Temperature and pressure dependence of molar volume in solid phases of ammonia near the melting point

Temperature and pressure dependencies of the molar volume are studied here along the transition curve between solid I and solid II phases near the melting point in ammonia. The molar volumes are calculated in the temperature range of 217 to 224 K and in the pressure range of 3 to 8 kbar with respect to the triple point (T-t=217.34 K. P-t=3.070 kbar) where the melting curves of solid I and solid II coincide with the transition curve in ammonia

Calculation of a generalised smectic–hexatic phase diagram in liquid crystals

This study gives a generalised smectic-hexatic phase diagram calculated by the Landau mean field theory. Our calculated phase line equations are fitted to the experimental phase lines, which represent first-order transition for the liquid crystals studied here. The temperature-and concentration-dependent phase lines calculated in this study describe the observed T-X phase diagram for the smectic-hexatic transitions. This indicates that the method of calculating the phase line equations from the mean field theory is general and it can be applied to other phase transitions in liquid crystals

LINEWIDTH OF PHONONS IN ORDER-DISORDER PHASE TRANSITIONS WITH RANDOM TELEGRAPH NOISE

We derive an expression for the linewidth of phonons involved in the order phase transitions in solids. The linewidth is obtained as a function of the dwell time from which the activation energy can be computed. A new viewpoint is presented on the assumption that the fluctuations in the random variable of the noise, which is carried by the random particles involved in the order disorder mechanism, affect the linewidth of the phonons

A temperature-concentration (T-X) phase diagram calculated using the mean field theory for liquid crystals

Phase-line equations for smectic-hexatic phase transitions in liquid crystals were derived using the Landau phenomenological theory. In particular, second-order transitions for the smectic-A-smectic-C (SmA-SmC) and hexatic-B-hexatic-F (or HexI) transitions were studied and the tricritical points for these transitions were located. The calculated phase-line equations were fitted (using experimental data for various liquid crystals) to construct a generalized T-X phase diagram. It was shown that the T-X phase diagram calculated from the free energy adequately describes the observed behavior of liquid crystals during smectic-hexatic transitions

Calculation of the Raman frequency and the damping constant of a coupled mode in the ferroelectric and paraelectric phases in KH(2)PO(4)

The Raman frequency of the coupled mode omega_ associated with the order parameter is calculated as a function of pressure from the mean field theory in the ferroelecric phase of KDP. Using the frequencies of the coupled mode, the pressure dependence of the damping constant Gamma_ is calculated by the proton spin lattice relaxation time and by the energy fluctuation of this mode of KDP in the ferroelectric (FE) phase. For the paraelectric (PE) phase, the Raman frequency of the coupled mode omega is calculated as a function of temperature at 6.54 kbar. The temperature dependence of the damping constant Gamma_ is performed only by the energy fluctuation of the coupled mode omega_ in the PE phase of KDP. The values of the activation energy U are extracted from the pressure and temperature dependence of the damping constant Gamma_ of this coupled mode omega_ for both FE and PE phase of KDP. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Raman linewidths calculated as a function of temperature in NaNO(2)

The temperature dependence of the damping constant is calculated for some Raman modes in NaNO(2) using the soft mode-hard mode coupling model and the energy fluctuation model. The damping constant is fitted to the measured Raman bandwidths of those modes, which is well described by the soft mode-hard mode coupling model. The values of the activation energy are extracted from the damping constant for the Raman modes, which are much greater than k(B)T(C) = 0.04 eV for NaNO(2) close to the transition temperature (T(C) = 436 K). This is an indication that NaNO(2) undergoes an order-disorder phase transition. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

CALCULATION OF THE DAMPING CONSTANT AND ACTIVATION ENERGY FOR RAMAN MODES IN (NH4)(2)SO4

The temperature dependence of the damping constant is calculated for various Raman modes in (NH4)(2)SO4 by the expressions derived from the soft mode hard mode coupling model and the energy fluctuation model. The expressions for the damping constant are fitted to the measured Raman bandwidths and then the activation energies are extracted, which are equal to similar to 0.2 eV for the Raman modes studied. The damping constant of a soft mode is also calculated and the activation energy (similar to 0.1 eV) is deduced in this crystal. Values of the activation energies obtained here, can be compared with the value of k(B)T(C) = 0.02 eV for (NH4)(2)SO4. This indicates that this ferroelectric material undergoes an order disorder transition. It is concluded that the observed behaviour of (NH4)(2)SO4 can be described reasonably by the soft mode hard mode coupling model considered here

Calculation of the Spontaneous Polarization and the Dielectric Constant for the Ferroelectric N(CH3)(4)HSO4 Using the Mean Field Model

The temperature dependences of the spontaneous polarization and the dielectric constant (susceptibility) are calculated using themean fieldmodel for the ferroelectric N(CH3)(4)HSO4. Expressions derived from the mean fieldmodel for the spontaneous polarization and the inverse susceptibility are fitted to the experimental data from the literature. The fitting parameters in the expansion of the free energy in terms of the spontaneous polarization are determined within the temperature intervals in the ferroelectric and paraelectric phases of N(CH3)(4)HSO4. Our results show that the temperature dependences of the spontaneous polarization and the dielectric constant as predicted from our mean field model, describe adequately the observed behavior of N(CH3)(4)HSO4 in the ferroelectric and paraelectric phases

Analysis of the Specific Heat of Ru Doped LiKSO4 Close to Phase Transitions

The temperature dependence of the specific heat, C (p) , is analyzed for different percentages of ruthenium (Ru) content in LiKSO4 using a power-law formula deduced from Ising model. For this analysis, data observed are taken from the literature and the values of the critical exponent for C (p) are extracted in the vicinity of the transition temperature (T (c) = 708 K) within the incommensurate phase of Ru doped LiKSO4. Obtained values of the critical exponent of C (p) confirm in most cases above and below C (p) the critical exponent value predicted by the 3D Ising model for the doped LiKSO4. On this basis, the specific heat, C (p) , of some other molecular crystals can also be analyzed using the same power-law formula as studied here