C2-isomer of [Pd(tfd)]6[tfd is S2C2(CF3)2] as its benzene solvate: a new member of the small but growing class of homoleptic palladium(II) monodithiolenes in the form of hexameric cubes

The title compound, hexakis[μ3-1,2-bis(trifluoromethyl)ethene-1,2-dithiolato]-octahedro-hexapalladium(II), [Pd(C4F6S2)]6, crystallizes as its benzene solvate, [Pd(tfd)]6·2.5C6H6, where tfd is the dithiolene S2C2(CF3)2. The molecular structure of [Pd(tfd)]6is of the hexametallic cube type seen previously in three examples of hexameric homoleptic palladium monodithiolene structures. All structures have in common: (a) the cluster closely approximates a cube containing six PdIIatoms, one at the centre of each cube face; (b) 12 S atoms occupy the mid-points of all 12 cube edges, providing for each PdIIatom an approximately square-planarS4environment; (c) each S atom is part of a dithiolene molecule, where the size of the dithiolene ligand necessitates that only sulfur atoms on adjacent cube edges can be part of the same dithiolene. This general cube-type framework has so far given rise to two isomeric types: anS6-symmetric isomer and aC2-chiral type (two isomers that are enantiomers of each other). The structure of [Pd(tfd)]6is of theC2-type. Out of the 12 CF3groups, three are rotationally disordered over two positions. Further, we answer the question of whether additional, previously undiscovered, isomers could follow from the cube rules (a) through (c) above. An exhaustive analysis shows that no additional isomers are possible and that the list of isomers (oneS6isomer, twoC2enantiomers) is complete. Each isomer type could give rise to an unlimited number of compounds if the specific dithiolene used is varied.

Precipitation of Gamma-Prime and Gamma-Double Prime Coherent Phases in Three-Phase Ni-V-Si Alloys

Phase separation of γ (A1) supersaturated solid solution into A1, γ’ (L12) and γ” (D022) phases was investigated in two Ni-rich Ni-V-Si ternary alloys by means of transmission electron microscopy. When the alloys are annealed at 1073K, two different sequences of the phase separation are observed, depending on the chemical composition of the alloy: In Ni-17.0at%V-6.9at%Si alloy (A) at the D022 corner of three-phase field, first many D022 particles precipitate aligning along the <110> direction of the matrix and the so-called chessboard pattern is observed, followed by the formation of L12 phase at the interface between D022 and A1 phases. In Ni-12.1at%V-11.3at%Si alloy (B) at the L12 corner of the Gibbs triangle, cuboidal L12 particles precipitate arranging along the <100> direction, and then D022 phase is formed. As the phase separation proceeds, a selective growth/formation of the third phase (L12 in the alloy A, D022 in the alloy B) occurs: In the alloy A, L12 phase grows into D022 particle inside along the diagonal direction of D022 cube which is parallel to the a-axis of D022 tetragonal phase. In the alloy B, D022 forms on the {100} cube face of cuboidal L12 particle, arranging the c-axis of D022 perpendicular to the {100} cube face of L12 phase. As a result of such a selective growth/formation, the first phase D022/L12 is split off into two particles, which results in the formation of laminated structure consisting of D022 and L12 phases. The selective growth/formation is considered to occur so as to maintain the less elastic strain state.

Pressure crack-figures on diamond faces II. The dodecahedral and cubic faces

The study of pressure figures on diamond is extended to the cubic and dodecahedral faces. The formation of cracks on a sawn but unpolished dodecahedral (two-point) face is first studied. The loads required for the initiation of cracks, using a diamond ball, were between 7 and 24 Kg. Two different types of cubic face were available, one on a natural cubic boart stone, the other a polished approximation to a cube face secured by truncating an octahedral stone. The loads required to initiate cracks were of the order 20 to 30 Kg. All the cracks observed were specifically oriented in accordance with crystallographic expectations of easy cleavage directions. The cracks were accompanied by permanent surface distortions which were studied by multiple-beam interference methods. The permanent distortions broadly resemble those found in part I for octahedral faces. There is one important difference in that for both the cube faces studied, the surface level within the perimeter of the ring crack is appreciably depressed, being some 800 Å below the outer undisturbed level. This is considered to offer further evidence for the existence of plastic flow in diamond at room temperature. For the polished cube face studied observations could also be made on the accompanying internal cracking effects within the body of the crystal.

The mode of formation of Neumann bands. Part I.–The mechanism of twinning in the body-centred cubic lattice

1. The existence of Neumann lamellæ as a characteristic feature of hexahedral meteoric iron and of the kamacite of octahedral meteorites has been known for many years. The traces of these lamellæ upon polished and etched surfaces were at first regarded as Widmanstätten figures; but it was shown by Neumann that they were distinct from such figures, that they were character­ istic of single cubic crystals, and that their outcrops upon a cube face, which he determined, were inconsistent with the assumption that they were octahedral lamellæ. Neumann, Tschermak and other mineralogists inferred from the geometrical relations between the outcrops of the bands that they were a consequence of an interpenetrating-cube twin structure within the meteorite, of a type known to occur in various minerals, e. g ., in fluor spar.

The mode of formation of Neumann bands. Part II.–The evidence that the bands are twins

1. Any discussion of the significance of Neumann bands must involve the geometrical relationships between them and the matrix. The orientation of the cube lattice of a meteorite having been determined by X-ray or other methods, a section parallel to a cube face can be cut, polished and etched, and the angles which the traces of the Neumann bands make with the sides or the diagonals of the cube face can be measured. If the bands are parallel to the twelve planes of the icositetrahedron {112}, their traces will lie in the directions shown in fig. 5, PVUT representing the orientation of the cube face. The spatial relationships of the planes producing such {112} traces can be visualised by means of fig. 6, an isometric projection of a cube, in which the planes producing the traces shown in fig. 5 are indicated by their traces on three mutually perpendicular planes, e. g ., PVUT (010), PTQX (001) and PXWV (100).