, stable below 250 K, has been characterized by means of x‐ray diffraction on powder and differential scanning calorimetry experiments. Lattice of this phase is orthorhombic and its molecular packing is isomorphous to ruthenocence one, so configuration of ferrocene molecules is eclipsed (D5h symmetry). Experimental conditions to obtain this orthorhombic phase are discussed in connection with crystallite size.

Structural study of an MFI/naphthalene system by X-ray powder diffraction, 29Si MAS n.m.r, and energy-minimization calculations

Abstract Combining solid-state 29Si MAS n.m.r. studies, energy-minimization calculations using the atom—atom potential method (Buckingham and Lennard-Jones models), and X-ray profile-fitting structure refinements (Rietveld method with geometric constraints) shows that it is possible to locate the naphthalene (NPH) molecules sorbed at room temperature in a saturated silicalite-4 C10H8 complex. Interpretation of the n.m.r. spectrum and of the diffraction profile is consistent with orthorhombic Pnma framework symmetry. The entrapped sorbate molecules, which are localized at the channel intersections of the MFI framework, have their longer symmetry axis oriented parallel to the y-axis direction [010] and form infinite and loosely associated polymeric chains in the straight channels. The NPH does not enter the zigzag channels, where the presence of an additional extraframework water molecule could be seen. NPH adsorption provokes an elliptical deformation of the straight-channel sections (6.3 × 4.8 A). The zigzag channel sections remain almost circular. For the first time, an inversion in the a and b unit cell parameters is observed ( a b = 0.9984 ) and the NPH acronym is proposed for this deformation of the MFI framework. Time-resolved X-ray diffraction shows that at lower pore fillings (0 a b ratio is also observed for several other MFI/aryl systems.

Etude des phases ferroelastiques de Pb3V2O8 : Determination de la maille cristalline de la phase α ferroelectrique basse temperature

Abstract The cell parameters of the ferroelastic and ferroelectric phase of Pb 3 V 2 O 8 (T a = 7.460 (1) A , b = 6.191 (1) A , c = 9.348 (1) A and β = 116.63 (1) A . This monoclinic cell is characterized by Z = 2 and the space group is A2. Thermal expansion tensors have been measured versus temperature for the three phases α (T 360 K). The intermediate phase β is characterized by an important thermal expansion anisotropy and its molecular volume is higher than that of the two other phases α and γ.

Evolution des tenseurs de dilatation thermique en fonction de la temperature. II. Etude calorimetrique et cristallographique de la transition de phase ordre-desordre du thiophene chrome tricarbonyle C4H4SCr(CO)3

Abstract Benzenechromium tricarbonyl C 6 H 6 Cr(CO) 3 and thiophenechromium tricarbonyl C 4 H 4 SCr(CO) 3 are isomorphous at room temperature. We have measured, in the range 77–295°K, the variations of specific heat and of the tensor of thermal expansion for these two molecular compounds. The first one exhibits quite normal behavior; on the other hand, the second one exhibits a first order phase transition, monoclinic ⇆ triclinic at T = 185°K, associated with an order-disorder phase transition. Structural evolution of thiophene-chromium tricarbonyl, during phase transition, has been explained from the variation of its tensor of thermal expansion.

Powder diffraction for solid state reaction studies

Abstract A position sensitive detecting system for time resolved diffraction measurement is described. Investigation of the thermal decomposition of lime shows that quantitative results can be obtained studying solid state chemical reactions whose duration lasts about one second.

Thermodynamic and X-Ray Studies on Arene Chromium Carbonyls

Abstract The heat capacities of benzene chromium tricarbonyl and benzene chromium dicarbonyl thiocarbonyl have been investigated in the range 4-300 K, and the thermodynamic functions are reported. Results are compared with the Cp values calculated from spectroscopic data. Selenophene- and tellurophene-chromium tricarbonyl have been studied in the range 80-300 K by calorimetry and X-Ray diffraction. The unit cells of these last two complexes have been determined (orthorhombic, Z = 4) as well as the variations of the lattice parameters. No annomaly has been detected in the heat capacity curves of the four compounds. Molecular motions (ring reorientation) are discussed in the light of these results and those of Raman spectroscopic studies.

Synchrotron high resolution powder study of molecular packing in hydrate calcium stearate

Abstract The structure of (C18H35O2)2Ca,H2O was completely solved using synchrotron high resolution powder diffraction data. This work, which was long delayed because of severe overlapping in powder scans obtained on conventional diffraction instruments, was made possible by the use of synchrotron radiation and a suitable parallel beam geometry with a crystal analyzer. The unit cell is monoclinic, and consists in the stacking of four molecular layers, c being roughly 100 A, with space group P 2 1 c . The organization of atoms is quite original: the local packing mode is monoclinic, which is very scarcely encountered; the chain orientation parallel to b reverses in the vicinity of polar groups, which are concentrated near z = 1 4 and z = 3 4 planes; the organization of hydrogen bonds is also explained.

Etudes calorimetrique et radiocristallographique du cobaltocene et du ruthenocene entre 77 et 298 K

Two metallocenes M(C5H5)2, (M = Co, Ru), with an ordered structure at room temperature have been investigated by calorimetry and X-ray diffraction measurements between 77 and 300 K. They show no phase transition in this temperature range, in contrast to the disordered metallocers (M = Fe, Ni, Cr). Values of thermodynamic function and Crystallographic parameters are given for the temperature range studied.

Four-dimensional structure determination of the incommensurate modulated compound (Nd4Ca2)Ti6O20

The structure of the incommensurate modulated compound (Nd4Ca2)Ti6O20 has been analyzed on the basis of a four-dimensional description in the space group or S(1–2)2mc+e4 (a=3.832A, b=36.64A, c=5.436A, q∗=0.49a∗, Z=2). The modulation involves rotations of the TiO6 octahedra of the perovskite-like structure around the [010] direction, coupled with a substitutional modulation of the NdCa species. R-factors are respectively 0.087, 0.070 and 0.113 for all, main and satellite reflections(1699 observed intensities).

Structural investigation of the incommensurate modulated phase of BaMnF4

The incommensurate modulated structure of BaMnF4 has been studied at 100 K by X-Ray diffractometry and described on the concept of the four dimensional spacegroup, Wp21nb199 The final R-factor of the refinement is 0.062 for 3501 reflections. Atomic displacements are given and discussed. The modulation involves the rotation of the MnF6 octahedra about the a-axis and a correlated translation of the Ba-atoms in the plane (a,b). The analogy between the anisotropic temperature factors in the high temperature phase and the directions of the atomic displacements in the modulated phase has been discussed.