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Unformatted text preview: 35Sketch the Ewald sphere construction for a 200 diffraction with Mo Kradiation and a polycrystalline specimen of a simple cubic substance with a= 3.30 . Graphically determine the angular rotation required to orient the sample for 300 diffraction if a 2diffractometer is being used.SolutionThere is a strong probability that this problem statement is in error, and the author wanted the sample to be a singlecrystal, with the xray beam directed along a [001] zone axis orientation. This would have made for a more meaningful calculation of a reorientation angle, with a sketch very similar to Figure 36 of the text.The reason for this suspicion is because a polycrystalline specimen of a material with a simple cubic structure (all reections are "allowed" by structure factor) will always satisfy 200 and 300 diffraction conditions simultaneously, provided the grain size is small enough. The reciprocal lattice Ewald sphere construction shows this. To establish the scale of the drawing, the wavelength of Mo Kradiation is 0.710739 (Appendix 7, page 629). Ewalds sphere has a radius given by the reciprocal of the wavelength orr= 1.407 1. A simple cubic crystal with a lattice constant of 3.30 will exhibit diffraction from all families of planes. Reciprocal lattice vectors will therefore follow a sequence according to the interplanar spacing relationThese values are 0.303 1 for 100, 0.428 1for 110, 0.525 1 for 111, 0.606 1, for 200, etc. including 0.909 1 for 300 (as called for in the problem statement).For polycrystalline samples of small grain size in a perfectly random orientation, all of these reciprocal lattice vectors will sweep out a sphere of radius rhkl*, centered about the origin of the reciprocal lattice. Ewalds sphere also terminates on the origin of the reciprocal lattice, generating a construction as shown in the following. r*hkl=1dhkl=k2+k2+l2a=k2+k2+l23.30APROBLEM 35B.D. Cullity and S.R. Stock, Elements of XRay Diffraction, 3rdEd., Prentice Hall, (2001)MSE 104 Materials CharacterizationProfessor R. Gronskypage 1 of 2Note that there is no signifcance to the incident direction oF the Mo Kbeam since the polycrystalline sample has no specifc orientation, but rather all orientations. This is indicated by the reciprocal "lattice" in all orientations, sweeping out circles (spheres in 3D) with radii given by the r* values above. IF the sample were single crystalline, the construction would appear as shown on the right For the 200 diFFraction condition. Note that both the origin and the 200 reciprocal lattice vectors lie on the Ewald sphere. The origin always does, and in this case the 200 reciprocal lattice point does because it represents an exact Bragg diFFraction condition....
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This homework help was uploaded on 04/07/2008 for the course MSE 104 taught by Professor Gronsky during the Spring '08 term at University of California, Berkeley.
 Spring '08
 GRONSKY

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