Crystallographic Points, Directions, and Planes. ISSUES TO ADDRESS... How to define points, directions, planes, as well as linear, planar, and volume densities – Define basic terms and give examples of each: Points (atomic positions) Vectors (defines a particular direction - plane normal) Miller Indices (defines a particular plane) relation to diffraction 3-index for cubic and 4-index notation for HCP

Points, Directions, and Planes in Terms of Unit Cell Vectors All periodic unit cells may be described via these vectors and angles, if and only if a, b, and c define axes of a 3D coordinate system. coordinate system is Right-Handed! But, we can define points, directions and planes with a “triplet” of numbers in units of a, b, and c unit cell vectors. For HCP we need a “quad” of numbers, as we shall see.

POINT Coordinates To define a point within a unit cell…. Express the coordinates uvw as fractions of unit cell vectors a, b, and c (so that the axes x, y, and z do not have to be orthogonal). origin pt. coord. x ( a )y ( b )z ( c ) /201/2 pt.

Crystallographic Directions Procedure: 1.Any line (or vector direction) is specified by 2 points. The first point is, typically, at the origin (000). 2.Determine length of vector projection in each of 3 axes in units (or fractions) of a, b, and c. X (a), Y(b), Z(c) Multiply or divide by a common factor to reduce the lengths to the smallest integer values, u v w. 4.Enclose in square brackets: [u v w]: [110] direction. a b c DIRECTIONS will help define PLANES (Miller Indices or plane normal). 5. Designate negative numbers by a bar Pronounced “bar 1”, “bar 1”, “zero” direction. 6. “Family” of [110] directions is designated as.

Self-Assessment Example 1: What is crystallographic direction? a b c Along x: 1 a Along y: 1 b Along z: 1 c [1 1 1] DIRECTION = Magnitude along X Y Z

Self-Assessment Example 2: (a) What is the lattice point given by point P? (b) What is crystallographic direction for the origin to P? The lattice direction [132] from the origin. Example 3: What lattice direction does the lattice point 264 correspond?

Symmetry Equivalent Directions Note: for some crystal structures, different directions can be equivalent. e.g. For cubic crystals, the directions are all equivalent by symmetry: [1 0 0], [ 0 0], [0 1 0], [0 0], [0 0 1], [0 0 ] Families of crystallographic directions e.g. Angled brackets denote a family of crystallographic directions.

Families and Symmetry: Cubic Symmetry x y z (100) Rotate 90 o about z-axis x y z (010) x y z (001) Rotate 90 o about y-axis Similarly for other equivalent directions Symmetry operation can generate all the directions within in a family.

Designating Lattice Planes Why are planes in a lattice important? (A) Determining crystal structure * Diffraction methods measure the distance between parallel lattice planes of atoms. This information is used to determine the lattice parameters in a crystal. * Diffraction methods also measure the angles between lattice planes. (B) Plastic deformation * Plastic deformation in metals occurs by the slip of atoms past each other in the crystal. * This slip tends to occur preferentially along specific crystal-dependent planes. (C) Transport Properties * In certain materials, atomic structure in some planes causes the transport of electrons and/or heat to be particularly rapid in that plane, and relatively slow not in the plane. Example: Graphite: heat conduction is more in sp 2 -bonded plane. Example: YBa 2 Cu 3 O 7 superconductors: Cu-O planes conduct pairs of electrons (Cooper pairs) responsible for superconductivity, but perpendicular insulating. + Some lattice planes contain only Cu and O

How Do We Designate Lattice Planes? Planes intersects axes at: a axis at r= 2 b axis at s= 4/3 c axis at t= 1/2 How do we symbolically designate planes in a lattice? 1. Take the reciprocal of r, s, and t. Here: 1/r = 1/2, 1/s = 3/4, and 1/r = 2 2. Find the least common multiple that converts all reciprocals to integers. With LCM = 4, h = 4/r = 2, k= 4/s = 3, and l= 4/r = 8 3. Enclose the new triple (h,k,l) in parentheses: (238) 4. This notation is called the Miller Index. * Note: If a plane does not intercept an axes (I.e., it is at ∞), then you get 0. * Note: All parallel planes at similar staggered distances have the same Miller index.

Self-Assessment Example What is the designation of this plane in Miller Index notation? What is the designation of the top face of the unit cell in Miller Index notation?

Families of Lattice Planes The Miller indices (hkl) usually refer to the plane that is nearest to the origin without passing through it. You must always shift the origin or move the plane parallel, otherwise a Miller index integer is 1/0, i.e.,∞! Sometimes (hkl) will be used to refer to any other plane in the family, or to the family taken together. Importantly, the Miller indices (hkl) is the same vector as the plane normal! Given any plane in a lattice, there is a infinite set of parallel lattice planes (or family of planes) that are equally spaced from each other. One of the planes in any family always passes through the origin.

z x y Look down this direction (perpendicular to the plane) Crystallographic Planes in FCC: (100) Distance between (100) planes Distance to (200) plane

Crystallographic Planes in FCC: (110) Distance between (110) planes

Crystallographic Planes in FCC: (111) z x y Look down this direction (perpendicular to the plane) Distance between (111) planes

Note: similar to crystallographic directions, planes that are parallel to each other, are equivalent

Comparing Different Crystallographic Planes 1 For (220) Miller Indexed planes you are getting planes at 1/2, 1/2, ∞. The (110) planes are not necessarily (220) planes! For cubic crystals: Miller Indices provide you easy measure of distance between planes. Distance between (110) planes

Directions in HCP Crystals 1.To emphasize that they are equal, a and b is changed to a 1 and a 2. 2.The unit cell is outlined in blue. 3.A fourth axis is introduced (a 3 ) to show symmetry. Symmetry about c axis makes a 3 equivalent to a 1 and a 2. Vector addition gives a 3 = –( a 1 + a 2 ). 4.This 4-coordinate system is used: [a 1 a 2 –( a 1 + a 2 ) c]

Directions in HCP Crystals: 4-index notation Example What is 4-index notation for vector D? Projecting the vector onto the basal plane, it lies between a 1 and a 2 (vector B is projection). Vector B = (a 1 + a 2 ), so the direction is [110] in coordinates of [a 1 a 2 c], where c-intercept is 0. In 4-index notation, because a 3 = –( a 1 + a 2 ), the vector B is since it is 3x farther out. In 4-index notation c = [0001], which must be added to get D (reduced to integers) D = Self-Assessment Test: What is vector C? Easiest to remember: Find the coordinate axes that straddle the vector of interest, and follow along those axes (but divide the a 1, a 2, a 3 part of vector by 3 because you are now three times farther out!). Check w/ Eq. 3.7 or just use Eq. 3.7 a2a2 – 2a 3 B without 1/3

Directions in HCP Crystals: 4-index notation Example What is 4-index notation for vector D? Projection of the vector D in units of [a 1 a 2 c] gives u’=1, v’=1, and w’=1. Already reduced integers. Using Eq. 3.7: Check w/ Eq. 3.7: a dot-product projection in hex coords. In 4-index notation: Reduce to smallest integers: After some consideration, seems just using Eq. 3.7 most trustworthy.

Miller Indices for HCP Planes As soon as you see [1100], you will know that it is HCP, and not [110] cubic! 4-index notation is more important for planes in HCP, in order to distinguish similar planes rotated by 120 o. 1.Find the intercepts, r and s, of the plane with any two of the basal plane axes (a 1, a 2, or a 3 ), as well as the intercept, t, with the c axes. 2.Get reciprocals 1/r, 1/s, and 1/t. 3.Convert reciprocals to smallest integers in same ratios. 4.Get h, k, i, l via relation i = - (h+k), where h is associated with a 1, k with a 2, i with a 3, and l with c. 5.Enclose 4-indices in parenthesis: (h k i l). Find Miller Indices for HCP: r s t

Miller Indices for HCP Planes What is the Miller Index of the pink plane? 1.The plane’s intercept a 1, a 3 and c at r=1, s=1 and t= ∞, respectively. 1.The reciprocals are 1/r = 1, 1/s = 1, and 1/t = 0. 2.They are already smallest integers. 3.We can write (h k i l) = (1 ? 1 0). 4.Using i = - (h+k) relation, k=–2. 5.Miller Index is

Yes, Yes….we can get it without a 3 ! 1.The plane’s intercept a 1, a 2 and c at r=1, s=–1/2 and t= ∞, respectively. 2.The reciprocals are 1/r = 1, 1/s = –2, and 1/t = 0. 3.They are already smallest integers. 4.We can write (h k i l) = 5.Using i = - (h+k) relation, i=1. 6.Miller Index is But note that the 4-index notation is unique….Consider all 4 intercepts: plane intercept a 1, a 2, a 3 and c at 1, –1/2, 1, and ∞, respectively. Reciprocals are 1, –2, 1, and 0. So, there is only 1 possible Miller Index is

a1a1 a2a2 a3a3 Parallel to a 1, a 2 and a 3 So, h = k = i = 0 Intersects at z = 1 Name this plane… Basal Plane in HCP

z a1a1 a2a2 a3a3 ( ) plane +1 in a 1 -1 in a 2 h = 1,l = 0i = -(1+-1) = 0,k = -1, Another Plane in HCP

(1 1 1) plane of FCC z x y z a1a1 a2a2 a3a3 ( ) plane of HCP SAME THING!*

SUMMARY Crystal Structure can be defined by space lattice and basis atoms (lattice decorations or motifs). Only 14 Bravais Lattices are possible. We focus only on FCC, HCP, and BCC, i.e., the majority in the periodic table. We now can identify and determined: atomic positions, atomic planes (Miller Indices), packing along directions (LD) and in planes (PD). We now know how to determine structure mathematically. So how to we do it experimentally? DIFFRACTION.