Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)2_2PF6_6 salts

We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)$_2$PF$_6$. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge ordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened in the deuterated one where the transition temperature has increased by more than thirty percent. Anomalies in the dielectric function are also observed in the spin-Peierls ground state revealing some intricate lattice effects in a temperature range where both phases coexist. The variation of the spin-Peierls ordering temperature under magnetic field appears to follow a mean-field prediction despite the presence of spin-Peierls fluctuations over a very wide temperature range in the charge ordered state of these salts.Comment: 7 pages, 6 figure

Exploring the Charge-Ordering Transition in (TMTTF)2_2X via Thermal Expansion Measurements

We report results of high-resolution measurements of the \emph{c$^*$}-axis expansivity ($\alpha_{c^{*}}$) at the charge-ordering (CO) transition for the quasi-1D (TMTTF)$_{2}$X compounds with X = SbF$_6$ and Br and make a comparison with previous results for the X = PF$_6$ and AsF$_6$ salts. For X = SbF$_6$, due to the screening of the long-range Coulomb forces, a sharp $\lambda$-type anomaly is observed at $T_{CO}$, which contrasts with the step-like mean-field anomaly at $T_{CO}$ for PF$_6$ and AsF$_6$, where CO occurs in the Mott-Hubbard charge-localized regime. For the latter two salts, a negative contribution to $\alpha_{c^{*}}$ is observed above $T_{CO}$. This feature is assigned to the anions' rigid-unit modes, which become inactive for $T$ $<$ $T_{CO}$. Our $\alpha_{c^{*}}$ results for the X = Br salt, where such rigid-unit modes are absent, reveal no traces of such negative contribution, confirming the model based on the anions' rigid-unit modes for the X = PF$_6$ and AsF$_6$ salts.Comment: Submitted to Special Issue - ISCOM 2009 - Physica B Condensed Matte

Retrosynthetic analysis of fullerene C-60: structure, stereochemistry and calculated stability of C-30 fragments

The total number of possible retrosynthetic bisections Of C-60 leads to nine different isometric C-30 fragments. These molecules include five chiral units, four of which derive from partitions corresponding to four distinct "Coupes du Roi". The energies, curvatures, and homodesmotic stabilization energies of the C-30 fragments are evaluated at the ab initio 6-31G level

Photochemistry with circularly polarized light-III. Synthesis of helicenes using bis(arylvinyl) arenes as precursors

A systematic study of the photochemical ring-closure of bis(arylvinyl)arenes, using circularly polarized light, is described and commented. In the two series of compounds 5a-e and 6a-e, the observed optical yields follow a similar trend; no asymmetric syntheses were observed in the case of the higher benzologues of [10] helicene, using a 290-370 nm irradiation band, circularly polarized at 313 nm. © 1975.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

A VARIABLE MECHANISM FOR THE NUCLEOPHILIC VINYLIC SUBSTITUTIONS IN A SERIES OF GEM-DIHALOGENATED ALKENES BY A BIDENTATE SULFUR NUCLEOPHILE - AN EXPERIMENTAL AND AM1 THEORETICAL-STUDY

The nucleophilic substitutions of a series of gem-dihalogenated alkenes 3,5,7,8, and 9 (RS)2C=CX2 (X = Cl, F) with 1,2-benzenedithiolate b have been studied. Depending on the structures of the R groups (alkyl, saturated and unsaturated cycloalkyls, aromatic ring), the course of the reactions and the structures of the yielded products are modified. In the frame of the addition-elimination-type mechanism for these nucleophilic substitutions, the energy contents of the anionic intermediates, resulting from the additions of the nucleophile b to the unsaturated centers, is calculated at the AM1 method level. For the compounds 5, 7, 8, and 9, the calculated energies nicely corroborate the experimental results. For 3, anionic intermediates are no longer found by calculation, and a synchronous single-step substitution is strongly suggested

Conducting-Probe AFM nanoscale joule heating yields charge-density-wave transition detection

International audienc

Growth and transport properties of HT-LixCoO2 thin films deposited by pulsed laser deposition

International audienceThe successful in situ growth of single phase, c-axis oriented, layered structured LixCoO2 thin films on (0 0 0 1) Al2O3 substrates by pulsed laser deposition is reported. Thin films were grown in an oxygen pressure of 1 mbar and substrate temperatures varying from 300 up to 600 °C. It is found that the surface roughness and electrical resistance of the films depends strongly on the deposition temperatures