Presentation on theme: "Determination of Crystal Structure (Chapt. 10) Guess Correct? Done Yes No 1.Use the angular positions of the diffraction lines to determine the shape and."— Presentation transcript:
Determination of Crystal Structure (Chapt. 10) Guess Correct? Done Yes No 1.Use the angular positions of the diffraction lines to determine the shape and size of the unit cell. Assume either cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic, or triclinic. Assign Miller indices to each reflection (“index the pattern”). If a match is not obtained, the assumption of should be changed and the pattern indexed again. Calculate the size of the unit cell based on the positions and Miller indices of the diffraction lines. 2.With the measured density of the material, the chemical composition, and the size of the unit cell, calculate the number of atoms per unit cell. 3.Find the positions of the atoms in the unit cell by using the relative intensities of the diffraction lines.
Preliminary Treatment of Data We want the values of sin 2 for each diffraction line (in order to find the cell size and shape), however, there can be errors to what we measure, including extraneous lines in the diffraction pattern, and systematic errors (misalignment, film shrinkage, absorption, etc.) Extraneous Lines 1.X-ray beam with multiple wavelengths: 2.Contaminants or impurities in the sample, or the specimen mount! Systematic Errors 1.Film shrinkage (see Fig. 6-5 Cullity) 2.Specimen is off-centered in Debye-Scherrer camera (see Fig Cullity) 3.Absorption in the sample Answer mix in a reference material and calibrate. (see Fig Cullity)