Powder+Prep+for+XRD - Chapter 12 Collecting Quality Powder...

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Chapter 12 Collecting Quality Powder Diffraction Data Many factors affect quality of powder diffraction data (e.g., see Figs.11.1, 11.5, and 11.25), and the state of the specimen used in a powder diffraction experiment is one of them. Further, a number of data acquisition parameters may, and should be properly chosen. Here we consider issues related to both the preparation of the spec- imen and selection of instrument-related parameters in order to achieve the highest possible quality of the resulting powder diffraction pattern. 12.1 Sample Preparation It is dif±cult to overemphasize the importance of proper sample preparation, espe- cially because it is always under the complete control of the operator carrying out the experiment. Poorly prepared samples will inevitably result in unusable experimental data, which will require additional effort to repeat everything from the beginning, thus both time and resources are wasted. On the other hand, a high quality sample for powder diffraction may take longer to prepare, but this is time well-spent! 12.1.1 Powder Requirements and Powder Preparation The true powder diffraction pattern can only be obtained from a specimen contain- ing an in±nite number of individual particles realizing an in±nite number of orienta- tions in the irradiated volume (e.g., see Fig. 8.1). In other words, the particles in the specimen should have a completely random distribution of crystallographic orienta- tions of grains or crystallites with respect to one another. Clearly, this ideal situation is impossible to achieve. However, if one considers a 10 mm diameter and 0.1 mm deep cylindrical sample holder ±lled with 50 μ m diameter spherical particles, it is easy to estimate that it will hold nearly 9 × 10 4 of such particles at 74.05% packing density, that is, assuming close packing of the spheres (Fig. 12.1). When the particle V.K. Pecharsky, P.Y. Zavalij, Fundamentals of Powder Diffraction and Structural 301 Characterization of Materials, DOI: 10.1007/978-0-387-09579-0 12, c ± Springer Science+Business Media LLC 2009
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302 12 Collecting Quality Powder Diffraction Data 0 2 04 06 08 0 1 0 0 10 10 10 9 10 8 10 7 10 6 10 5 10 4 d 3 D 2 h N = 1.1108 Particle diameter, d ( μ m) Number of particles, N N Fig. 12.1 The total number of spherical particles in a cylindrical specimen 10 mm in diameter and 0.1 mm deep as a function of particle size assuming close packing of the spheres. D – specimen diameter, h – specimen depth and d – particle diameter. size is reduced to 30 μ m, the same volume will contain 4 × 10 5 particles, and when the particles are 10 μ m in diameter, it will take a total of 1 × 10 7 different particles to Fll the same volume. These are large numbers, which may be considered sufF- cient to approximate the inFnite quantity of particles required for collecting powder diffraction data.
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Powder+Prep+for+XRD - Chapter 12 Collecting Quality Powder...

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