Thermoelectrets formed by the phase transition in cholesteryl caproate and its derivatives

Phase transitions have been measured by the thermodielectric effect, i.e. by the production of thermoelectrets in liquid crystals when cooling the samples from the isotropic state to the solid state. By recording the current across the samples as a function of temperature it is possible to specify very exactly the temperature at which the isotropic-liquid crystal phase transition begins.     

Determination of phase transitions in binary systems of mesogenic compounds by optical absorption spectroscopy

The use is described of absorption spectroscopy for determining phase transitions in the mesogenic compound cholesteryl myristate (CM) and in binary mixtures of terephthalbis-butyl-aniline (TBBA) and cholesteryl myristate. The temperature dependence of the optical densities in the visible region at phase transitions is reported. The transition temperatures obtained with this method are in good agreement with the results obtained with other methods.     

Positron lifetimes in five phases of BEA

Positron lifetimes have been measured as a function of temperature in 4-butyloxybenzal-4′-ethylaniline (BEA). BEA has been previously reported to have two liquidcrystalline phases (smectic and nematic) with transition temperatures as follows: solid → smectic, 40.5° C; smectic → nematic, 51.0° C; and nematic → isotropic liquid, 65.5° C. Positron life time spectra were resolved into two components, with the shorter component τ₁ remaining approximately constant in all phases, and τ₂ exhibiting reversible changes at all of the above transitions. In addition, an irreversible discontinuity in the τ₂ lifetime was observed in the vicinity of 28° C, indicating the presence of a new phase (phase X) of BEA. The τ₂ andI ₂ values obtained for the various phases of BEA are: solid (1.25 nsec, 7.1%), phase X (1.97 nsec, 26.2%), smectic (2.36 nsec, 23.6%), nematic (2.72 nsec, 28.3%), and isotropic liquid (2.69 nsec, 29.8%).     

Novel approach to study liquid crystal phase transitions using Legendre moments

A novel approach is proposed to investigate the phase transitions of cholesteric liquid crystals using the Legendre moments. The textures of cholesteric liquid crystals (cholesteryl butyrate, cholesteryl n-valerate, cholesteryl decanoate, and cholesteryl myristate) are captured as a function of temperature using high-resolution camera attached to the arthroscopic mode of polarizing optical microscope with hot stage. A recurrence formula is used to compute the Legendre moments of the liquid crystal textures based on the Legendre polynomial using MATLAB software. The abrupt change in the values of Legendre moments as a function of temperature gives the phase transitions of liquid crystals. The investigated transition temperatures of cholesteric liquid crystals are compared with other techniques.     

Logarithmic divergence of the shear viscosity in the isotropic phase of two cholesterics

The transition from the isotropic liquid to the cholesteric blue phase in cholesteryl oleyl carbonate and cholesteryl oleyl carbonate and cholesteryl oleate is accompanied by an anomaly in the shear viscosity which follows a pseudocritical logarithmic divergence as a function of temperature.     

Peculiarities of biphasic regions at phase transitions in oriented textures of 4-cyano-4′-n-pentylbiphenyl

Peculiarities of the biphasic regions at the direct and reverse phase transitions between homeotropic oriented nematic mesophase and isotropic liquid have been investigated in detail in this work. The shift of phase transition temperatures to the low temperatures by the reverse phase transitions have been observed for this mesophase. Nonlinearity of thermotropic and thermo-optical properties at the phase transitions between nematic mesophase and isotropic liquid have been found.     

First order phase change during isotropic-nematic and nematic-smectic a transitions of NPOB

The temperature variation of the magnetic susceptibility (χ) of NPOB are reported in isotropic, nematic and smectic A phases, χ and Δχ undergo sudden changes at the phase transitions. The isotropic-nematic and nematic-smectic A transitions are assigned to be first order phase transition.     

Propagation of hypersound and acoustic relaxation in the isotropic phase of cholesteryl miristate

The features of the sound propagation at a frequency of 1.1 GHz in the isotropic phase of cholesteryl miristate are studied. The analysis of experimental data in terms of the theory of interacting modes shows that the peculiarities of the temperature behavior of sound velocity and absorption coefficient are determined by the interaction between the hypersonic wave and the fluctuations of the cholesteric helix pitch.     

Phase behavior and nucleation kinetics of Octaphenylcyclotetrasiloxane

Phase behaviour and certain aspects of phase-transformation kinetics for Octaphenylcyclotetrasiloxane (OPCTS) have been determined by means of differential scanning calorimetry and optical microscopy. Three solid phases have been observed: 3 (stable at room temperature), 2 (stable above 76.6°C), and 1 (stable above 189.5°C). The 3-and 2-phases are optically anisotropic and mechanically brittle; the 1-phase is optically isotropic and soft or plastic. Although the 3-phase is thermodynamically unstable above 76.6°C, its transformation to the 2-phase is sufficiently slow that the 3 → 1 transition can be studied. The transition temperatures (and corresponding latent heats) for the 3 → 2, 3 → 1, 2 → 1, and melting transitions are: 76.6°C (～2.9 KJ/mol), 186.0°C (47.3 KJ/mol), 189.6°C (43.76 KJ/mol), and 204.9°C (1.95 KJ/mol). The entropy of melting (ΔS_m = 0.491 R) is one of the lowest measured for any solid. The low-melting entropy, isotropy, and plasticity of the 1-phase lead to the conclusion that it is an orientationally-disordered crystal mesophase (plastic crystal).

The kinetic behavior of the 3 → 2 phase transformation, as determined by DSC, supports a theoretical model taking homogenous nucleation as the rate-determining process.     

Field-induced magnetic phase transitions in the Yafet-Kittel model

The Yafet-Kittel model for a two-sublattice ferrimagnet with an antiferromagnetic exchange interaction in one of the sublattices was developed to describe magnetic-field-induced phase transitions in the isotropic and Ising cases. Depending on the relative values of the exchange parameters of the inter-sublattice interaction and the intra-sublattice interaction in the isotropic case, two types of magnetic phase diagrams with two types of second-order phase transitions are possible: to the noncollinear phase and to the spin-flip phase, and, in the Ising case, three types of magnetic phase diagrams with first-order phase transitions are possible. Fiz. Tverd. Tela (St. Petersburg) 41, 1797–1799 (October 1999)     

Isotropic to ferroelectric and antiferroelectric liquid crystals phase transitions

This review is an attempt towards a unified picture of the direct transitions from the isotropic liquid to ferroelectric and antiferroelectric liquid crystalline phases formed by rod-like and bent-core molecules. The Landau–Ginzburg theories of the phase transitions between the isotropic liquid to ferroelectric and antiferroelectric liquid crystalline phases in compounds composed of chiral rod-like molecules and achiral bent-core molecules are presented. This includes a discussion of the nature of the order parameters and the nature of the various types of phase transitions. The various predictions are compared with the available experimental results.     

Phase studies and thermal stability of binary systems of cholesteric liquid crystals

The binary mixture of cholesteryl oleyl carbonate (COC) and cholesteryl nonanoate (CN) with different ratios has been characterized by differential scanning calorimetry (DSC) to determine the phase diagrams. Their normal shelf-life and accelerated stability studies were also examined. A novel microscopic Fourier-transform infrared (FT-IR) spectrometer equipped with DSC was also used to measure simultaneously the chemical structural variation and the thermal response of these liquid crystals. The results indicate that the binary mixture might transform from smectic to cholesteric and then to isotropic liquid (CN composition <50%); from solid to cholesteric and then to isotropic liquid (CN composition >50%). Therefore, the 50% CN composition might play a critical role in the phase transition of the binary COC-CN mixture. The microscopic FT-IR/DSCsystem could also confirm the phase transition, but could measure the phase trasition temperature of liquid crystals more easily and sharply than the DSC method. The long-term accelerated thermal stability of the COC-CN mixture seemed to be more constant when stored at 4 °C than in any other temperature conditions.     

Studies of the heat capacity and thermal expansion of the Na0.95K0.05NbO3 solid solution

The heat capacity and thermal expansion of ceramic samples of the Na_0.95K_0.05NbO₃ solid solution have been investigated over a wide temperature range of 100–750 K. The observed anomalies in the heat capacity and thermal expansion at T ₄ = 297 K, T ₃ = 535 K, T ₂ = 665 K, and T ₁ ≈ 710 K correspond to the sequences of phase transitions N → Q → G → S → T1. It has been shown that, as a result of the phase transitions, the unit cell volume at T ₄ and T ₂ decreases, and at T ₃ and T ₁, increases with increasing temperature. The directions of the shift of the phase transition temperatures induced by hydrostatic pressure have been determined. It has been established that all structural transformations are accompanied by relatively small variations in the entropy. Different mechanisms of the structural distortions have been discussed.     

Methanol as a sensitive probe for spatial and temporal variations of the proton-to-electron mass ratio

The 6.7 and 12.2 GHz masers, corresponding to the 5(1) → 6(0)A+ and 2(0) → 3(-1)E transitions in methanol (CH3OH), respectively, are among the brightest radio objects in the sky. We present calculations for the sensitivity of these and other transitions in the ground state of methanol to a variation of the proton-to-electron mass ratio. We show that the sensitivity is greatly enhanced due to a cancellation of energies associated with the hindered internal rotation and the overall rotation of the molecule. We find sensitivities of K(μ) = -42 and K(μ) = -33, for the 5(1) → 6(0)A+ and 2(0) → 3(-1)E transitions, respectively. The sensitivities of other transitions in the different isotopologues of methanol range from -88 to 330. This makes methanol a sensitive probe for spatial and temporal variations of the proton-to-electron mass ratio.     

Sound propagation and relaxation processes in the isotropic phase of cholesteryl miristate

The propagation and absorption of high-frequency sound in the isotropic phase of cholesteryl miristate is studied in a wide frequency range using the data of both Brillouin spectroscopy and acoustooptic measurements. The parameters of the relaxation process associated with the volume viscosity relaxation are calculated using the experimental data on the velocity and absorption of sound.     

Induced birefringence in the isotropic phase of cholesteric liquid crystals

The birefringence induced by an acoustic field in the isotropic phase of the cholesteryl myristate cholesteric liquid crystal has been measured. The measurements were performed at sound frequencies of 3.2 and 9.8 MHz in a wide temperature range. Based on the results of the measurements of the induced birefringence, the relaxation time of the orientation of the liquid crystal molecules is estimated.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

Thermal studies of N-(p-Hexyloxybenzylidene)-p-Toluidene (HBT)

The effect of quenching on phase transitions in monotropic liquid crystal HBT is investigated using DSC, DTA, X-ray diffraction and microscopic techniques. The phases which are normally seen only during the cooling cycle are observed when the substance is quenched from the isotropic liquid phase at appropriate rate.     

Reconstructive structural phase transitions in dense Mg

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.     

Superconductivity and ferromagnetism in the Hubbard model

In the framework of the Hubbard model at U → ∞, the phase transitions related to the lower underfilled subband are studied. The self-consistent equations determining the superconducting and ferromagnetic order parameters are derived in the one-loop approximation. The phase boundaries within which ferromagnetic and superconducting phenomena can coexist are discussed.