Features of the oxidative polymerization of aminothiazole in the presence of iron(III) chloride

A new conjugated polymer with semiconducting characteristics-polyaminothiazole-was obtained by oxidative polymerization of 2-aminothiazole in the presence of iron(III) chloride. A reaction scheme is proposed on the basis of the data from IR, ESR, and electronic spectroscopy. It involves the formation of an intermediate aminothiazole-FeCl₃ complex with coordination of the initiator at the nitrogen atom and subsequent oxidation of another molecule of aminothiazole by this complex with the formation of a radical-cation, leading to polymerization. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 91–95, March–April, 2007.     

Magnetostriction of Hexagonal HoMnO<Subscript>3</Subscript> and YMnO<Subscript>3</Subscript> Single Crystals

We report on the magnetostriction of hexagonal HoMnO₃ and YMnO₃ single crystals in a wide range of applied magnetic fields (up to H = 14 T) at all possible combinations of the mutual orientations of magnetic field H and magnetostriction ΔL/L. The measured ΔL/L(H, T) data agree well with the magnetic phase diagram of the HoMnO₃ single crystal reported previously by other authors. It is shown that the nonmonotonic behavior of magnetostriction of the HoMnO₃ crystal is caused by the Ho³⁺ ion; the magnetic moment of the Mn³⁺ ion parallel to the hexagonal crystal axis. The anomalies established from the magnetostriction measurements of HoMnO₃ are consistent with the phase diagram of these compounds. For the isostructural YMnO₃ single crystal with a nonmagnetic rare-earth ion, the ΔL/L(H, T) dependences are described well by a conventional quadratic law in a wide temperature range (4–100 K). In addition, the magnetostriction effect is qualitatively estimated with regard to the effect of the crystal electric field on the holmium ion.     

Conductivity and persistent photoconductivity in GaAs epitaxial films containing single and double delta-doped layers

Electrophysical parameters of single and double delta-doped layers in GaAs epitaxial films grown by the metal-organic chemical vapor deposition have been systematically investigated in the temperature range of 4.2 to 300 K. The 2D electron gas density distribution is affected by the overlap of wave functions in neighboring quantum wells, as a result of which the peak on the curve of the Hall mobility in the 2D electron gas versus the separation between the quantum wells shifts. The persistent photoconductivity in delta-doped layers is due to the change in the surface potential caused by the neutralization of the negative charge of surface states by photoexcited holes. A method for comparing delta-doped layers grown under different conditions at different depths from the sample surface has been suggested. Zh. éksp. Teor. Fiz. 113, 693–702 (February 1998)     

Crystal structure and electronic spectrum of SnS2

It is shown that layered metal dichalcogenides are quasi-one-dimensional molecular crystals and form a new class of crystal structures — molecular close packed. Since the minimum structural unit in these crystals is a monomolecular layer, using the symbols employed in atomic close packing to describe them gives a mistaken representation of their structure and symmetry. A new system of notation is proposed which provides complete and exact information about the ordering of the atomic layers in different polytype modifications and about their symmetries. It is found that in molecular close packing and, especially, in tin disulfide, there is not one (as in atomic close packing), but two, simplest structures, 1T and 1H, containing one molecule each in a unit cell and, therefore, two series of superlattices constructed on their basis. An energy model is constructed for the natural superlattice in tin disulfide crystals and the electronic spectra of the 2H, 4H, and 9R polytype modifications are calculated in the Kronig-Penney approximation with rectangular potential barriers as superstructure perturbations of the simplest structures. These results make it possible to explain the observed complicated dependence of the band gaps of these crystals on polytype structure. Fiz. Tverd. Tela (St. Petersburg) 40, 1712–1718 (September 1998)     

Tunneling resonances in structures with a two-step barrier

Electron transport through an asymmetric heterostructure with a two-step barrier N⁺GaAs/N⁻GaAs/Al_0.4Ga_0.6As/Al_0.03Ga_0.97As/N⁻GaAs/N⁺GaAs was investigated. Features due to resonance tunneling both through a size-quantization level in a triangular quantum well, induced by an external electric field in the region of the bottom step of the barrier (Al_0.03Ga_0.97As layer), and through virtual levels in two quantum pseudowells of different width are observed in the tunneling current. The virtual levels form above the bottom step or above one of the spacers (N⁻GaAs layer) as a result of interference of electrons, in the first case on account of reflection from the Al_0.4Ga_0.6As barrier and a potential jump at the Al_0.03Ga_0.97As/N⁻GaAs interface and in the second case — from the Al_0.4Ga_0.6As barrier and the potential gradient at the N⁻GaAs/N⁺GaAs junction, reflection from which is likewise coherent. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 814–819 (25 May 1998)     

Regularities of the formation of polytype structures in layered metal dichalcogenides

Possible sequences of alternating triple layers X-M-X in MX₂ dichalcogenides (where M is a metal and X is a chalcogen) that are consistent with the close packing conditions are considered, and the structure and symmetry of the unit cells of the main polytypic modifications of these compounds are determined. Reasoning from the molecular structure of the crystals and the fact that their smallest structural unit is an MX₂ molecule, the notation of each triple layer can be represented in a reduced form in which only the position of the central metal atom and the orientation of the molecule are specified. In the proposed notation, allowances are made for two types of coordination of the metal atom in the molecule, namely, the octahedral coordination, in which the nearest X layers occupy different positions and form close packing, and the trigonal prismatic coordination, in which both X layers occupy identical positions. The use of the reduced (molecular) form of the notation significantly simplifies the analysis of the polytype structures and makes it possible to distinguish several structural groups differing in structure and symmetry among the crystals with unit cells including two, three, and four layers.     

On the electron mobility of δ layers in the presence of diamagnetic “ejection” of size-quantization levels

The Hall mobility of electrons is investigated as a function of the population of size-quantization subbands in the two-dimensional electron gas of a δ-doped layer in GaAs with constant total electron density N _s =3.2×10¹² cm⁻² (three initially filled subbands) at T=4.2 K. The population of the subbands is varied by diamagnetic “ejection” of size-quantization levels (i.e., pushing them over the Fermi level) by a magnetic field oriented parallel to the plane of the δ-doped layer. The measurements are made in magnetic fields making small angles (5°) with the plane of the doping. The magnetic field component normal to the plane was used to measure the Hall mobility and density. It is found that the measured Hall mobility as a function of the ejecting magnetic field has a distinct maximum. This maximum is due to an increase in the electron mobility in the first subband (the ground subband is assigned the index 0) and electron redistribution between subbands with in increasing ejecting magnetic field parallel to the plane of the δ layer. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 704–708 (10 December 1997)