F. Bénière

Vacancy pairs and correlation effects in kCl and NaCl single crystals

Abstract We compare the self-diffusion coefficients, respectively determined with radioactive tracers and from the ionic conductivity, in the intrinsic range of pure KCl and NaCl single crystals. The correlation effect and the vacancy pair effect originate deviations between both values. The vacancy pair effect is successfully measured by studying the influence of divalent anions and divalent cations on the self-diffusion coefficients in KCl. We derive the value of the correlation factor f = 0.75 ±0.04, which provides a direct experimental verification of the correlation theory ( f = 0.78). The whole set of the recent measurements of transport processes and isotope effects in NaCl are reexamined by considering this value for the correlation factor. We derive the whole set of properties of the free and associated vacancies in NaCl and KCl.

Doping of polyacetylene by diffusion of iodine

Abstract The solubility limit and diffusion coefficient of I 2 in (CH) x polymer have been measured with a radiotracer technique. The concentration of iodine in the polymer is a function of the surrounding concentration outside the film at equilibrium (free enthalpy of solubility = − 0.20 eV). The data are consistent with the following mechanism: liquid state diffusion of the solution in between the fibrils and solid state diffusion of iodine inside the fibrils. The macroscopic diffusion coefficient throughout the film is equal to 10 −9 cm 2 sec −1 . These results restrict the present (CH) x polymer to heterolithic device applications after encapsulation. Monolithic substrate application will require a further inhibition of diffusion.

Effect of rare-earth Sm3+ substitution on the superconducting properties of the Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy system

Abstract The superconducting properties of the Bi1.7Pb0.3Sr2Ca2−xSmxCu3Oy system have been investigated by X-ray diffraction, electrical resistance, AC susceptibility and DC magnetization. Substitution of diluted quantities of Sm3+ for Ca2+ is found to change drastically the superconducting properties of the system, despite the weak paramagnetic nature of the Sm3+ ion. X-ray diffraction results show that the volume fraction of the high-Tc (2:2:2:3) phase decreases and that of the low-Tc (2:2:1:2) phase increases as the samarium concentration increases from x=0 to 0.05. At higher samarium concentrations, i.e. for x=0.075 and 0.1, only the low-Tc phase exists. AC susceptibility measurements (χ′ versus T) show that Tc onset does not change up to x=0.02, and then decreases down to 100 K for x=0.1. The loss peak (χ″ versus T) decreases from 108 to 104 K for x=0 to x=0.02, while for higher Sm concentrations, the χ″ peak decreases more significantly. The same trend was observed by resistivity measurements and can be explained on the basis of granular superconductivity. Magnetization measurements show that the superconducting volume (slope d M d H ) decreases with increasing samarium concentration. These results show that the Ca2+→Sm3+ substitution changes the hole carrier concentration, which in turn lowers Tc and decreases the volume fraction of the superconducting phase.

Atomistic simulations of the structures of the pristine and doped lattices of polypyrrole and polythiophene

Abstract The crystal lattices of the conductive polymers polypyrrole and polythiophene and their BF 4 − and ClO 4 − doped modifications were investigated using atomistic lattice simulation. The method implements an energy-variational relaxation mechanism applied to intra- and inter-molecular modifications of initial lattice structures. The calculated lattices are found to exhibit polymorphism, the lowest-energy forms of which for polythiophene (the only one of the two polymers for which an experimental lattice structure is available) agrees with the results of two apparently inconsistent diffraction studies. In their doped states the polymers form parallel planes, the dopant ions assuming intercalation sites between the chains.

Atomistic simulation calculations on the structures of conducting polymers: Part I. Pristine polyacetylene and polyparaphenylene

Abstract Calculated atomistic simulation investigations of the lattices of polyacetylene and polypara-phenylene are reported. When the lattice energy is minimized with respect to bond lengths and bond angles as well as to the overall positions of the polymer chains stable structures are found which are not revealed by rigid-chain calculations. The calculated structures for polyacetylene are consistent with the different setting angles found by different experimental groups. Polypara-phenylene shows a metastable structure which is planar, reflecting the observed tendency of the lattices of some shorter chain polyphenyls to stabilize all-planar conformations.

Energy of migration of monovalent ions in NaCl: An experimental and theoretical study

Abstract Experimental diffusion measurements show that migration enthalpies of Cl − , Br − and I − in NaCl are comparable, while that of F − is considerably lower. Earlier studies had shown that migration enthalpies of Na + , K + , Rb + and Cs + in NaCl were similar. The polarised point ion model predicts migration energies of ions (by vacancy mechanism) to monotonically increase with ion size, contrary to experiment. Inversely, the shell model calculations rightly predict the variation of migration energies with ionic size. Thus, migration energies by vacancy mechanism do not vary significantly for ions larger than the host ions. However, in the case of the small ions, Li + and F − , the migration energies by vacancy mechanism are much lower and in good agreement with experiment for F − .

Anion diffusion mechanism in strontium chloride single crystals

Abstract Simultaneous measurements of the anion self-diffusion coefficient and of the ionic conductivity have been carried out in the intrinsic range (440–630°C) of pure Sr Cl 2 single crystals. The microtome sectioning method using the radiotracer 36 Cl has been successfully applied to this difficult matrial. Accurate values of the correlation factor have thus been derived by applying the Nerst-Einstein relation. Results suggest that the Frenkel defects are predominant in the anion sub-lattice with the enthalpy and entropy of formation of 2.98 eV and 22 K, respectively. The chloride anions diffuse via the vacancy mechanism to which is added to a lesser extent an exchange mechanism where the ions successively occupy lattice and interstitial sites.

Atomistic simulation investigations of the structures of conducting polymers: Part II. Doped polyacetylene and polyparaphenylene

Abstract The structures of conducting doped polymer lattices of polyacetylene (PA) and polyparaphenylene (PPP) have been investigated using an atomistic simulation method with relaxation of bond angles and bond lengths. Agreement with available experimental structural data is obtained for potassium-doped PA and with PPP doped with lithium, potassium and arsenic pentafluoride, and structural features at present not available from experiment are deduced. As well as bonded atom-pair potentials, a non-bonded carbon-potassium potential is presented which has been fitted to the experimental structure.

Self- and heterodiffusion in crystalline solid solutions

Abstract We present a self-consistent analysis of a complete set of atomic diffusion data in mono-crystalline alkali halide mixed crystals. New measurements of the heterodiffusion coefficients of Na + and Rb + in KCl z Br (1− z ) are combined with previous data for the self-diffusion coefficients of the three host ions constitutive of the solid solutions: KCl z Br (1− z ) , K z Rb (1− z ) Cl and K z Rb (1− z ) I. The correlatio have been accounted for by making a systematic use of Mannings equations of the correlation factors for vacancy diffusion in random alloys for unbound free vacancies. The justification and generalization of those equations is discussed in the light of recent simulation and analytical studies. The diffusivity is increased in the mixed crystals. The enhancement effect for both the self- and heterodiffusion coefficients reaches a factor of about 2 around the middle composition ( z

A lattice simulation of the migration of the BF4− ion in polythiophene and polypyrrole lattices

0.5) in the three systems. The major part of the enhancement originates from an increase of the vacancy concentration relative to the pure end members. This effect is qualitatively interpreted in thermodynamic terms. The present conclusions derived in alkali halides may be extended to other similar solids: metallic alloys, solid solutions, etc.