Crystallographic Directions And Planes

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Crystallographic directions and planes OutlineCrystallographic directionsCrystallographic planesLinear and planar atomic densitiesClose-packed crystal structuresChapter 3 -

Point CoordinateszPoint coordinates for unit cellcenter are111ca/2, b/2, c/2000aybPoint coordinates for unit cellcorner are 111xz2cb½½½yTranslation: integer multiple oflattice constants identicalposition in another unit cellbChapter 3 - 2

Crystallographic DirectionsAlgorithmzy1. Vector repositioned (if necessary) to passthrough origin.2. Read off projections in terms ofunit cell dimensions a, b, and c3. Adjust to smallest integer values4. Enclose in square brackets, no commas[uvw]xex: 1, 0, ½ 2, 0, 1 [ 201 ]-1, 1, 1 [ 111 ]where overbar represents anegative indexfamilies of directions uvw Chapter 3 - 3

ExamplesSketch the following directions : [110], [-1-21], [-1 0 2]Chapter 3 -

HCP Crystallographic DirectionszAlgorithma2-a3a11. Vector repositioned (if necessary) to passthrough origin.2. Read off projections in terms of unitcell dimensions a1, a2, a3, or c3. Adjust to smallest integer values4. Enclose in square brackets, no commas[uvtw]a2ex:½, ½, -1, 0-a3a22Adapted from Fig. 3.8(a), Callister 7e. [ 1120 ]a3dashed red lines indicateprojections onto a1 and a2 axesa12a1Chapter 3 - 5

HCP Crystallographic Directions Hexagonal Crystals– 4 parameter Miller-Bravais lattice coordinates arerelated to the direction indices (i.e., u'v'w') asfollows.z[ u 'v 'w ' ] [ uvtw ]1(2 u ' - v ')31 v(2 v ' - u ')3t - (u v )u a2-a3a1Fig. 3.8(a), Callister 7e.w w'Chapter 3 - 6

Crystallographic Planes Miller Indices: Reciprocals of the (three) axialintercepts for a plane, cleared of fractions &common multiples. All parallel planes havesame Miller indices. Algorithm1. Read off intercepts of plane with axes interms of a, b, c2. Take reciprocals of intercepts3. Reduce to smallest integer values4. Enclose in parentheses, nocommas i.e., (hkl)Chapter 3 - 7

Crystallographic PlaneszcbaxyzcabyxChapter 3 - 8

Crystallographic PlaneszcabyxChapter 3 - 9

Crystallographic planes (continued) Construct planes by Miller indices of planes (0 -1 -1) and (1 1 -2)Chapter 3 -

Atomic arrangements The atomic arrangement for a crystallographic plane dependson the crystal structureFCC: (a) reduced sphere(b) atomic packing ofan FCC (110) planeBCC: (a) reduced sphere(b) atomic packing ofan BCC (110) planeChapter 3 -

Atomic arrangements A family of planes contains all the planes that arecrystallographically equivalent. In cubic system, planes with same indices, irrespectiveof order and sign, are equivalent– (111), (111), (111) belong to {111} family– (100), (100), (010), and (001) belong to {100} family– (123), (123), (312) in cubic crystals belong to {123} family In tetragonal, (001) (001) are not as same family as (100),(100)Chapter 3 -

Linear and planar atomic density Linear Density of Atoms LD Number of atomsUnit length of direction vectorAtomic planar density: number of atoms centered on aplane/area of plane#atomsPlanar Density Area(2D repeat unit)Chapter 3 -

Planar Density of (100) IronSolution: At T 912 C iron has the BCC structure.2D repeat unit(100)Adapted from Fig. 3.2(c), Callister 7e.Chapter 3 - 14

Close-packed crystal structuresClose-packed plane stackingsequence for HCPClose-packed plane stackingsequence for HCPChapter 3 -

Chapter 3 - Atomic arrangements A family of planes contains all the planes that are crystallographically equivalent. In cubic system, planes with same indices, irrespective of order and sign, are equivalent – (111), (111), (111) belong to {111} family – (100), (100), (010), and (001) belong to {100} family – (123), (123), (312)

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