M. Rachwalska

Study of the Phase Situation in 4-n-pentylphenyl-4′-n-heptyloxythiobenzoate (7S5)

Abstract Results of measurements of the specific heat of 4-n-pentylphenyl-4′-n-heptyloxythiobenzoate in the temperature range from about 90 K to about 370 K are presented. They were carried out with an adiabatic calorimeter, a differential scanning calorimeter and a polarizing microscope. Adiabatic calorimetry revealed four anomalies in the specific heat vs. temperature curve viz., two associated with transitions in the solid phase, one with melting and one with the liquid-crystalline nematic-to-isotropic phase transition. Also, two modifications of the solid, stable and metastable, were found to exist. In addition, the DSC method applied in the 285 K to 370 K range revealed a smectic C phase, but only when the sample is being cooled. The polarizing microscope showed the structural polymorphism of the solid phase in the 297 K to 327 K range to be more complex than would stem from adiabatic calorimetry and DSC measurements alone. It also confirmed the occurence of the Sc phase on cooling as ascertained by ...

An adiabatic calorometry study of [Mg(H2O)6](ClO4)2

Abstract Specific heat vs. temperature was measured for polycrystalline [Mg(H2O)6](ClO4)2 in the temperature range 90-350 K. Four anomalies in specific heat curve are found at about 108 K, 168 K, 273 K and 325 K. The entropy changes indicate a configurational disorder of ClO-4 ions in the high temperature phases. The results are compared to those previously obtained for [Ni(H2O)6](ClO4)2. On the basis of analogy one may expect an existence of a phase transition below 90 K for [Ni(H2O)6](ClO4)2 not yet detected.

Calometry, proton magnetic resonance and quasielastic neutron scattering studies of [Mg(NH3)6] (NO3)2

Abstract Adiabatic calorimetry measurements show that [Mg(NH3)6](NO3)2 goes from a room temperature phase I to an intermediate phase II below 221 K and then to a phase III below about 110 K, when cooled. On heating, the III to II transition occurs around 145 K. The structural phase transitions and the hysteresis are also reflected in the correlations times of the NH3 reorientation motion as derived from NMR relaxation and quasielastic neutron scattering.

An adiabatic calorimetry study of [Ni(NH3)6](NO3)2

Abstract Specific heat vs. temperature was measured for polycrystalline [Ni(NH3)6](NO3)2 in the temperature range 83–256 K. Two anomalies, one at ≃197 K and one at ≃247 K were found. The corresponding entropy values are 1.21 and 6.05 e.u. In this respect the substance to be similar to the previously measured [Ni(NH3)6](BF4)2, [Ni(NH3)6](ClO4)2 and [Mg(NH3)6](ClO4)2. However, further foundings are different: the intermediate phase, normally existing between 197 and 247 K, may be undercooled until ≃100 K, being metastable below 197 K.

DSC study of the phase transitions in [Ni(D2O)6](ClO4)2 and [Ni(H2O)6](ClO4)2

DSC measurements were carried out for [Ni(H2O)6](ClO4)2 (sampleH) and [Ni(D2O)6](ClO4)2 (sampleD) in the temperature range 300–380 K. For both compounds two anomalies on the DSC curves were detected. The results for sampleH are compared to those previously obtained using adiabatic calorimetry method. For both compounds studied in this work the high-temperature transition appears at the same temperature while the low-temperature one is shifted towards higher temperatures in sampleD. Disorder connected with H2O or D2O groups is suggested in the intermediate phase between the low- and high-temperature transitions.ZusammenfassungAn [Ni(H2O)6](ClO4)2 (ProbeH) und [Ni(D2O)6](ClO4)2 (ProbeD) wurden DSC-Messungen im Temperaturintervall 300–380°C durchgeführt. Für beide Verbindungen wurden an den DSC-Kurven zwei Anomalien festgestellt. Die Ergebnisse für die ProbeH wurden mit denen verglichen, die unlängst mittels adiabatischer kalorimetrie ermittelt wurden. Für beide in vorliegender Studie untersuchten Verbindungen erscheint die Umwandlung bei höherer Temperatur bei der gleichen Temperatur, während die bei niedriger Temperatur im Falle der ProbeD in Richtung höherer Temperaturen verschoben wird. Für die Übergangsphase zwischen den Nieder- und Hochtemperaturmwandlungen wird eine Fehlordnung bezüglich der H2O oder D2O Gruppen angenommen.

Adiabatic calorimetry and neutron diffraction studies of phases and phase transitions in [Ni(ND3)6](NO3)2

Abstract Calorimetry, X-ray and neutron diffraction measurements have been performed on [Ni(ND 3 ) 6 ](NO 3 ) 2 on heating and cooling to study the phase transitions which occur, and to determine the structure of the different phases which are formed. The high temperature phase, phase I, has a face centered cubic structure of space group Fm3m and transforms on cooling to a phase II of simple cubic structure, space group Pa3. On further cooling an orthorhombic deformation leads to phase III. Below 80 K a further splitting of the neutron peaks gives evidence of a phase IV of monoclinic structure.

Calorimetric Study of NPOB

Abstract In the case of NPOB two solid modifications, stable and metastable, were found. Thermodynamic parameters were determined for the solid-smectic A, smectic A-nematic and nematic-isotropic liquid phase transitions. The nature of these transitions is analyzed. Special attention was paid to the kinetics of the metastable-to-stable phase transition.

Mixed Mode, Sequential and Relaxation Oscillations in the Belousov-Zhabotinsky System

The behaviour of a system composed of malonic acid (MA), KBrO3, H2SO4, and ferroin was investigated in batch experiments at various concentrations of oxygen above the chemical mixture when changing the concentration of MA. We could observe that at 10% of oxygen or more and for initial concentrations of malonic acid [MA]0 between 0.15 M and 0.6 M the system attains an equilibrium by some of mixed mode oscillations. Such a behaviour of the system could be described by a model used lately. Additionally for [MA]0 = 0.1 M or less at 0 - 20% of oxygen a region of sequential oscillations was discovered (observed for the first time in the system with ferroin) and a trial of understanding of the event is suggested. For rather small [MA]0 ca. 0.025 M no mixed mode oscillations, no sequential ones but the so-called relaxation oscillations were observed.

FIR spectra of [Ni(H2O)6](ClO4)2 and [Ni(D2O)6](ClO4)2 at various temperatures

Low frequency vibrations in [Ni(H2O)6](ClO4)2 and [Ni(D2O)6](ClO4)2 were measured by the FIR Fourier Transform Spectroscopy method. They were compared with those obtained by the IINS method. No evidence of proton glass discovered by the IINS was observed via the FIR method. By applying the band contour method, the barrier to rotation Uact for H2O groups in [Ni(H2O)6](ClO4)2 crystals was determined. A III–II phase transition mechanism was proposed.

The effect of oxygen on time-dependent bifurcations in the Belousov–Zhabotinsky oscillating chemical reaction in a batch

We have studied the effect of oxygen on the time-dependent bifurcations of transient oscillations in the Belousov-Zhabotinsky oscillating chemical reaction in a closed system. Experiments show that oscillations disappear through different bifurcations depending on the oxygen concentration in gas phase above the reaction solution. Oscillations disappear through the time-delayed Hopf bifurcation at low oxygen concentrations, whereas at high oxygen concentrations they disappear through the time-dependent SNIPER (saddle-node infinite period) bifurcation. We propose a kinetic scheme that describes the effects observed in experiments. Good agreement between the experimental data and simulations is obtained.