Transformations crystallographic

X-Ray and electron diffraction measurements have been most usually used to characterize the phases present in any reactant mixture, and provide a means of identification of solid reactants, intermediates and products. In addition to such qualitative analyses, the method can also be used quantitatively, with suitable systems, to determine the amounts of particular solids present [111], changes in lattice parameters during reaction, topotactical relationships between reactants and products, the presence of finely divided or strained material, crystallographic transformations, etc. 

Undoped LaMn03 is orthorhombic at room temperature and shows an orthorhombic/ rhombohedral crystallographic transformation at 600°C [6], This transformation has been attributed to the oxidation of some Mn3+ to Mn4+ ions. Thus, the orthorhombic/... 

In Region IV, performance again increases. Crystallographic transformations in phosphate conversion coatings at 180 C are known to adversely affect phosphate crystal adhesion (26). However, nitrile cyclization may be important at these higHer temperatures. Cyclization is believed to enhance barrier properties of high nitrile polymers (27,28.29). 

Data derived from pressure experiments on semi-conducting elements by Klement and Jayamaran (1966) and Minomura (1974) have also been useful in obtaining confirmation that the entropies associated with transitions in Si, Ge and Sn form a consistent pattern, supporting the concept that each crystallographic transformation tends to have a characteristic associated entropy change (Miodownik 1972a, 1972b). Similarly, extrapolations from pressure data on alloys can be used to obtain estimates of lattice stabilities at P = 0, which can then be compared with estimates obtained by other routes, such as SFE measurements. 

Kolaczkowska and T. Urbanski [32] on the basis of X-ray investigations attempted to explain the crystallographic transformations suffered by starch as result of nitration. They used nitrated potato starch and soluble starch in this investigation. 

It is well established that all the known martensitic transitions, before Nitinol, occurs through crystallographic transformation [11], As described above [10], single crystal X-ray diffraction work shows a crystallographic distortion (rather than transformation) and therefore is a second-order transformation [12], This conclusion has the support of heat capacity investigation [13] that gave a latent heat of transition AH >... 

A wide variety of spectroscopic techniques [1-4] are used to provide information about the identities and amounts of reactant, the solid product(s), or the solid phase(s) present at intermediate stages of the decomposition. The cooling of reactant samples after partial or complete decomposition may be accompanied by changes such as crystallographic transformations, solidification of any molten material and particle disintegration. Interpretations of observations must take account of these possibilities. 

The crystallographic transformations which follow water removal firom Group IIA metal oxalate hydrates have been studied in some detail, but structural [131] changes have not always been considered in kinetic investigations of water removal from transition-metal oxalate hydrates. 

Judd and Pope [36] conclude that because the activation energies for decompositions of CaCOj, SrCO, and BaCOj are all close to the corresponding enthalpies of dissociation (apparent values of , are 180,222 and 283 kJ mol and A//, 178, 235 and 269 kJ mol, respectively) the mechanisms of decomposition in all three substances are the same as that proposed by Hills [18] for calcite. Strontium carbonate [37] generally resembles the calcium salt in that an increase in sample size results in a decrease in reaction rate. Differences in behaviour were ascribed [37] to the occurrence of a crystallographic transformation and to fusion. 

The thermal decomposition of AP proceeds to completion in two distinct stages. The low temperature decomposition (420 to 600 K) is an autocatalytic nucleation and growth process. Kinetic behaviour is influenced by a crystallographic transformation (orthorhombic to cubic) at 513 K. Reaction rates just above 513 K are significantly slower than rates immediately below this temperature. The residual... 

Many other workers [18-24] have similarly concluded that the first step in decomposition of NH4CJ04 is the transfer of a proton from the cation to the anion. This explains [4] the increase in induction period to reaction following exposure of salt to ammonia and the more rapid onset of reaction of reactant contaiiung small amounts of HQO4. The inhibiting effect of NHj changes across the crystallographic transformation [25]. The stability of the residual phase in the low temperature... 

J, Ragai and W, Lotfi, Effect of preparative ph and aging media on the crystallographic transformation of amorphous TiOz to anatase and ratile. Colloids Surf., 61 97-109, 1991 S, Han, S, H, Choi, S, S, Kim, M, Cho, B, Jang, D, Y, Kim, J, Yoon, and T, Hyeon, Low-temperature synthesis of highly crystalline Ti02 nanocrystals and their application to photocatalysis, Small, 1 812-816, 2005... 

By crystallographic transformations one generally means that, when a new phase (product) forms from the old (parent), it bears certain deHnite geometrical relationships. The most widely studied crystallographic transformation is the martensitic transformation, the prototype of which occurs in quenched steels. Actually, martensite (in honor of Professor A. Martens) was the name given by Osmond in 1895 to the microstructure observed in quenched steels, but in more modern times the word martensite designates a transformation mechanism, now known to be associated with many metals, alloys, ceramics, and even some polymers. These transformations occur in a variety of intermetallics, most notably the Hume-Rothery electron compounds as found, for example, in /3-brass of near-equiatomic composition, and many similar alloys of Cu, Ag, and Au. 

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