D. Weigel

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.

Evolution des tenseurs de dilatation thermique en fonction de la temperature. I. Loi generale d'evolution de la symetrie du tenseur

Abstract In studying the variation of the symmetry of the thermal expansion tensor as a function of temperature, we first define a measure of this symmetry, called aspherism index . Then we analyze experimental results of thermal expansion measurements obtained from various authors, for a set of 8 crystallized solids which show order-disorder phase transitions and for a set of 38 solids which do not. A law relating to variation of the aspherism index with respect to temperature is given.

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.

Introduction to description of phase diagram of solid wüstite: II. Structural review

Abstract A review of the most significant studies achieved by X-ray and electron diffraction and by Mossbauer effect gives descriptions of the structure of wustite. It is roughly a NaCl type structure with Fe II octahedral vacancies. Fe III on tetrahedral interstitial sites are necessarily associated with vacancies on octahedral sites. The most likely clusters (vacancies/interstitials) at high temperature would be ( 4 1 ) and some larger clusters built from ( 4 1 ). Some imperfect superlattices are also described, principally those found after quenching. They are usually cubic with a parameter that is a multiple of the parameter of the NaCl type structure.

Report of a Subcommittee on the Nomenclature of n-Dimensional Crystallography. II. Symbols for arithmetic crystal classes, Bravais classes and space groups

The Second Report of the Subcommittee on the Nomenclature of n-Dimensional Crystallography recommends specific symbols for R-irreducible groups in 4 and higher dimensions (nD), for centrings, for Bravais classes, for arithmetic crystal classes and for space groups (space-group types). The relation with higher-dimensional crystallographic groups used for the description of aperiodic crystals is briefly discussed. The Introduction discusses the general definitions used in the Report.

Modèle de décomposition thermique des bioxydes de plomb: rôle des protons, relation avec la structure de l'oxyde pseudocubique PbO1.57

Abstract The thermal decomposition of lead dioxides α-PbO2 and β-PbO2 is explained according to a complex model; postulating the existence of Pb24O48 and Pb24O40(OH)8 groups in the initial dioxide, it is possible to explain the various steps of the thermal decomposition from Pb24O44(OH)4 to β-PbO in relation with the various structures of these oxides. Structural data of the pseudocubic oxide PbO1.57 are used to justify this model; PbO1.57 appears as a juxtaposition of Pb24O36 and Pb24O40 zones with similar structures. The influence of the grain size and the speed of heating are also explained.

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Abstract Several samples of Pb 3 O 4 have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase ( T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb 3 O 4 exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).

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.

Crystal families and systems in higher dimensions, and geometrical symbols of their point groups. II. Cubic families in five- and n-dimensional spaces.

This paper is devoted to the study of the crystal families with cubic symmetries and to the mathematical construction of all their point-symmetry groups. The mono cubic crystal families of n-dimensional space (E(n)) are defined and a list of these families is given for spaces E4, E5, E6 and E7 with the Weigel-Phan-Veysseyre (WPV) symbols of their holohedries. The cubic and iso cubic crystal point-symmetry groups of space E(n) are also defined together with their properties and their WPV symbols. Some examples of these point groups are given. The 16 point groups of the three isomorphic mono cubic crystal families, the cubic-al family of space E4 (No. 19), the cube oblique (or cube parallelogram) family of space E5 (No. XVIII) and the triclinic cubic family of space E6 (No. 21) are listed. All the WPV symbols of the point-symmetry groups of all the mono cubic crystal families of space E5, i.e. the cube rectangle family (No. XXII), the cube square family (No. XXVI) and the cube hexagon family (No. XXVII), are given together with an explanation of the mathematical construction of these point-symmetry groups. All the di cubic crystal families of spaces E6, E7 and E8 are predicted and the symbols of their holohedries are given. Finally, some tri cubic crystal families of spaces E9, E10 and E11 are listed.

Crystallography, geometry and physics in higher dimensions. VII. The WPV symbols of the 38 cyclic crystallographic point symmetry groups in the five-dimensional space %E5

This paper is the first of a series of three devoted to crystallography in the five-dimensional space \bb E5. The 38 types of point symmetry operations (PSO for short) are described i.e. 19 types of PSO+s or rotations and 19 types of PSO-s or improper rotations; each of them generates a cyclic point group. A WPV (Weigel, Phan, Veysseyre) symbol is given both to the PSOs and to the cyclic groups. There is a generalization of the well known symbols of \bb E3. For instance, \bar 6 is the symbol of a point group of \bb E3 (and \bb E4), and \bar 6 has application in \bb E5 (and \bb E6); but new symbols such as \bar {\bar 6}, \bar {\bar 6} \bar {\bar 6} are also required.