Electronic properties and phase transitions in low-dimensional semiconductors

We present the first review of the current state of the literature on electronic properties and phase transitions in TlX and TlMX2 (M = Ga, In; X = Se, S, Te) compounds. These chalcogenides belong to a family of the low-dimensional semiconductors possessing chain or layered structure. They are of significant interest because of their highly anisotropic properties, semi- and photoconductivity, non-linear effects in their I-V characteristics (including a region of negative differential resistance), switching and memory effects, second harmonic optical generation, relaxor behavior and potential applications for optoelectronic devices. We review the crystal structure of TlX and TlMX2 compounds, their transport properties under ambient conditions, experimental and theoretical studies of the electronic structure, transport properties and semiconductor-metal phase transitions under high pressure, and sequences of temperature-induced structural phase transitions with intermediate incommensurate states. Electronic nature of the ferroelectric phase transitions in the above-mentioned compounds, as well as relaxor behavior, nanodomains and possible occurrence of quantum dots in doped and irradiated crystals is discussed.Comment: 70 pages, 38 figure

Assessment of 2-0 resistivity structures using 1-0 inversion

ABSTRACT Schlumberger and Wenner array resistivity soundings over 2-D resistivity structures are interpreted using apparent resistivity pseudosections and cross-sections constructed from I-D inversions in order to determine the effectiveness of I-D interpretations over such structures. Cross-sections contoured from resistivities of inverted "layers" show distinct differences from the apparent resistivity pseudosections and may be used as interpretational aids. Contour lines in the cross-sections locate the horizontal interfaces of the 2-D structures quite well. The vertically oriented segments of the cross-section contours are relatively undistorted in the inversion process and are similar to the vertically oriented portions of contours in the apparent resistivity pseudosection. A simple, empirically determined formula is used to separate the sections into resistive and conductive zones and helps to define the geometry of the anomaly. In order to apply the formula, it is necessary to know whether the target is a relative conductor or a relative resistor. Except for the case of a square prism, the Schlumberger array appears to hold advantages over the Wenner in qualitatively assessing an anomaly. The primary drawback of the Wenner array is that its expanding potential electrodes create false anomalous zones and complicate interpretation. As might be expected, structures with long horizontal interfaces, i.e. those more nearly I-D, yield the most accurate interpretations

A crystalline H-bond cluster of hexafluaroisopropanol (HFIP) and piperidine : Structure determination by X ray diffraction

International audiencePiperidine and 1,1,1-3,3,3 hexafluoro-2-propanol (HFIP) have been co-crystallized and X-ray crystal structure has been explored. Single-crystal X-ray analysis displays the existence of hydrogen bonding aggregates through dimers 1 of the complex (one piperidine/two HFIP) where the heteroatoms form a six-center ring. In this cluster 1, each heteroatom (N, O) is multiple H-bond donor and acceptor. Surprisingly the strongest H-bond of the network is where HFIP acts as an acceptor from the amine. In this complex HFIP adopts a conformation different from that of HFIP aggregates. The supramolecular architecture is also based on discrimination between polar and hydrophobic parts that allows the alignment of molecules and the formation of parallel channels. NMR experiments show that strong interactions between piperidine and HFIP are maintained in solution

Covalently bonded infinite zigzag chain structure in a novel Zn(II) complex of 2.5-dihydroxy-1.6-benzenedicarboxylic acid

International audienceReaction of the disodium salt of 2,5-dihydroxy-1,6-benzenedicarboxylic acid (DHBDC) and zinc sulfate in aqueous solution at room temperature yields complex [Zn(H2O)2(DHBDC)]n. The coordination tetrahedron around Zn center exhibits a highly irregular geometry, two carboxylato oxygen atom and two water–ligand oxygen atoms occupying the coordination sites. Structural analyses of this complex reveal that the individual zero-dimensional units (0D) are covalently linked through their metal ends, forming infinite 1D zigzag chains. These chains, in turn, are engaged in 2D hydrogen bonding, resulting in a 3D supramolecular architecture

In vivo inhibition of nitric oxide synthesis by bisisothiouronium and bisguanidinium salts

Abstract The ability of two S,S'-(alkane-1,ω-diyl) bisisothiouronium dibromides, three N,N'-(alkane-1,ω-diyl) bis guanidinium dinitrates and N,N'-bis (3-guanidinopropyl)piperazine dinitrate to inhibit constitutive (i.e. endothelial and neuronal forms) and inducible forms of nitric oxide synthases has been evaluated in vivo. These compounds, synthesized by two of us (J.C.L. and C.S.), have been tested in vivo; they were administered simultaneously with an irritant (carrageenan λ) into the pleural cavity. The amount of nitrites collected 0.5 and 7 hours after this injection can be considered as an indicator of nitric oxide (NO) production. According to previous data, the first harvesting time can be related to activation of constitutive NO synthases and the second to activation of inducible NO synthases. These substances significantly inhibited nitrite production as did 2-methyl-2-thiopseudourea sulphate, previously described as a potent inhibitor of NO synthases and considered as the reference compound. The inhibiting effect varied according to the chemical structure of the compounds. Results were significantly different from controls at 0.5 h only with the S,S'-(octane-1,8-diyl) bisisothiouronium dibromide and the S,S'(nonane-1,9-diyl) bisisothiouronium dibromide at the highest concentration, N,N'-(heptane-1,7-diyl) bisguanidinium dinitrate and N,N'-bis (3-guanidinopropyl)piperazine dinitrate. At 7 h, all the results were significantly different from controls, with a major effect observed with N,N'-(heptane-1,7-diyl) bisguanidinium dinitrate. The most active substances exerted similar effects to the reference substance

Crystal structures and physico-chemical properties of Zn(II) and Co(II) tetraaqua(3-nitro-4-hydroxybenzoato) complexes: their anticonvulsant activities as well as related (5-nitrosalicylato)-metal complexe

International audiencePurposes of these studies were to synthesize Zn(II) and Co(II) complexes of 3-nitro-4-hydroxybenzoic acid, determine their structures through X-ray crystallography, and obtain their anticonvulsant activities. Thermogravimetric, differential scanning calorimetry, impedance of aqueous solutions and magnetic properties analyses were also determined. Anticonvulsant and related activities of these complexes as well as Zn(II), Co(II), Ni(II) and Mg(II) (5-nitrosalicylato) complexes were determined by the National Institutes of Health, Antiepileptic Development Program. Results of these analyses are presented to document unique bonding features and physical properties of these compounds and their anticonvulsant activities. It is concluded that these compounds have chemical and physical properties that can be used to account for their anticonvulsant activities