Crystalline structures spectra

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

The X-ray diffraction spectrum in Figure 8 shows the crystalline structure of a normal cellulosic membrane. Diffraction peaks appeared around 10, 11, 16, and 21 degrees of 20. This spectrum... 

The problem with limited selectivity includes some of the minerals which are problems for XRD illite, muscovite, smectites and mixed-layer clays. Poor crystallinity creates problems with both XRD and FTIR. The IR spectrum of an amorphous material lacks sharp distinguishing features but retains spectral intensity in the regions typical of its composition. The X-ray diffraction pattern shows low intensity relative to well-defined crystalline structures. The major problem for IR is selectivity for XRD it is sensitivity. In an interlaboratory FTIR comparison (7), two laboratories gave similar results for kaolinite, calcite, and illite, but substantially different results for montmorillonite and quartz. 

In the same samples, a second absorption feature was detected that is associated with the dopant ions themselves. These ligand-field transitions allow distinction among various octahedral and tetrahedral Co2+ species and are discussed in more detail in Section III.C. The three distinct spectra observed in Fig. 4(b) correspond to octahedral precursor (initial spectrum), tetrahedral surface-bound Co2+ (broad intermediate spectrum), and tetrahedral substitutional Co2+ in ZnO (intense structured spectrum). Plotting the tetrahedral substitutional Co2+ absorption intensity as a function of added base yields the data shown as triangles in Fig. 4(b). Again, no change in Co2+ absorption is observed until sufficient base is added to reach critical supersaturation of the precursors, after which base addition causes the conversion of solvated octahedral Co2+ into tetrahedral Co2+ substitutionally doped into ZnO. Importantly, a plot of the substitutional Co2+ absorption intensity versus added base shows the same nucleation point but does not show any jump in intensity that would correspond with the jump in ZnO intensity. Instead, extrapolation of the tetrahedral Co2+ intensities to zero shows intersection at the base concentration where ZnO first nucleates, demonstrating the need for crystalline ZnO to be... 

If the molarity is further increased, and the material is allowed to precipitate on a substrate, the liquid crystalline structure of the precipitate will exhibit a highly anisotropic absorption spectrum at the resonant wavelength of its chromophore as well as its normal isotropic molecular absorption spectrum. The relaxation characteristic will either remain unchanged or be impacted by the electronic characteristics of the substrate. 

The emission spectrum is composed of the A2 emission which is characteristic for Cr in the crystals with a large field strength, and of the wide T2 A2 emission in the glass and low field crystalline sites. The E —> A2 emission reveals a complex structure which is well resolved at 4 In order to understand the origin of various bands we need to know the crystalline structure of spinels and sites into which Cr can enter. 

Cellulose pyrolysis has been studied in detail from a variety of points of view mainly related to chemical utilization of wood pyrolysis products or to fire related problems. Analytical pyrolysis of cellulose is not often used as a tool for cellulose detection, but it is a common procedure for studying the pyrolysis products. A variety of analytical procedures have been applied for this study, pyrolysis/gas chromatography/mass spectrometry (Py-GC/MS) being the most common [11-16]. Besides Py-GC/MS, other analytical procedures also have been utilized, such as Py-MS [17,18], Py-IR [19], and off-line Py followed by HPLC [20]. The Py-MS spectrum of cellulose was shown in Figure 5.4.1 (B). Some procedures applied GC/MS on derivatized pyrolysis products (off-line), the derivatization being done by silylation [21], permethylation, perbenzoylation [22], etc. Information about cellulose also has been obtained from the analysis of pyrolysis products of several cellulose derivatives, such as O-substituted cellulose [23]. Also the study of cellulose crystalline structure with X-ray during pyrolysis has been used [23a] to generate information about the pyrolysis mechanism. 

The crystalline structure was characterized by X-ray diffraction (XRD) using a Rich-Seifert Iso-Debyeflex 2002 diffractometer. Radiation was Cu K(i, filtered with Ni the diffraction spectrum range was 0 < 2( < 60°. 

X-rays are electromagnetic radiation of wavelength about lA (10 m), which is about the same size as an atom. They occur in that portion of the electromagnetic spectrum between gamma rays and the ultraviolet. The discovery of X-rays in 1895 enabled scientists to probe crystalline structure at the atomic level. X-ray diffraction has been in use in two main areas for the fingerprint characterization of crystalline materials and the determination of their structure. Each crystalline solid has its unique characteristic X-ray powder pattern, which may be used as a "fingerprint" for its identification. Once the material has been identified. X-ray... 

Prior to these studies, the Raman spectrum of the photo-induced sulfur, p-S, has never been reported. Based on the discrete features of the Raman spectrum shown in Fig. 23 a certain molecular and crystalline structure for... 

Once again, it should be pointed out that the exact shape of the modulus-temperature curve of a crystalline polymer depends on the thermal history of the sample, particularly on the rate of cooling from the melt and annealing treatment. Two crystalline polymers are mechanically equivalent," for practical purposes, if they have the same values of Tg, Tm chain length, percentage of crystallinity, and crystalline structure. Because this is rarely the case, semicrystalline polymers exhibit a wide spectrum of properties. 

The technique of Raman scattering (RS) to study vibrational spectra in the numerous polytypes of SiC will be described. An explanation of the various notations used to describe the stacking sequences in these polytypes will then be given. Section C discusses the various optical phonons studied by RS and the concept of a common phonon spectrum for all polytypes will be introduced. Raman studies are also used to assess crystalline structure and quality of epitaxial layers of SiC on Si and SiC substrates. Section D outlines several other excitations of interest, e.g. polaritons, plasmons, and electronic RS, as well as impurity and defect recognition in irradiated and ion implanted material. 

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