Phase-transition compounds

Table 4. Properties of phase-transition compounds used for NMR temperature calibrations in the solid state...

The last problem is relative to the crystallographie phases. They are often deduced from electron diffraction patterns (EDP s). Various phenomena whieh oeeurred in the course of the study of thin samples are sometimes badly known, or else the patterns can be misinterpreted and the indexation becomes wholly wrong. Reeently Z. Li et al. (1988) have claimed to the formation of new polymorphic erbium oxide phases. These were in fact the well-known ErH2, C- and B-Er203 compounds (Gasgnier 1980, 1990). Other misinterpretations result from decided opinions on chemical reactivity, phase transitions, compound formation (as Lu(OH)4 for example) (Gasgnier 1991). .. and/or on disorder between two crystallographic phases. The rare earth series display basie chemical and physical properties which are now well established. Moreover, the new micro- (and even nano-) analysis apparatus should be used in a systematic way to insure accurate determination of the specific properties of the materials. 

Figure 3. Electric field strength dependence for the C -G phase transition (compound A7, [191).

Identification of unknown compounds in solutions, liquids, and crystalline materials characterization of structural order, and phase transitions... 

The appearance of spontaneous polarization in the case of LuTaO is related to volumetric irregularities and ordering of the Li+ - Ta5+ dipoles, as is in the case of the similar niobium-containing compound Li4Nb04F. It can be assumed that the main difference between the two compounds is that the irregularities and the Li+ - Ta5+ dipoles are thermally more stable compared to the niobium-containing system. This increased stability of the dipoles leads to the reversible phase transition at 660°C. 

As with other crystalline substances, on heating coordination compounds may melt, sublime, decompose, or undergo a solid phase transition. The greater complexity of the constituents present increases the number of types of bond redistribution processes which are, in principle, possible within and between the coordination spheres. The following solid-state transitions may be distinguished (i) changes in relative dispositions... 

The semiconducting properties of the compounds of the SbSI type (see Table XXVIII) were predicted by Mooser and Pearson in 1958 228). They were first confirmed for SbSI, for which photoconductivity was found in 1960 243). The breakthrough was the observation of fer-roelectricity in this material 117) and other SbSI type compounds 244 see Table XXIX), in addition to phase transitions 184), nonlinear optical behavior 156), piezoelectric behavior 44), and electromechanical 183) and other properties. These photoconductors exhibit abnormally large temperature-coefficients for their band gaps they are strongly piezoelectric. Some are ferroelectric (see Table XXIX). They have anomalous electrooptic and optomechanical properties, namely, elongation or contraction under illumination. As already mentioned, these fields cannot be treated in any detail in this review for those interested in ferroelectricity, review articles 224, 352) are mentioned. The heat capacity of SbSI has been measured from - 180 to -l- 40°C and, from these data, the excess entropy of the ferro-paraelectric transition... 

The compound in Fig. 3b exhibits two smectic phases (Sm and Sm ) in addition to nematic, whereas the compound in Fig. 3a exhibits only a nematic phase. The substitution of an alkoxy for an alkyl tail is known to shift phase transition temperatures considerably. In the cyano-biphenyls (Fig. 4), substitution of an alkoxy tail raises the melting point from 24 to 48 °C and T from 35 to 68 °C [22]. 

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

Fig. 14. Temperature dependences of the layer spacing in the smectic Ad and Cd phases for terminally polar polyphilic compounds F4H11OCB (squares), FsHnOCB (circles), FgHioOCB (triangles) and FioHnOCB (diamonds). The arrows indicate the smectic Ad - smectic Cd phase transition points (Ostrovskii et al. [45])...

Following the high-temperature syntheses used for these compounds according to the reaction (4), a phase transition into the low-temperature modification may occur on slow cooling. The high-temperature phase, on the other hand, may be frozen out at room temperature through fast quenching (Fig. 8.5). 

The Nobel prize in Chemistry for the year 1996 was awarded for the discovery of the fullerenes, the third allotropic form of carbon, with Cgo and C70 as the two most prominent representatives. While the fullerenes of course are the epitome of carbon-rich molecular compounds, it is an irony that their synthesis is more of a physical phase transition, taking place under drastic conditions [1]. 

When the free enthalpy of reaction AG for the transformation of the structure of a compound to any other structure is positive, then this structure is thermodynamically stable. Since AG depends on the transition enthalpy AH and the transition entropy AS, and AH and AS in turn depend on pressure and temperature, a structure can be stable only within a certain range of pressures and temperatures. By variation of the pressure and/or the temperature, AG will eventually become negative relative to some other structure and a phase transition will occur. This may be a phase transition from a solid to another solid modification, or it may be a transition to another aggregate state. 

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