Temperature dependence of the phase structure

B. Molecular Weight Dependence of the Phase Structure at Room Temperature... 

We have known by examining the room temperature spectrum of the bulk-crystals that the phase structure depends strongly on the molecular weight but it is generally composed of the crystalline, interfacial, and interzonal regions, of which molecular mobilities differ with each other. The temperature dependency of the phase struc-... 

Stezowski, J. Phase transition effects a crystallographic characterization of the temperature dependency of the crystal structure of the 1 1 charge transfer complex between anthracene and tetracyanobenzene in the temperature range 297 to 119 K. J. Chem. Phys. 73, 538-547 (1980). 

Fig. 20. Temperature dependence of the phase transition from the NaGdCU to the NaEtCU type of structure in NaRCl4-type chlorides (R=Eu-Yb, Y).

The phase transition described above can be characterized as a second order transition. However, the nature of the metal-insulator-transition in FA2X-salts is still an unsolved question a a matter of continuing research. In DSC experiments two transitions at approximately 200 K an 180 K are observed of which the first one is the transition which can be monitored crystallographically. However, the electronic properties change abruptly at 180 K. The assumptic of the existence of a lower transition is also supported by some results of ESR- and NMR-experiments with a pronuonced line broadening at and below the phase transition around K as well as by measurements of the static susceptibility Wd the electrical conductivity. At thi temperature no anomalies in the temperature dependence of the crystal structures can be detected... 

The temperature dependent formation and transformation of meso-structures in water-[bmim][BF4] mbctures has been investigated using nitroxide spin probes. The temperature dependence of the solution structure was monitored via a spin probe. Additionally the phase behaviour on cooling and reheating was probed by differential scanning calorimetry. Thermal hysteresis and memory effects were observed. EPR data before and after freezing revealed a transformation of the mesostructures, probably triggered by crystallization of water pools to ice. 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

A number of other thermodynamic properties of adamantane and diamantane in different phases are reported by Kabo et al. [5]. They include (1) standard molar thermodynamic functions for adamantane in the ideal gas state as calculated by statistical thermodynamics methods and (2) temperature dependence of the heat capacities of adamantane in the condensed state between 340 and 600 K as measured by a scanning calorimeter and reported here in Fig. 8. According to this figure, liquid adamantane converts to a solid plastic with simple cubic crystal structure upon freezing. After further cooling it moves into another solid state, an fee crystalline phase. 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

As in the case of the diffusion properties, the viscous properties of the molten salts and slags, which play an important role in the movement of bulk phases, are also very structure-sensitive, and will be referred to in specific examples. For example, the viscosity of liquid silicates are in the range 1-100 poise. The viscosities of molten metals are very similar from one metal to another, but the numerical value is usually in the range 1-10 centipoise. This range should be compared with the familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for the temperature dependence of the viscosity of liquids as an Arrhenius expression is... 

Thus the temperature dependency of the three components for this sample with a molecular weight of 90000 has been adequately explained in terms of the a-, 0-, and 7-relaxation phenomena associated with the detailed phase structure of the sample. 

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