National Institute Of Materials Physics - Romania

The local electric properties at K and Zn sites in the normal, incommensurate and commensurate phases of K2ZnCl4, as derived from a numerical computation of the lattice contributions to the electric potential V(r), electric field intensity E(r) and electric field gradient tensor V-alpha beta(r) are reported. The numerical data obtained at each cationic position were correlated with the experimental K-39 NMR, Cu2+ and Mn2+ EPR and Fe-57 Mossbauer results in pure and doped K2ZnCl4. A proportionality between crystal field and zero-field splitting was taken into account for Mn2+, whereas for K+, Cu2+ and Fe3+ ions the electric field gradient is directly related to the crystal field parameter. By this comparison, on computations done in the ionic fractional charge and relaxed lattice approximations, the insertion of probe-species of iron, copper and manganese ions on off-center Zn sites is proposed. The K-39 electric field gradient tensor calculations in the incommensurate phase fit well with the NMR data reported recently.

Two Fe+ centres have been identified by ESR after X-ray irradiation at 80 K of chlorinated SrCl2 : Fe2+ crystals. Both centres, with tetragonal symmetry, consist of an Fe+ ion with a neighbouring charged defect along the [001] symmetry axis. The substitutional Fe+ ion of the Fe+(I) centre is strongly off-centre displaced in the Fe+(II) centres, in the middle of a square of four nearest neighbour chlorine ligands.

In case of a metal finely dispersed in a C-60 matrix, the metal-to-C-60 charge transfer lowers the interface energy. The total cohesive energy per metal atom E-tot, was estimated for Au clusters in fullerite. The results show that, for a charge Q(f) greater than or equal to 1 electron (e) transferred from the metal clusters to each C-60 cage, a minimum appears in E-tot, defining a most stable cluster size. Thereby, the equilibrium dispersion state of the metal in the fullerite matrix depends on Q(f).

Data on the dielectric properties of the liquid crystal 4-octyl-4'-cyanobiphenyl (8CB) have been determined onto unaligned samples, for a large scale of frequency, as function of temperature. The components of the complex dielectric permittivity as well as the corresponding dielectric parameters were evaluated using Havriliak-Negami (HN) functions. The dielectric characteristics were discussed and were compared with the literature, a good agreement being observed. The method allows to obtain for the first time the dielectric strength for a large range of the temperature and gives the two functions describing the relaxation processes in the isotropic range.

Orientational transitions induced by temperature variation, in cells with nematic liquid crystal having limiting substrates with controlled topography, are presented. The variable topography was obtained by SiOx deposition in vacuum at an incidence angle around 85 degrees, or by depositions of polyvinyl alcohol (PVA) with different thicknesses, on glasses with the same SiOx deposition. The experimental results are explained by a phenomenological model based on the Landau-de Gennes free energy expansion, taking into account the anisotropy of the substrate.

The effect of niobium and lithium doping on the structure and piezoelectric properties of some samples with the following compositions PbZr0.051 Nb3Li)(x)Ti0.49-xO3, with 0.015 less than or equal to x less than or equal to 0.050, was investigated. The materials were prepared by the usual ceramic technique using high purity raw materials. Disc shaped samples of each composition were sintered at temperatures between 1200 and 1300 degrees C, for 4 h. The samples structure was determined by X-ray diffractometry and the piezoelectric properties were determined by means of resonance-antiresonance method, using a HP 4194A impedance gain phase analyser. A morphotropic phase boundary (MPB) with the optimum piezoelectric properties was found around the compositions with x=0020. The optimum values for relative dielectric constant epsilon(r), planar electromechanical coupling factor k(p), and mechanical quality, factor Q(m) of about 1700, 0.59 and 75, respectively, were found for these compositions. The lattice constants a and c of rhombohedral and tetragonal phases were determined by means of an X-ray CuK alpha diffractometer. The results are discussed in terms of the domain easy switching phenomena and high poling degree, brought about by the A-vacancies produced by the NbLi pairs doping. (C) 1999 Elsevier Science Limited. All rights reserved.

Three ceramic magnetic compositions within the CuZnTi ferrite system, having the Curie temperatures centered around 60, 80 and 100 degrees C, respectively, were investigated in order to determine the time-temperature stability of their main magnetic properties. Permeability and the slope of permeability around their Curie points were determined on ring shaped samples, before and after being subjected to long term (over 6000 h) ageing at their Curie temperature and for a shorter time of about 50h at higher temperatures up to 700 degrees C. Rates of changes of 33, 66 and 100 ppm h-l of the maximum slopes of permeability were found for the samples aged at 60, 80 and 100 degrees C, respectively. The samples aged at temperatures higher than 200 degrees C show a rather sudden decrease of maximum slope of permeability, but no shift of the Curie temperature and the working point, corresponding to the temperature where the slope has the maximum value. This is the most interesting result as concerns the use of such magnetic temperature sensors for the construction of highly sensitive temperature controllers, for example ultrathermostats. The results are discussed in terms of the migration processes of the cations, especially Cu2+,from metastable positions on which they were frozen during the rapid cooling of the sample from the sintering temperature, to the more stable ones, namely the octahedral sites into the spinel lattice. (C) 1999 Elsevier Science Limited. All rights reserved.

Tl2+(6s(1))-type of paramagnetic centres, produced by low temperature X-ray irradiation, were observed in the low temperature ferroelectric phases of Rb2ZnCl4 : TICl crystals. The difference between the spin-Hamiltonian parameters of the main centre, determined in the two phases, is attributed to the symmetry lowering at phase transition.

The structure of the heteronuclear iron acetate [Fe2CoO(CH3COO)(6)(3-Cl-Py)(3)]. 1/4 . 3-Cl-PyX1/4(CH3)(2)CO . 1/2H(2)O was determined by X-ray structure analysis. The crystal has a molecular structure and is monoclinic with lattice parameters a = 21.034(4), b = 8.398(2), c = 23.360 Angstrom, beta = 98.28(3)degrees, R = 0.0656, space group P2(1)/c. The trinuclear complex [Fe2CoO(CH3COO)(6)(H2O)(3)] has a structure typical for iron(III) mu(3)-O compounds with iron atoms lying at the vertices of the equilateral triangle centered by an oxygen atom. The metal atoms are each coordinated to four oxygen atoms of the four bridging carboxy groups, the bridging oxygen atom (mu(3)-O), and the coordinated 3-chloropyridine molecule which is trans relative to the latter atom. According to Mossbauer spectroscopy data, the iron(III) ions are in the high-spin state. The value of (mu(ef))(mole)/mu(3) at room temperature and its temperature dependence suggest that the resulting magnetic exchange interaction between the paramagnetic centers of the cluster is antiferromagnetic.

Aliquots of waste water from power stations were analyzed by a FT-IR spectroscopic procedure and showed both qualitatively and quantitatively the presence of cyclohexylamine and morpholine. The detection limit and the sensitivity of the method were investigated and the validity of the Lambert-Beer law was checked. To maintain the selectivity of the method the pH must be maintained around 6.