Jerry A. Cowen

New Insight into the Nature of Cu(TCNQ): Solution Routes to Two Distinct Polymorphs and Their Relationship to Crystalline Films That Display Bistable Switching Behavior

Syntheses and characterization of two polymorphs of Cu(TCNQ) have been carried out and the results correlated to films of the materials. Reactions of CuI with TCNQ or [Cu(CH3CN)4][BF4] with TCNQ- lead to blue-purple needles of Cu(TCNQ) phase I (1). A slurry of this kinetic product in CH3CN yields a second crystalline phase of Cu(TCNQ), phase II (2), which exhibits a platelet morphology. Powder X-ray diffraction and scanning electron microscopy data revealed that both phases are present in films of Cu(TCNQ) formed by oxidation of Cu foil by TCNQ in CH3CN. X-ray photoelectron spectra of the two phases are indistinguishable from each other and are indicative of the presence of Cu(I). Single-crystal X-ray studies were undertaken on very small crystals of the two samples, the results of which reveal that subtle geometrical changes for the nitrile arrangements around the four-coordinate Cu(I) centers lead to major changes in the architectural framework of the polymers. Phase I was indexed in the tetragonal crys...

Electron spin-lattice relaxation in molecular crystals: S = 12

Abstract The results of an extensive investigation of electron relaxation in radiation-damaged organic materials are presented. The recovery of electron magnetization following pulsed saturation is determined (a) by Heisenberg exchange or molecular reorientation-induced spectral diffusion (cross relaxation) and (b) by the spin—lattice relaxation (SLR) processes characteristic of the material examined. Spectral diffusion quenching procedures were employed to minimize the effects of (a), thus permitting measurement of SLR times. Local mode effects, torsional oscillations and/or exchange interactions between radicals, as well as direct and Raman spin—phonon processes contribute to the observed temperature dependence of the SLR times for the approximately twenty radical systems studied. The frequently observed quadratic temperature dependence of the SLR rates is particularly noteworthy. Two mechanisms that give rise to such behavior have been explored theoretically and are discussed briefly.

Electron spin-lattice and cross relaxation in irradiated malonic acid☆

Abstract Electron spin-lattice and cross relaxation processes in X-ray and electron irradiated single crystals of malonic acid have been investigated over the temperature range 1.1° to 300°K as a function of radiation dosage and as a function of thermal aging. The recovery of the longitudinal magnetization following pulsed saturation is due to cross relaxation among the radicals R2⋯X2, R⋯X2, and R⋯X2CO (where R = COOH or COOD and X = H or D) and to the characteristic spin-lattice relaxation rates of these radicals. The spin-lattice relaxation times in these radicals appear to be determined by phonon modulation of the spin-orbit coupling rather than of the hyperfine interaction.

Ln2Al3Si2 (Ln = Ho, Er, Tm): New silicides from molten aluminum - Determination of the Al/Si distribution with neutron crystallography and metamagnetic transitions

Highly metallic compounds with a quasi-one-dimensional structure, the new ternary compounds Ln2 Al3 Si2 (Ln=Ho, Er, Tm) are synthesized in molten aluminum from lanthanoid and silicon as reagents. Their structures show a formally [Al3 Si2 ]6- framework that contains infinite Al zigzag chains and Si-Si dimers and accommodates rows of Ln3+ ions in parallel tunnels. The compounds exhibit metamagnetic transitions at high magnetic fields.

Ln2Al3Si2 (Ln = Ho, Er, Tm): neue Silicide aus Aluminiumschmelzen – Bestimmung der Al/Si-Verteilung mit Neutronenkristallographie und metamagnetische Übergänge

Ausgepragt metallische Verbindungen mit einer quasi-eindimensionalen Struktur sind die ternaren Verbindungen Ln2Al3Si2 (Ln = Ho, Er, Tm), die in Aluminiumschmelzen ausgehend von metallischem Lanthanoid und Silicium synthetisiert wurden. In ihnen liegt ein formales [Al3Si2]6−-Gerust aus unendlichen Al-Zickzackketten und Si-Si-Dimeren vor, das Reihen von Ln3 +-Ionen in parallelen Tunneln beherbergt. Die Verbindungen weisen bei hohen Magnetfeldstarken metamagnetische Ubergange auf.

The Use of Organic Acceptors as Ligands for Paramagnetic Metal Centers: a New Spin on Charge-Transfer Solids

Abstract Low temperature solution routes are being used to prepare solids with organic acceptors directly coordinated to paramagnetic transition metals. This research is currently focused on the elaboration of “hybrid” solids that contain TCNQ radicals. Magnetic and conductivity data regarding the nature of Cu(TCNQ) and several M(TCNQ)2 compounds will be summarized and discussed.