Crystal structure transformation

Fig. 11.21 shows the results of TG and DTA measurements on mixtures of AP particles and catalysts. The endothermic peak observed at 513 K is caused by the crystal structure transformation of AP from orthorhombic to cubic. A two-stage exothermic decomposition occurs in the range 573-720 K. The decomposition of the AP is seen to be drastically accelerated by the addition of catocene. The exothermic peak accompanied by mass loss occurs before the AP crystal transformation. Although the AP is sensitized by the addition of carborane, no effect is seen on the AP decomposition. The results indicate that carborane acts as a fuel component in the gas phase but does not catalyze the decomposition of AP. Thus, the critical friction energy is lowered due to the increased reaction rate in the gas phase. The results imply that the initiation of ignition by friction is caused by the ignition of the gaseous products of the AP pyrolants.PI... 

Rao, C.N.R., Prakash, B., and Natarajan, M. (1975) Crystal Structure Transformation in Inorganic Nitrites, Nitrates and Carbonates, National Standard Reference Data Series (NSRDS)-Nafional Bureau of Standards (NBS), Washington, D.C. USA. 

Theoretical treatment of order-disorder structural phase transitions in hydrogen-bonded ferroelectrics implies, in principle, the determination of the transition critical temperature Tc and accompanying crystal structure transformations. 

The particle size and crystallite size of anatase increase markedly in the region of the crystal structure transformation. The unit cell of anatase seems to expand prior to the transformation to rutile. This expansion has been attributed to a displacive transformation of the type defined by Buerger. Smaller particle size and larger surface area seem to favour the transformation. 

The results discussed show that crystal structure transformations are considerably dependent on the thermal history of the samples to be more specific, the crystallite size, particle size and surface area have measurable effects on the transformation. It would, therefore, probably be difficult to reproduce strictly transformation data with different samples. The magnitudes of these effects are, however, not too great to result in the wide variability of temperatures of polymorphic transformations. The wide variations in transformation temperature can only be due to other factors... 

It can be seen that Ni/BaTiOj catalyst possesses high activity and does not deactivate for a long time under high reactive temperature. Whereas, though Ni based catalysts such as Ni/y-AljOj also show high activity, they often deactivate rapidly due to carbon deposition or the crystal structure transformation of the support occurring at high reactive temperature, which hinders the process from industrial applications. 

Rao CNR (1961a) Kinetics and thermodynamics of the crystal structure transformation of spectroscopically pure anatase to rutile. Can J Chem 39 498-500... 

The most useful rule in describing the effect of pressure on solids is the so-called Pressure-Coordination Rule (i, 2) according to which the coordination number is increased with pressure. In Table 1 and 2 examples are listed for various crystal structure transformations which follow this qualitative mle at different pressures and temperatures. An exception to this mle is known, however, for ytterbium (2) the cubic face-centered modification (coordination number = 12) of the metal is transformed at 40 kbar into a cubic space-centered stmcture (coordination number = 8). 

Mixed Oxides. The crystal structure transformations of inorganic chromates have been reviewed and data on the thermodynamic, crystallographic, and spectroscopic properties listed. ... 

Rajeshwar et al. (180) used DSC to determine the enthalpy of crystal structure transformations of MC104 salts, where M — Na, K, Rb, Cs, and NH4. As illustrated in Table 7.15, inconsistencies in the literature data are striking, especially with M = Na, K, Rb, and Cs. Most of the earlier studies used DTA techniques to determine the enthalpy changes. However, the literature values for NH4CI04 are in good agreement with the author s results. 

Transition Metal Hydrides , ed. E. L. Muetterties, Dekker, New York, 1971. Crystal Structure Transformations in Binary Halides , C. N, R, Rao and... 

Yu ZJ, Huang XW, Zhuang WD, Cui XZ, Li HW (2005) Crystal structure transformation and luminescent behavior of the red phosphor for plasma display panels. J Alloys Compd 390 220... 

The endothermic peak at 226 C belongs to the crystal structure transformation of 3"< y< lodextrin. The separated peaks, attributed to the decomposition of the PGI -Me and the 3-< yclodextrin are clearly seen on the TG and DTG curves. The exothermic decomposition of the complex begins above 200 C and decomposes in one step ... 

Here, Bq is the bulk modulus and Bq is its pressure derivative. The solid line in Figure 1 shows the result of fitting, in which the observed data are well reproduced. The obtained values are Vq = 42.68 A, Bq = 69.7 GPa, and Bq = 3.97. It is interesting to note that the initial compressibility is very close to the average compressibility of Gd and Cu elements. The authors suggested possible crystal structural transformation above 11 GPa to a low-symmetry structure. This result confirms the hypothesis of Degtyareva et al. (1997) in which the existence of an orthorhombic structure with the AuCd-type at high pressure has been proposed. 

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