National Institute Of Materials Physics - Romania

Alignment layers containing copper(II) complexes with acrylic acid or poly polyvinylpyrrolidone show homeotropic orientational properties. This behaviour is supposed to be due to the particular structure of the complex copper(II) ions. We give spectroscopic arguments to support the anchoring of liquid crystal (LC) molecules by coordination to the copper ion within the alignment layer.

An impedance spectroscopy experimental characterization of a new synthetized Palladium complex organo-metallic liquid crystal is reported. It has been found that although organo-metallic complexes modify some of the physical properties of the liquid crystals, namely transition temperatures, values of the relaxation times and conductivity, they still have, in the nematic phase, the general behaviour of the usual liquid crystals. In particular the low frequency conductivity is very small, so one can conclude that the considered organo-metallic liquid crystal behaves like a good insulator. The temperature dependence of the static parallel and perpendicular components of the dielectric tensor are reported.

This paper presents a new method for obtaining electric twins in quartz crystal: thermal treatment in a temperature gradient at about 500 degrees C, that means under phase transition point. + 5X quartz crystal plates for resonators were used; a temperature gradient of 15-25 degrees C was applied for 4-5 hours along the Y axis (the place length). For + 5X resonators, the diminution of the resonant frequency and the increase of the inductance were plotted as a function of the level of twinning.

The electron paramagnetic resonance spectra of Cr3+ in LiNbO3 co-doped with ZnO or CaO (approximate to 6%) have been studied. Both impurities give rise to the same effect as that observed in MgO-doped samples: a strong diminution of the anisotropic (axial) spectrum reported for LiNbO3:Cr and the appearance of an intense and isotropic line. The origin of this line in the co-doped samples is discussed.

In this work, we perform a study of the off-center effect: on the diffusion coefficient and we propose a new mechanism for fast diffusion. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability [1]. The nonlinear paths between two consecutive positions characterize even the selfdiffusion and furthermore, in the off-center diffusion this path is appropriate. The main results is that the off-center position considerably enhances the diffusion. The theoretical diffusion coefficients have been obtained by modeling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. We emphasize the good agreement of the model with the experimental data for KCl:Cu+ and show that if the impurity is placed close to the central site, the appropriate diffusion coefficient is close to that for the cationic self-diffusion: We proceeded in the Li+ cation case as in the case of Cu+ cation.