Solid-state reactions phase transitions

Time-resolved X-ray diffraction has been used for a long time to study solid-state reactions. With the emergence of the new radiation sources it is now possible to follow solid-state reactions, phase transitions and physical changes caused by perturbations on much shorter time-scales. 

Pb5Al3Fi9 results from the reaction of Pbp2 and AIF3 in the solid state. Four phase transitions occur at 160, 295, 320 and 370 K. All varieties contain trans-PAFs chains and isolated AlFg octahedra (Figures 12.24a and b). They differ by the relative rotations or... 

Since an analyte and interferent are usually in the same phase, a separation often can be effected by inducing a change in one of their physical or chemical states. Changes in physical state that have been exploited for the purpose of a separation include liquid-to-gas and solid-to-gas phase transitions. Changes in chemical state involve one or more chemical reactions. 

Phase transitions Spin and magnetic transitions Dynamic solid-state phenomena Solid-state reactions (e.g., thermolysis, radiolysis)... 

A further method of producing amorphous phases is by a strain-driven solid-state reaction (Blatter and von Allmen 1985, 1988, Blatter et al. 1987, Gfeller et al. 1988). It appears that solid solutions of some transition metal-(Ti,Nb) binary systems, which are only stable at high temperatures, can be made amorphous. This is done by first quenching an alloy to retain the high-temperature solid solution. The alloy is then annealed at low temperatures where the amorphous phase appears transiently during the decomposition of the metastable crystalline phase. The effect was explained by the stabilisation of the liquid phase due to the liquid—>glass... 

Glass transition determinations Decomposition reaction Reaction kinetics Phase diagrams Dehydration reactions Solid-state reactions Heats of absorption Heats of reaction Heats of polymerization Heats of sublimation Heats of transition Catalysis... 

Chapters 6 and 7 dealt with solid state reactions in which the product separates the reactants spatially. For binary (or quasi-binary) systems, reactive growth is the only mode possible for an isothermal heterogeneous solid state reaction if local equilibrium prevails and phase transitions are disregarded. In ternary (and higher) systems, another reactive growth mode can occur. This is the internal reaction mode. The reaction product does not form at the contacting surfaces of the two reactants as discussed in Chapters 6 and 7, but instead forms within the interior of one of the reactants or within a solvent crystal. 

For a solid-state reaction, one of the solutions of Equation 3.1 is the Avrami-Erofeev equation [3], The phase transition model that derives this equation supposes that the germ nuclei of the new phase are distributed randomly within the solid following a nucleation event, grains grow throughout the old phase until the transformation is complete. Then, the Avrami-Erofeev equation is [3]... 

Prasad et al. (1982) have described Raman investigations of solid state reactions. They show that it is advantageous to obtain the spectra at lower temperature, typically at 120 K. Lower frequency phonon spectra show considerable broadening at room temperature, so that details of spectral features are lost. Raman phonon spectra of 2-benzyl-5-benzilidene cyclopentanone at room temperature and at 77 K (see Fig. 6.8-18) indicate that lower temperature improve.s the resolution of the spectral features. This phenomenon can be used for a detailed analysis in order to elucidate the reactivity of a compound at room temperature. However, it is necessary to make sure that neither the reactant nor the product undergoes a structural phase transition between room temperature and the low temperature at which the spectrum is recorded. This is confirmed by studying the phonon spectra as a function of the temperature. 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

Figure 16. Illustration of change of solid-state reaction rate constant in phase transition. Soft-mode splitting out occurs at point equalling 0,7i/4a, njla, 3 t/4o and n/a for curves 1-5, respectively. K( T) jump at Kq = 0 determined by difference in amplitude in two phases.

The kinetics associated with the thermally induced phase transformations of phenanthrene and caffeine monohydrate were studied using hot-state quantitative XRPD.29 Using a single non-isothermal experiment conducted at a constant heating rate, it was possible to obtain the activation parameters for the solid-state reactions. In another study, quantitative XRPD was used to study the tetrahydrate to monohydrate transition of the sodium salt of 5-(4-oxo-phenoxy-4H-quinolizine-3-carboxamide)-tetra-zolate.30... 

In recent years interest in these materials has grown mainly for physical reasons. The layer perovskites are looked at as model compounds for the study of magnetic properties in two-dimensional systems (J2) and as models for the study of structural phase transitions in lipid bilayer-type arrays ( ). The use of layer perovskites as a matrix for organic solid state reactions represents a fairly new research topic. First experiments were carried out studying the photolysis of butadiyne (diacetylene) derivatives (li-ZSl) For a corresponding study of the butadiene derivatives the compounds listed in Table I were synthesized. 

The reduction occurs by direct oxygen removal from the solid oxides (solid-state diffusion). The basic underlying mechanism is not known (diffusion of O, OH, H2O) and is likely to vary for different for different phase transitions. On the final reduction the metal remains pseudomorphous to the starting oxide, forming a polycrystalline metal sponge. Solid-state reactions are characteristic for low reduction temperatures (<750 °C) and the early WO3 - WO2 9 transition ( crystallographic shear transition). 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

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