Single Crystalline Lattice Research Articles

Miscibility and isomorphic cocrystallization in blends of ferroelectric copolymers of vinylidene fluoride and trifluoroethylene

AbstractThe miscibility in binary blends of ferroelectric copolymers of vinylidene fluoride (VF2) and trifluoroethylene ( F3E) has been investigated over the full range of blend compositions for three different random copolymers containing 52, 65, or 73 mol % VF2. We found these copolymers miscible in the melt with each other but not with the parent homopolymer, PVF2. In thin solid films we demonstrated, for all blend compositions, the very rare phenomenon of isomorphic cocrystallization in both the ferroelectric and paraelectric phases of the 52 + 65 and the 65 + 73 mol % VF2 copolymers. However, the blend having the largest compositional difference (52 + 73 mol % VF2) was found to consist of two separate ferroelectric and paraelectric phases. These results demonstrate the very close chemical and structural similarities required for coexistence of two different macromolecules within a single crystalline lattice. Moreover, they allow us to probe the compositional limits for such similarity in both the ferroelectric and the paraelectric lattices. Phase diagrams of these blends have been constructed and are discussed. The Curie transition is shown to be an exceptionally sensitive probe of compatibility in the ferroelectric phase.

Properties of (Ge2)x(GaAs)1−x alloys grown by molecular beam epitaxy

Single-crystalline (Ge2)x(GaAs)1−x alloys have been grown by molecular beam epitaxy on GaAs (100) substrates at substrate temperatures of 550 and 430 °C. The structure of these alloys has been studied using transmission electron microscopy. We observe that growths done at 550 °C show a compositional phase separation, within a structurally single-crystalline lattice, into Ge-rich and GaAs-rich domains whose sizes are of the order of 100 Å. Growths at 430 °C appear to be both single phase and single crystalline. Room-temperature Hall measurements on the single-phase alloys show them to be n type.

Radiationless transitions in p-terphenyl crystals doped with anthracene, tetracene and pentacene

The fluorescence spectra and lifetimes of anthracene, tetracene and pentacene molecules in a host p-terphenyl single crystalline lattice have been measured over the temperature range 10–300 K. At low temperatures the molecules occupy different sites in the host lattice: five for anthracene (O1, O2, O3, O4, O5), six for tetracene (O1, O2, A1, A2, A3, A4) and four for pentacene (O1, O2, O3, O4). The high-temperature fluorescence spectra are dominated by particular sites (O4/O5 for anthracene, A2/A4 for tetracene and O1/O2 for pentacene) and the temperature dependence of the fluorescence lifetime of these sites is recorded. Neither O4/O5 for anthracene nor A2/A4 for tetracene exhibit a marked temperature dependence whereas the lifetime of O1/O2 for pentacene decreases with increasing temperature with an activation energy of 60–90 cm–1. This temperature dependence is associated with an increased probability for intersystem crossing from O1/O2 to a triplet level (T2) of the pentacene molecule which is located at 16 960 ± 15 cm–1. For anthracene and tetracene T2 is inaccessible from all sites over the entire temperature range and for anthracene the short lifetime reflects the facile transfer of excitation from S1 to T1.

Characterization of crystalline filtrate tetanus toxin

Filtrate tetanus toxin has been crystallized in the form of a thin platelet. The crystallized material is very similar to the uncrystallized, purified toxin preparations with respect to amino acid composition, immunological properties, and toxicity. An electron microscopic examination of the thin crystals reveals that the crystals are mostly stacked together in random orientations to form Moire-type patterns. Optical diffraction analysis of these electron micrographs indicates that the crystals have a hexagonal symmetry with a two-dimensional reciprocal lattice constant of 1/150 A −1 . Computer processing of electron micrographs of stacked crystals allows the separation of Moire-type images and the reconstruction of the projected density of a unit cell in a single crystalline lattice. Refinement of the phases of the reflections in a single reciprocal lattice indicates that the crystals belong to the two-dimensional plane group p6, suggesting that each unit cell is composed of 6, or a multiple of 6, toxin molecules.

Optical Constants and Exciton States in KCl Single Crystals. I. The Low Temperature Properties

Optical measurements were carried out on the reflectivity and absorption tails of KCl single crystals over the region 6.0 to 12 ev in the temperature range 10 to 573°K. With use of these data, Kramers and Kronig analysis was made on reflectivity spectra for three different temperatures 10, 78 and 295°K. The analysed spectra of important optical constants are presented in this article, and their properties are discussed in order to clarify the excitonic picture in a single crystalline lattice. The result is analysed along the theories of Toyozawa and Elliot. It was found that the first exciton line shape and the plateau in the higher energy side of the step are explained by them. The two overlapping components of the first band can be well separated at low temperatures in the case of single crystals, and this permits us to determine an oscillator strength for each component as 0.27 and 0.375 for \(\left(\frac{3}{2},\frac{1}{2}\right)\)- and \(\left(\frac{1}{2},\frac{1}{2}\right)\)-transition, respectively. Attempts were made to express the spectrum by an analytic expression in the first exciton band region.