Powder X-ray Diffraction Technique

1. Powder X-ray Diffraction Technique Dr. Anjani Bhatt Bhavnagar

2. Keywords • History and Brief Introduction of Powder x-ray Diffractometer • Importance of the Technique • Information obtained from a Diffractogram • Instrumentation of PXRD • Analysis and interpretation of PXRD output. • Few examples of PXRD applications.

3. A bit of History • William Roentgen discovered X-rays in 1895 and determined they had the following properties • Travel in straight lines • Are exponentially absorbed in matter with the exponent proportional to the mass of the absorbing material • Darken photographic plates • Make shadows of absorbing material on photosensitive paper • X-ray range: 0.01 to 10 nanometers • Roentgen was awarded the Nobel Prize in 1901. Debate over the wave vs. particle nature of X-rays led the development of relativity and quantum mechanics

4. Discovery of Diffraction • Max von Laue theorized that if X-rays were waves, the wavelengths must be extremely small (on the order of 10-10 meters) • If true, the regular structure of crystalline materials should be “viewable” using X-rays • His experiment used an X-ray source directed into a lead box containing an oriented crystal with a photographic plate behind the box • The image created showed: • The lattice of the crystal produced a series of regular spots from concentration of the x-ray intensity as it passed through the crystal and demonstrated the wave character of the x-rays, proved that x-rays could be diffracted by crystalline materials.

5. Interaction of X-rays with Matter

6. Bragg’s “Extensions” of Diffraction • Lawrence Bragg and his father W.H. Bragg discovered that diffraction could be treated as reflection from evenly spaced planes if monochromatic x-radiation was used • Bragg’s Law: n = 2d sinwhere n is an integer  is the wavelength of the X-radiationd is the interplanarspacing in the crystalline material and is the diffraction angle • The Bragg Law makes X-ray powder diffraction possible

7. Empyrean powder X-ray diffractometer What is a Powder? • A perfect powder sample consists of an infinite number of small, randomly oriented crystallites • Note that this is the underlying definition for many quantitative analysis methods! • In real life, the number is of course not infinite, but should be large to give good averaging • Small particle size: 1-5 m is ideal • “Powder samples” can come in many different forms: • Loose powders, Films, sheets, blocks, wires… • Basically any “polycrystalline” sample can be used in PXRD – if it is not a single crystal, it is considered a “powder sample”

8. Form of Diffractograms of various states of Matter Comparative x-ray scattering by crystalline solids, amorphous solids, liquids and Monatomic gases. The three vertical scales are not equal.

9. Why We Use Powder Diffraction? • Originally, powder diffraction was mainly used for phase identification • Advantages over single crystal methods: Can be used on ANY sample • If you can mount it, you can measure it • For some materials, single crystal growth is difficult or impossible • Powder methods are the only option • “Real life samples” rarely come as single crystals: Engineering materials, formulations etc. • Powder diffraction can be used on mixtures of compounds • Peak shape analysis gives insights into size, stress and defects

10. Diffraction Method Method λ Ɵ Laue Variable Fixed Rotating crystal Fixed Variable (in part) Powder Fixed Variable Factors influencing angle of diffraction • Shape and size of the unit cell and orientation of the plane determines the directions of diffraction. 10/25/2017 10

11. Factors influencing shape of the peak • Ideal / perfect crystal should give a diffraction line. • Real crystals give diffraction peak with symmetric/assymmetric shape. • Defects in crystal are responsible for broadening and shaping of diffraction peaks. • Forms of the defects- - Grain size - Point, line and planar defects - Important consequences of defects is change in the mechanical, optical and electrical properties of material 10/25/2017 11

12. Importance of the Technique • Powder X-ray diffraction technique is used to get structural information about polycrystalline materials for which single crystal isolation is not possible or is difficult. • This is a good technique to get Qualitative and quantitative information about all the polycrystalline phases present in the material. • Good technique to study polymorphism.

13. Information obtained from a diffractogram

14. Notes on Units of Measure 1) an angstrom (Å) is 10-10 meters 2) a nanometer (nm) is 10-9 meters 3) a micrometer (m) or micron is 10-6meters 4) a millimeter (mm) is 10-3 meters In X-ray crystallography, d-spacing and X-ray wavelengths are commonly given in angstroms

15. Generating X-rays for Diffraction • To get an accurate picture of the structure of a crystalline material requires X-radiation that is as close to monochromatic as possible. • The function of the x-ray tube and associated electronics is to produce a limited frequency range of high-intensity x-rays. • Filters, monochromators, specially tuned detectors and software are then used to further refine the frequency used in the analysis.

16. The X-ray Tube • The anode is a pure metal. Cu, Mo, Fe, Co and Cr are in common use in XRD applications. Here in our instrument, we are using CuKα monochromatic radiation. (1.54Å) • The tube is cooled by water and housed in a shielding aluminum tower

17. Applications of Powder X-ray Diffraction technique • Single and mixed Phase identification • Identification of Polymorph • Impurity phase identification • Determination of Unit cell Parameters • Crystallite size determination • Crystallite strain determination • Quantification of Analytephase from a binary mixture of Drug Polymorph • Structure determination from powder Pattern using rietveld refinement • % Crystallinity determination

18. Modern Use: What Information Can We Get From Powder Diffraction Data? • Phase identification (qualitative phase analysis) • Most important/frequent use of PXRD • Qualitative analysis tool • Search pattern against database to identify phases present • Starting materials, known target compounds, likely impurities • Assumption: The material, or an isostructural material, is in the database • Phase fraction analysis (quantitative phase analysis) • Applied to mixtures of two or more crystalline phases • Compare intensities of selected peaks of all phases • Theoretically only requires one peak/phase, but better with multiple peaks • Accurate analysis requires standardization • Mix known quantities of two phases in several different ratios • Caution: Possibility of amorphous components

19. What Information Can We Get From Powder Diffraction Data? (Cont’d) • Lattice parameters • Two modes of analysis: • Accurate lattice parameters for a compound of known structure • Unit cell determination for an unknown compound through indexing • ACCURATE peak positions are crucial! • Phase transition behavior • In situ diffraction experiments • Temperature-induced phase transitions • Pressure-induced phase transitions • Kinetic studies • Requires specialized setups

20. What Information Can We Get From Powder Diffraction Data? (Cont’d) • Line shape analysis • Width of Bragg peaks is inversely related to crystallite size • Often used for crystallite size estimates for nanoparticles • Requires use of a standard to determine instrument contribution first • Microstrain(non uniform strain) also results in peak broadening • Due to atomic disorder, dislocations, vacancies etc. • Different angular dependence than size effects • Residual stress can be determined • Defects like stacking faults and antiphase boundaries also affect line shape

21. Powder Pattern Analysis Beyond Search/Match • As stated previously, early use of powder methods and most common use today, was for phase Identification. • 1966-1969: Hugo Rietveld introduced a whole pattern fitting approach for neutron data • Nowadays it is known as “Rietveld method” And Soon applied to X-ray data (1977). • Became more feasible with increasing computer power • “Routine” powder tool by now • The Rietveld method can be used to verify structures, determine accurate lattice parameters, microstructural sample characteristics, phase fractions in mixtures etc.

22. Rietveld refinement for structural analysis • Least squares based minimization algorithm to obtain the best fit between a structural model and a powder pattern • Starting model necessary to apply this method • Applicable to simple and complicated structures, single phase and multi-phase samples • Automatically gives phase fractions and lattice parameters from ALL peaks • Requires good data for meaningful results

23. Examples of Powder XRD Measurements . PXRD spectra of Bio Materials

24. Crystallite size and Strain Crystallite size and strain of some samples have been determined with the help of sherrer formula using Pxrd technique :

25. Quantitative Analysis of Inorganic Materials Quantification analysis of clay based materials Using Rietveld refinement technique with proper background quantification has been done.

26. Variable Temperature Powder X-ray Diffraction analysis Variable Temp. PXRD of MOF samples.

27. PXRD analysis of Textiles waste sample Quantification studies showed (1) Thenardite is Sodium Sulphate (8.2%) and (2) Halite is Sodium Chloride (91.8%)

28. Structure determination of NaCl using Powder patterns

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