H. Yurtseven

Study of Phase Transitions in Polymorphic Liquid Crystals

This work presents our investigations of mezomorphic properties of two polymorphic liquid crystals, namely, 4-nonyloxy-4-butoxyphenyl benzoate and N-(-4-heptyloxybenzylidene-4-butylaniline) in a wide temperature range, particularly, in the phase transition regions. By means of an original experimental method, the heterophase regions and also the phase transition temperatures have been determined for these materials with high accuracy. These phase transition intervals have been analyzed using a mean field model.

Spectroscopic modifications of Pippard relations for ammonium halides

Abstract Spectroscopic modifications of the Pippard relations have been established for ammonium halides near their λ-phase transitions. A linear relationship has been obtained between the specific heat and the frequency shifts using our observed Raman frequencies of the ν5TO(134 cm−1) and ν5LO(177 cm−1) lattice modes of NH4Br near its first-order phase transition (P = 0). Similar linear relationships have also been obtained using our observed Raman frequencies of the ν5TO(174 cm−1) lattice mode and the ν2(1708 cm−1) internal mode of NH4Cl near the tricritical ( P ⋍ 1.6 kbar ) and the second-order ( P ⋍ 2.8 kbar ) phase transitions in this crystal system. Our values of the slope ( d P d T ) λ deduced from the spectroscopic modifications of the Pippard relations are comparable with the literature value for the NH4Br crystal near its first-order phase transition. We also give in this study our ( d P d T ) λ values which are the first reported in the literature for the tricritical and the second-order phase transitions in NH4Cl.

Spectroscopic modifications of Pippard relations and their application to NH4Cl

Abstract The Pippard relations have been reformulated in terms of the spectroscopic parameters and applied to the NH 4 Cl system in the vicinity of its λ-transition. We have effectively tested some of these relations in an earlier study where we have used, for the frequency shift, data for the ν 5 lattice mode of NH 4 Cl. Here, using our observed frequency shifts for the internal mode, ν 2 of NH + 4 , we have obtained a good fit to the Pippard relations for temperatures below T λ for the NH 4 Cl system. We have then been able to extract the frequency shift data for the ν 2 internal mode as a function of temperature, using the specific heat and also thermal expansion data from the literature. The method of analysis for obtaining the thermodynamic data from the spectroscopically measured quantities or vice versa is given here.

T-X Br phase diagram for the NH4Br x Cl1−x system

Abstract In this study using a mean field model we calculate the phase line equations for the β-δ, β-γ and γ-δ phase transitions in the NH4BrxCl1 x system. We then fit our phase line equations to the experimentally observed T-X Br phase diagram for this system. Our calculated phase diagram agrees well with the observed one for the NH4BrxCl1 x system.

Temperature Dependence of the Raman Frequency, Damping Constant and the Activation Energy of a Soft-Optic Mode in Ferroelectric Barium Titanate

The Raman frequency of a soft-optic mode associated with the order parameter is calculated as a function of temperature from the mean field theory for barium titanate (TC =222 K). Using the Raman frequencies calculated, the damping constant is evaluated and it is compared with the observed one for this soft-optic mode. Using the temperature dependence of the damping constant which is calculated from the pseudospin-phonon coupled model and from the energy fluctuation model, activation energies are also deduced for the soft-optic mode of barium titanate. For the temperatures to 330 K, the activation energy U varies from 0.03 to 0.01 eV, which can be compared with the value of kBTC ≅ 0.02 eV. Above 330 K, it decreases as predicted from both models. It is shown here that the calculated Raman frequency and the damping constant of the soft-optic mode describe adequately the observed behaviour of barium titanate. The activation energies deduced are also reasonable.

TILT ANGLE AND THE TEMPERATURE SHIFTS CALCULATED AS A FUNCTION OF CONCENTRATION FOR THE AC* PHASE TRANSITION IN A BINARY MIXTURE OF LIQUID CRYSTALS

We study here the tilt angle and the temperature shifts as a function of concentration for the AC* phase transition in a binary mixture, using our mean field model with the biquadratic P2θ2 coupling — and also with the bilinear Pθ and P2θ2 couplings. By expanding the free energy in terms of the tilt angle and polarization, the tilt angle and the temperature shift are evaluated by using the coefficients given in the free energy expansion. By employing a concentration-dependent coefficient, the tilt angle and the temperature shift are calculated as a function of concentration of 10.O.4 for the SmAC* transition in a binary mixture of C7 and 10.O.4. Our calculated values of the tilt angle and the temperature shifts decrease as the concentration of 10.O.4 increases, as confirmed experimentally for the AC* transition in this binary mixture. This indicates that our mean field models studied here are satisfactory to explain the observed behavior of the AC* transition of the binary mixture of C7 and 10.O.4.

A phase diagram of ammonia close to the melting point

Abstract We calculate here the phase line equations using the mean field theory for the liquid-solid I - solid II phases in the ammonia close to the melting point. Our calculated phase line equations have been fitted to the experimental data. Our calculated phase diagram agrees very well with the experimentally obtained P-T phase diagram from the literature.

T–P phase diagram of ammonia solid phases I, II and III

In this study, using the mean field theory, we obtain temperature versus pressure phase diagram for the liquid and solid phases I, II, and III of ammonia. We fit our phase line equations to the experimentally observed data. There exists good agreement between the theoretical and experimental phase lines.

Calculation of the Damping Constant and the Relaxation Time for the Soft-Optic and Acoustic Mode in Hexagonal Barium Titanate

The temperature dependence of the damping constant is calculated below the transition temperature (T 0=222K) in the ferroelectric phase of hexagonal barium titanate. The damping constant of the coupled soft-optic and acoustic mode which causes an intense central peak in the light scattering spectra, is calculated using the soft mode-hard mode coupling model and the energy fluctuation model for barium titanate. By considering the bilinear coupling between acoustic and soft-optic modes, which is related to the order parameter, the inverse of relaxation time is estimated as a function of temperature below T0 in barium titanate. Our results are compared with the experimental data obtained from the light scattering spectra of this ferroelectric compound.

Some peculiarities of nematic–isotropic liquid phase transitions in monomorphic and polymorphic mesogens

In this study, the dynamics of the changes of the morphological properties in the nematic mesophase and in the heterophase regions of the phase transitions between the nematic mesophase and the isotropic liquid for the thermotropic mesogens have been investigated. Experimentally, we have obtained the thermal hysteresis for the investigated phase transitions. For the analysis of the phase transition temperatures and the peculiarities of the heterophase regions, the mean field theory has been used.