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Objectives By the end of this section you should: understand the concept of planes in crystals know that planes are identified by their Miller Index and their separation, d be able to calculate Miller Indices for planes know and be able to use the d-spacing equation for orthogonal crystals understand the concept of diffraction in crystals be able to derive and use Braggs law

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Lattice Planes and Miller Indices Imagine representing a crystal structure on a grid (lattice) which is a 3D array of points (lattice points). Can imagine dividing the grid into sets of planes in different orientations

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All planes in a set are identical The planes are imaginary The perpendicular distance between pairs of adjacent planes is the d-spacing Need to label planes to be able to identify them Find intercepts on a,b,c: 1/4, 2/3, 1/2 Take reciprocals 4, 3/2, 2 Multiply up to integers: (8 3 4) [if necessary]

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Exercise - What is the Miller index of the plane below? Find intercepts on a,b,c: Take reciprocals Multiply up to integers:

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Plane perpendicular to y cuts at, 1, (0 1 0) plane General label is (h k l) which intersects at a/h, b/k, c/l (hkl) is the MILLER INDEX of that plane (round brackets, no commas). This diagonal cuts at 1, 1, (1 1 0) plane NB an index 0 means that the plane is parallel to that axis

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Using the same set of axes draw the planes with the following Miller indices: (0 0 1) (1 1 1)

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Using the same set of axes draw the planes with the following Miller indices: (0 0 2) (2 2 2) NOW THINK!! What does this mean?

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Planes - conclusions 1 Miller indices define the orientation of the plane within the unit cell The Miller Index defines a set of planes parallel to one another (remember the unit cell is a subset of the infinite crystal (002) planes are parallel to (001) planes, and so on

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d-spacing formula For orthogonal crystal systems (i.e. = = =90 ) :- For cubic crystals (special case of orthogonal) a=b=c :- e.g. for(1 0 0)d = a (2 0 0)d = a/2 (1 1 0)d = a/ 2 etc.

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A tetragonal crystal has a=4.7 Å, c=3.4 Å. Calculate the separation of the: (1 0 0) (0 0 1) (1 1 1) planes A cubic crystal has a=5.2 Å (=0.52nm). Calculate the d-spacing of the (1 1 0) plane

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Question 2 in handout: If a = b = c = 8 Å, find d-spacings for planes with Miller indices (1 2 3) Calculate the d-spacings for the same planes in a crystal with unit cell a = b = 7 Å, c = 9 Å. Calculate the d-spacings for the same planes in a crystal with unit cell a = 7 Å, b = 8 Å, c = 9 Å.

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X-ray Diffraction

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Diffraction - an optical grating Path difference XY between diffracted beams 1 and 2: sin = XY/a XY = a sin For 1 and 2 to be in phase and give constructive interference, XY =, 2, 3, 4 …..n so a sin = n where n is the order of diffraction

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Consequences: maximum value of for diffraction sin = 1 a = Realistically, sin So separation must be same order as, but greater than, wavelength of light. Thus for diffraction from crystals: Interatomic distances Å so = Å X-rays, electrons, neutrons suitable

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Diffraction from crystals ? X-ray Tube Detector

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Beam 2 lags beam 1 by XYZ = 2d sin so 2d sin = n Braggs Law

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We normally set n=1 and adjust Miller indices, to give 2d hkl sin = 2d sin = n e.g. X-rays with wavelength 1.54Å are reflected from planes with d=1.2Å. Calculate the Bragg angle,, for constructive interference. = 1.54 x m, d = 1.2 x m, =? n=1 : = 39.9° n=2 :X (n /2d)>1

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Example of equivalence of the two forms of Braggs law: Calculate for =1.54 Å, cubic crystal, a=5Å 2d sin = n (1 0 0) reflection, d=5 Å n=1, =8.86 o n=2, =17.93 o n=3, =27.52 o n=4, =38.02 o n=5, =50.35 o n=6, =67.52 o no reflection for n 7 (2 0 0) reflection, d=2.5 Å n=1, =17.93 o n=2, =38.02 o n=3, =67.52 o no reflection for n 4

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Use Braggs law and the d-spacing equation to solve a wide variety of problems 2d sin = n or 2d hkl sin =

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X-rays with wavelength 1.54 Å are reflected from the (1 1 0) planes of a cubic crystal with unit cell a = 6 Å. Calculate the Bragg angle,, for all orders of reflection, n. Combining Bragg and d-spacing equation d = 4.24 Å

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n = 1 : = 10.46° n = 2 : = 21.30° n = 3 : = 33.01° n = 4 : = 46.59° n = 5 : = 65.23° = (1 1 0) = (2 2 0) = (3 3 0) = (4 4 0) = (5 5 0) 2d hkl sin =

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Summary We can imagine planes within a crystal Each set of planes is uniquely identified by its Miller index (h k l) We can calculate the separation, d, for each set of planes (h k l) Crystals diffract radiation of a similar order of wavelength to the interatomic spacings We model this diffraction by considering the reflection of radiation from planes - Braggs Law

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