Jacek Mayer

CALORIMETRIC AND DIELECTRIC STUDIES OF RELAXATION ACCOMPANYING A GLASS TRANSITION IN THE RIGHT-HANDED ISOPENTYLCYANOBIPHENYL (5*CB)

Abstract The specific heat measurements by means of the adiabatic calorimetry provided new information concerning the polymorphism of 5*CB. A relatively large enthalpy relaxation was observed in the vicinity of glass transition Temperature dependence of the relaxation time showed that the main contribution to the relaxation is due to the reorientations of 5*CB molecules around the short axes. The comparison of the calorimetric results with the dielectric relaxation times was performed. Data from both methods are well described by the Vogel-Fulcher-Tammann equation with the same fitting parameters.

Relaxation in glass and cholesteric phase of isopentylcyanobiphenyl

Abstract The paper presents adiabatic calorimetry investigations of slow relaxation processes (enthalpy relaxation) in isopentylcyanobiphenyl (right handed (s) 4–(2-methylbutyl) 4′–cyanobiphenyl) in a vicinity of the glass transition. The preliminary neutron scattering results concerning the fast relaxation processes and low-energy excitation detected as “boson peak” are also presented. The general behaviour observed in the experiments seems to correspond to the typical picture for the molecular glasses.

A DSC study of K1−x(NH4)x SCN mixed crystals

Abstract The phase transitions in mixed crystals of the system K 1− x (NH 4 ) x SCN for a wide range of x values have been studied using differential scanning calorimetry (DSC). On the basis of the experimental data a phase diagram was constructed. The diagram contains two curves: (a) - the line of the order-disorder phase transition separating regions of the orthorhombic (phase II) phases, (b) - the line of coexistence of the monoclinic (phase III) and the orthorhombic one. The enthalpies of both transitions were determined. Careful investigations of the thermal hysteresis of the phase II-phase I transition showed no systematic dependence on the concentration. For most concentrations Δ hyst. T ≈ 0 and does not excite 0.8 K for any concentration. For pure ammonium thiocyanate the kinetics of the spontaneous transition of undercooled phase II into stable phase III has been investigated.

Quasielastic neutron scattering (QNS) study of cation rotation in (CH3NH3)5Bi2Cl11, (CD3NH3)5Bi2Cl11 and (CH3NH3)5Bi2Br11

Abstract Rotation of (CH3NH3)+ cations in crystalline (CH3NH3)5Bi2Cl11, (CD3NH3)5Bi2Cl11 and (CH3NH3)5Bi2Br11 was studied by QNS. The model of instantaneous 120° jumps of whole cations around their C–N axes was fitted to the experimental data. The parameters describing reorientation obtained for two types of cations agree well with the results of NMR experiments.

Neutron quasielastic scattering by (CH3NH3)5Bi2Cl11

Abstract (CH3NH3)+-cation rotation in crystalline (CH3NH3)5Bi2Cl11 was studied by the QNS. The following (no doubt simplified) models were considered: 1. 1. All cations are equivalent and rotate as a whole around C-N axes 2. 2. Majority of cations behave as in point (1) and the remaining ones do not move (for the neutron observation window). 3. 3. All CH3 groups rotate, whereas all NH3 groups do not move. The QNS measurements exclude the possibilities (1) and (3). A transition at ca. 160–170 K was confirmed in the measurements, as connected with an increase of the quasielastic component intensity observed via a dip of the EISF.