Unit cell determination

Both compounds crystallize with the cadmium diiodide structure (space group P3ml) as previously reported on polycrystalline samples.3 For platinum disulfide, ao = 3.542(1) A and c0 = 5.043(1) A, and for platinum ditelluride, a0 = 4.023(1) A and c0 = 5.220(3) A. Direct chemical analysis for the component elements was not carried out. Instead, precision density and unit-cell determinations were performed to characterize the samples. The densities of both compounds as determined by a hydrostatic technique with heptadecafluorodeca-hydro-l-(trifluoromethyl)naphthalene as the density fluid4 indicated that they are slightly deficient in platinum. For platinum disulfide, = 7.86 g/cm3 and Pmeas = 7.7(1) gm/cm3, and for platinum ditelluride, p = 10.2 gm/cm3 and Pmeas = 9.8(1) gm/cm3. In a typical experiment an emission spectrum of the platinum disulfide showed that phosphorus was present in less than 5 ppm. A mass spectroscopic examination of the platinum ditelluride revealed a small doping by sulfur (less than 0.4%) and traces of chlorine and phosphorus (less than 100 ppm). 

Both calcium and strontium crystallize in face-centered cubic unit cells. Determine which metal is more dense, calcium or strontium, given that their radii are 197 pm and 215 pm, respectively. The molar mass of Ca is 40.08 g-mol 1 and that of Sr is 87.62 g-mol (See Section 5.10.)... 

Fig. 6. Extrapolated crystalline density for polytetra-fluoroethylene. (After Moynihan). qc = Crystalline density calculated from the unit cell determined by x-ray diffraction...

Faint, smeared (hkl) reflections are also observed, but are not resolved enough to be used for unit cell determination. Equatorial and meridional spacings indicate the following monoclinic cells ... 

A disadvantage of the back end monochromator is its inability to separate Ka 2 from Kai. For very precise unit cell determinations this separation is desirable. For this purpose a front end or primary beam monochromator is used (Figure 12c). This monochromator is the only t)q)e that can be used with area detectors. Their disadvantages include a high intensity loss, the need for precise alignment and nonremoval of fluorescent radiation. 

Many applications require accurate unit cell determinations such as solid solutions studies and thermal expansion coefficients. We discuss the case of cubic crystals first. Precision in ao, the unit cell parameter, depends upon accurate values of d. The key in this determination is to have accurate values of sinS. Fortunately, sinS varies very slowly for 6 angles near 90°. For example, an error of 1° in 0 leads to an error in sin 0 of about 0.3% at 80° but to a 1.0% error at 60°. Therefore the approach is to measure the high angle reflections very accmately and carry out a linear extrapolation of ao as a function of 0. ... 

Solid-state NMR can distinguish between lattice and nonlatdce species, but is difficult to use in some cases. Ion exchange capacity is simple however, caution must be used since ion exchange capability has been observed for high-silica materials. The size of the unit cell determined by x-ray diffraction is widely used to determine lattice concentrations, but may be unreliable if the unit cell size is affected by more than lattice concentrations. ... 

Unique unit cell determination of to H bonding. determining phase fusion/transition. birefringence. 

Figure 4.1. The flowchart illustrating common steps employed in a structural characterization of materials by using the powder diffraction method. It always begins with the sample preparation as a starting point, followed by a properly executed experiment both are considered in Chapter 3. Preliminary data processing and profile fitting are discussed in this chapter in addition to common issues related to phase identification and analysis. Unit cell determination, crystal structure solution and refinement are the subjects of Chapters 5,6, and 7, respectively. The flowchart shows the most typical applications for the three types of experiments, although any or all of the data processing steps may be applied to fast, overnight and weekend experiments when justified by their quality and characterization goals.

Thus, a fast experiment is routinely suitable for evaluation of the specimen and phase identification, i.e. qualitative analysis. When needed, it should be followed by a weekend experiment for a complete structural determination. An overnight experiment is required for indexing and accurate lattice parameters refinement, and a weekend-long experiment is needed for crystal structure determination and refinement. In some instances, e.g. when a specimen has exceptional quality and its crystal structure is known or very simple, all relevant parameters can be determined using data collected in an overnight experiment. Similarly, fast experiment(s) may be suitable for unit cell determination in addition to phase identification. In any case, one should use his/her own judgment and experience to assess both the suitability of the experimental data and the reliability of the result. 

Unit cell determination and refinement 5.8.1 Indexing example LaNi4,8sSno,is... 

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