Dielectric spectroscopy of T-shaped blue-phase-III liquid crystal.

Dielectric relaxation spectra of a liquid crystalline (LC) material showing blue-phase-III (BPIII) for a considerably large temperature regime consisting of T-shaped molecules are investigated. A low frequency relaxation mode is observed in the isotropic phase (I) as well as in BPIII of the investigating material which is attributed to the short axis rotation of the T-shaped molecules. The outcome of the temperature and dc bias field variation of dielectric strength (Δε) and relaxation frequency (ν(c)) in the vicinity of the I-BPIII transition is also discussed. The temperature dependence of ν(c) in BPIII with a minor deviation from Arrhenius activities in the fluctuation-dominated nonlinear region (FDNLR) is found to follow the unusual thermal behavior of the activation energy (E(A)). The growth of pretransitional fluctuations is found to be nonlinear in the vicinity of the I-BPIII transition. A moderate growth of transition fluctuation commences from the value of the exponent α(eff)=0.38/°C, which is obtained by an exponential variation of ν(c) with respect to temperature in BPIII. Observed dynamic phenomenon in the vicinity of the I-BPIII transition regions is explained on the basis of the Landau-de Gennes and Maier-Saupe Theories.

Observation of relaxor ferroelectricity and multiferroic behaviour in nanoparticles of the ferromagnetic semiconductor La(2)NiMnO(6).

We report a diffuse phase transition (extending over a finite temperature range of ∼50 K) in sol-gel derived nanoparticles (∼25 nm) of the ferromagnetic double perovskite La(2)NiMnO(6). The macroscopic polarization (P-E hysteresis loop), validity of the Vogel-Fulcher relation and high dielectric permittivity (∼9 × 10(2)) confirm relaxor ferroelectric phenomena in these magnetic nanoparticles. Compared to the corresponding bulk sample, appreciably large enhancement of the magnetocapacitive effect (MC  âˆ¼ 30%) is observed even under low magnetic field (0.5 T) around the broad relaxor dielectric peak temperature (∼220 K), which is close to the ferromagnetic transition temperature (θ(f) âˆ¼ 196 K). All of these features establish the multiferroic character of the La(2)NiMnO(6) nanoparticles. The inhomogeneities arising from chemical and valence mixing in the present La(2)NiMnO(6) nanoparticles and the inter-site, Ni/Mn-site disorder along with surface disorder of the individual nanoparticles resulting in local polar regions are attributed to the observed dielectric behaviour of the nanoparticles. The wave vector dependent spin-pair correlation is considered to be the plausible cause of the colossal magnetocapacitive response near the transition temperature. High permittivity and large magnetocapacitive properties make these ferromagnetic La(2)NiMnO(6) nanoparticles technologically important.

Correlation of structural, magnetic and transport properties with the tolerance factor in a low-doped La(0.875)Sr(0.125-x)Ca(x)MnO(3), (0 ≤x≤0.125) system: cross-over from Mott to Shklovskii-Efros variable range hopping conduction.

Samples of the low-doped manganite La(0.875)Sr(0.125-x)Ca(x)MnO(3) (0≤x≤0.125) have been synthesized and the effect on the structural, magnetic and transport properties of decreasing the tolerance factor by replacing larger Sr(2+) ions with smaller Ca(2+) ions are reported. For samples with x≥0.0625, a concentration (x) dependent structural transition (rhombohedral ([Formula: see text]) to orthorhombic (Pnma)) has been detected at room temperature and the Curie temperature T(C) is found to decrease with increased Ca doping level. For samples with x≤0.0625, a narrow metallic region exists and the corresponding insulator to metal transition temperature T(MI) decreases with increasing Ca content, i.e. decreasing tolerance factor. In the paramagnetic region, x dependent crossover from Mott variable range hopping (Mott-VRH) to Shklovskii-Efros variable range hopping (SE-VRH) occurs as the Ca content increases. The thermoelectric power (TEP) of the samples increases substantially, varying inversely with the tolerance factor. These results are analysed from the consideration of increased bending of the Mn-O-Mn bond with the decrease of the average ionic radius of the A-site element [Formula: see text] and the tolerance factor t, which causes narrowing of the bandwidth, decrease of mobility of e(g) electrons and weakening of the double exchange (DE) interaction associated with the substitution of Ca.

Dielectric relaxation behavior of a liquid crystal showing an unusual type of antiferroelectric-ferroelectric-antiferroelectric phase sequence.

The temperature and frequency dependent dielectric relaxation behavior of a liquid crystalline (S)-(+)-1-methylheptyl 4-[2-(4-alkoxyphenyl) thiophene-5-carbonylthiooxy] benzoate system is reported. Interesting successive antiferroelectric-ferroelectric-antiferroelectric (AF-FE-AF) phase transitions are observed in this system resembling the successive phase transitions observed in crystalline Rochelle salt. The smectic-C* (SmC*) to AF1 phase transition (around 103.0 degrees C) is first order in nature, predicted from the use of Orihara and Ishibashi theory. It is also found that a contribution of the ferroelectric SmC* phase ordering penetrates even in the antiferroelectric AF1 (SmC(*)(A)) and AF2 (SmC(*)(A)) phases very close to the SmC*-AF1 and SmC*-AF2 phase boundaries (critical regions). It is suggested that this type of mixing of AF and FE phases might cause surface induced ferroelectric- or ferroelectric-type ordering near the AF-FE phase transitions. A soft mode with Debye-type dispersion was observed in the SmA phase. The thermal behaviors of dielectric dispersion, absorption, and dielectric strength in different phases are also reported and discussed.

Dielectric investigation of electrically oriented ferroelectric smectic mixture CS-1013.

From dielectric spectroscopic study, a first-order ferroelectric phase transition has been observed in ferroelectric smectic mixture CS-1013 having the phase sequence Cr-SmC*-SmA-N*-Iso. Frequency (100 Hz-10 MHz) and temperature-dependent dielectric measurements have been performed on an electrically aligned sample (thickness 15+/-1 microm) gold coated on glass plates. In the unidirectionally aligned sample, two dielectric relaxation modes (Goldstone mode and soft mode) have been clearly observed in the ferroelectric SmC* phase while only one relaxation mode (soft mode) is visualized in the paraelectric SmA phase. Low-frequency molecular relaxation was also observed in the smectic phases. The experimental results have also been analyzed at different temperatures and biasing voltages for an understanding of the dynamics of dielectric processes in the ferroelectric phase. Finally, we proposed the "pseudospin" model for understanding the ferroelectric-antiferroelectric transition in liquid crystals. We associate the tilt angle straight theta and the pitch of the helix, respectively, with biaxial (b) and uniaxial (u) anisotropy parameters as fluctuating parameters around their stability limit (corresponding to the crystalline values). Here, the director acts as the pseudospin variable. This gives rise to a transverse Ising type (or anisotropic Heisenberg model under the mean-field approximation). It is then shown that such a model with fluctuations of (b) and (u) would explain the ferroelectric and antiferroelectric phase transitions in such liquid crystals. Using Landau theory and the stability conditions, we have also shown, in brief, the feasibility of different types of phase transitions in the ferroelectric liquid crystal system.

Enhanced anaerobic degradation of benzene by enrichment of mixed microbial culture and optimization of the culture medium.

A heterogeneous mixed culture, originally collected from two different sources, namely cow-drug and sludge from the city waste-water treatment plant, was grown in mineral medium containing 1% glucose and then adapted on benzene as the carbon and energy source. Under anaerobic conditions benzene was degraded via benzoic acid as a major intermediate in the benzene degradation pathway. The degradation rate of benzene was improved stepwise by the number of enrichments and optimization of the culture medium. The effects of microaerobic conditions and/or physicochemical treatment with H2O2 prior to anaerobic degradation were studied with respect to variations in benzene degradation rate, growth of biomass and gas composition. It was noticed that the amount of gas produced is less than the theoretical value expected and the percentage of methane in the product gas was very small (3%-3.5%). The reason for this is not well understood but it is presumed that the major group of benzene-degrading bacteria present in the culture medium are sulphate reducers and the mixed consortium is unable to degrade certain complex aromatic intermediates in the benzene degradation pathway under the experimental conditions. For an actual explanation of the situation arising in this study, further investigations must be carried out. However, the mixed culture is capable of oxidizing benzene more rapidly to intermediate compounds and also partly into gas under the culture conditions, compared to the published data for the anaerobic degradation of benzene.