High temperature X-ray analysis

At higher temperatures two nonstoichiometric phases, o-(Ce20j+j) and a(Ce02- ), dominate the phase diagram. These appear in high temperature X-ray analysis to be bixbyite-related or fluorite-related respectively with widely varying lattice parameters. 

In many cases, high-temperature modifications of sulfidic compounds cannot be quenched for room temperature examination. Inversion twinnings, crystal morphology, or other crystallographic features may indicate the appearance of polymorphism. Under these circumstances differential thermal analysis (DTA) can be suitable for the determination of the exact phase transition temperatures. DTA determinations are practically valuable if used in conjunction with high-temperature X-ray diffraction methods. DTA apparatus can operate up to 1100 °C and can be specially designed for sulfides2-4) individual experimental techniques are included in these references. 

Existence of five polymorphic forms was shown by high-temperature X-ray diffraction (1, 6, 7) and differential thermal analysis (6, 7). Both o- and 0-forms persisted at room temperature, but the a-form appeared only on quenching of high-temperature forms (1,). The o-form transformed to 0 near 490 K (J, 6), but the reverse transformation was not observed. Single-crystal X-ray diffraction showed the o-form to be orthorhombic (8). 

The recent development of solid state detectors has enabled highly efficient X-ray analysis to be carried out by energy dispersive methods. The detector unit consists of a silicon-lithium doped crystal cooled to liquid nitrogen temperature. 

The high temperature oxidation of (J-NiAl, undoped and doped with Ce, Y and Hf was studied in situ by thermogravimetry in He with p(02) = 5 10 6bar at 1000°C and by high temperature X-ray diffraction at 950 and 1000°C in air. After the in situ experiments the samples were analysed metallographically by optical microscopy and by scanning electron microscopy (SEM) with energy dispersive analysis (EDX). 

Combining a high-temperature X-ray camera with a thermobalance is difficult due to geometrical and focus problems. However, Wiedemann and Bayer (61) have described such a technique in which high-temperature X-ray diffraction patterns and information about its mass-loss can be obtained on a single sample. This new technique, which they called thermomolecular beam analysis (TMBA), is illustrated schematically in Figure 3.35. The... 

The methods used are differential thermal analysis (DTA) and X-ray diffraction (including high temperature X-ray diffraction). The experimental result is that there is indeed a 1 1 congruently melting compound (CsCaBra) formed. The melting point of CsCaBrs is 821 C. 

P. J. Freud and P. N. LaMori, Non-dispersive high pressure—high temperature X-ray diffraction analysis, Trans. Am. Cryst. Assoc. 5, 155-162 (1969). 

Next we studied high temperature bromination of benzobarrelene at 150 C. NMR analysis indicated that the reaction mixture was very complex and consisted of at least ten products. After repeated column chromatography combined with fractional crystallization we have been able to separate 18 compounds (Scheme 6). Four of them were bromoalcohol compounds 18, 12, 22 and 2fl. After high temperature bromination we expected three isomeric non-rearranged products with benzobarrelene skeleton and isolated 22, 22, and 24 in yields of 34, 9.3, and 6.2 %, respectively. Because of the very close structural similarity we were not able to make a clear-cut differentiation between the stereochemistry of 22 and 24-Therefore, we carried out an X-ray analysis (ref. 9) of the isomer 22-... 

As described in Section 9.4.7.1, some Rh and Ru carbenoid intermediates that undergo cyclopropanation reactions have been spectroscopically identified.294,254 Less reactive metal-carbenoid intermediates (108) and (109) have been isolated and their structures have been determined unequivocally by X-ray analysis.255 258 The isolated carbenoid intermediate (108) undergoes cyclopropanation at high temperature (110°C),255 and another intermediate (109) serves as the catalyst for asymmetric cyclopropanation (Figure 11).258... 

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