Intrinsic nanostructure in Zr2-xFe4Si16-y(x = 0.81, y = 6.06).

We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr2-xFe4Si16-y(x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric ß-FeSi2-derived slabs are separated by Zr-Si planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these Zr-Si layers consist of 3.5 nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in x-ray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2µB or larger down to 1.5 K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat γ = C/T suggests that quasi-particle masses are not particularly strongly enhanced. The onset of superconductivity at Tc ≃ 6 K results in a partial resistive transition and a small Meissner signal, although a bulk-like transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nano-sized superconducting grains are separated by a non-superconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a potentially rich source of new bulk superconductors.

Vibration-assisted electron tunneling in C140 transistors.

We measure electron tunneling in transistors made from C(140), a molecule with a mass-spring-mass geometry chosen as a model system to study electron-vibration coupling. We observe vibration-assisted tunneling at an energy corresponding to the stretching mode of C(140). Molecular modeling provides explanations for why this mode couples more strongly to electron tunneling than to the other internal modes of the molecule. We make comparisons between the observed tunneling rates and those expected from the Franck-Condon model.

Evaluation of compounds for insecticidal activity on adult mosquitos. 1. Responses of adult mosquitos to some new insecticides.

Responses shown by female mosquitos to topical applications of solutions, to deposits from water-dispersible-powder formulations, or to exposure to the vapour from such deposits, are described for a number of compounds that have shown promise as residual contact insecticides.The behaviour of Anopheles stephensi females in laboratory tests indicates that departures of mosquitos from treated buildings, and their survival rates, are likely to be increased as a result of irritation and consequent activity after only a brief period of contact with phthalthrin (OMS-1011) and other synthetic pyrethroids but not with dieldrin (OMS-18) and lindane (OMS-17) or the carbamates and organophosphorus compounds tested. Mosquitos were not disturbed by exposure to the vapour from deposits of any of the compounds.The rates of action of the different compounds following their application in solution directly to the insect cuticle or contact of the mosquitos with deposits on plywood and plaster emphasize the importance of the lipid-solubility and partitioning properties of an insecticide in determining its passage from a surface deposit into, and through, the cuticle to the site of action.

Evaluation of compounds for insecticidal activity on adult mosquitos. 2. Toxicity to adult mosquitos and the physical properties of some new carbamates.

The N,N-dimethylcarbamates are much less toxic and much more volatile than the corresponding N-methylcarbamates. N-acylation of N-methylcarbamates is also generally accompanied by a loss in intrinsic toxicity to adult mosquitos by topical application, but the reduction in toxicity may be relatively small and in some N-acetyl derivatives may not occur at all. N-acylation is also accompanied by changes in physical properties such as lipid solubility and partition coefficient, which influence contact toxicity, and volatility, which affects residual activity. In the series of compounds examined, contact toxicity is maximal in the N-acetyl derivatives but their volatility is often so high that they lack the persistence required of residual insecticides. This does not apply, however, to the N-acetyl derivatives of OMS-597 and OMS-708 because the volatility of the parent compound is so low that some degree of persistence can be sacrificed to obtain some improvement in contact activity.OMS-1064 is the only N-chloroacetyl derivative tested so far that shows promise as a residual insecticide.

Evaluation of compounds for insecticidal activity on adult mosquitos. 3. Properties of some N-methylthiolcarbamates.

N-methylcarbamates derived from thiophenols possess negligible insecticidal activity against the mosquitos Anopheles stephensi and Aedes aegypti. The probable basic reason for the inactivity is that these compounds are much less efficient inhibitors of cholinesterase than the oxygen analogues.

Evaluation of compounds for insecticidal activity on adult mosquitos. 4. Insecticidal activity and physical properties of some halogenated organophosphorus esters.

The effects of changes in structure upon toxicity to adult mosquitos and upon physical properties which are important in determining contact effectiveness and residual activity are described for a group of O,O-dialkyl O-halogenated-phenyl phosphates, phosphorothionates and phosphamido(thio)ates. This group includes such established insecticides as fenchlorphos, bromophos and iodofenphos.

Evaluation of compounds for insecticidal activity on adult mosquitos. 5. Toxicity to adult mosquitos and residual properties of some pyrethroids.

New pyrethrin-like compounds are compared with earlier synthetic pyrethroids and natural pyrethrins for intrinsic toxicity to adult mosquitos and for residual contact activity. Two of the compounds are at least as toxic as pyrethrin I to female Anopheles stephensi and Aedes aegypti. Residues of these compounds are very persistent in the dark or in very subdued lighting but they decompose on exposure to normal intensities of daylight and rapidly lose their insecticidal activity.