crystal

Crystallography and Structure ME 2105 R. R. Lindeke

Overview: 

Crystal Structure – Structure – matter assumes a periodic shape Non-Crystalline Non-Crystalline or Amorphous “structures” exhibit no long range periodic shapes  Xtal Systems  – not structures but potentials HCP  – common Xtal  FCC, BCC and HCP – Structures for metals 

Point, Direction and Planer ID‟ing in Xtals  X-Ray Diffraction and Xtal Structure 

Overview: 

Crystal Structure – Structure – matter assumes a periodic shape Non-Crystalline Non-Crystalline or Amorphous “structures” exhibit no long range periodic shapes  Xtal Systems  – not structures but potentials HCP  – common Xtal  FCC, BCC and HCP – Structures for metals 

Point, Direction and Planer ID‟ing in Xtals  X-Ray Diffraction and Xtal Structure 

Energy and Packing • Non dense, random packing

Energy typical neighbor  bond length

typical neighbor  bond energy

• Dense, ordered packing

r 

Energy typical neighbor  bond length

typical neighbor  bond energy

Dense, ordered packed structures tend to have lower energies & thus are more stable.

r 

CRYSTAL STRUCTURES Crystal Structure Means: PERIODIC ARRANGEMENT OF ATOMS/IONS OVER LARGE ATOMIC DISTANCES

Leads

to structure displaying LONG-RANGE ORDER that is Measurable and Quantifiable

All metals, many ceramics, and some  polymers exhibit this “High Bond Energy” and a More Closely Packed Structure

Materials Lacking Long range order 

Amorphous Materials

These less densely packed lower bond energy “structures” can be found in Metals are observed in Ceramic GLASS and many “plastics”

Crystal Systems – Some Definitional information Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal. 7 crystal systems of varying symmetry are known

These systems are built by changing the lattice parameters: a, b, and c are the edge lengths , , and  are interaxial angles Fig. 3.4, Callister 7e.

Crystal Systems Crystal structures are divided into groups according to unit cell geometry (symmetry).

Metallic Crystal Structures • Tend to be densely packed. • Reasons for dense packing: - Typically, only one element is present, so all atomic radii are the same. - Metallic bonding is not directional. - Nearest neighbor distances tend to be small in order to lower bond energy. - Electron cloud shields cores from each other 

• Have the simplest crystal structures. We will examine three such structures (those of  engineering importance) called: FCC, BCC and HCP – with a nod to Simple Cubic

Crystal Structure of Metals – of  engineering interest

Simple Cubic Structure (SC) • Rare due to low packing density (only Po – Polonium -has this structure)

• Close-packed directions are cube edges.

• Coordination No. = 6 (# nearest neighbors) for each atom as seen (Courtesy P.M. Anderson)

 Atomic Packing Factor (APF)  APF =

Volume of atoms in unit cell* Volume of unit cell

*assume hard spheres • APF for a simple cubic structure = 0.52 atoms

a 

unit cell

R =0.5a  close-packed directions contains (8 x 1/8) = 1 atom/unit cell  Adapted from Fig. 3.23, Callister 7e.

 APF =

volume atom 4 p (0.5a ) 3 1 3

a 3

volume unit cell

Here: a = Rat*2 Where Rat is the „handbook‟ atomic radius

Body Centered Cubic Structure (BCC) • Atoms touch each other along cube diagonals within a  unit cell . --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing.

ex: Cr, W, Fe ( ), Tantalum, Molybdenum

• Coordination # = 8 (Courtesy P.M. Anderson)

 Adapted from Fig. 3.2, Callister 7e.

2 atoms/unit cell: (1 center) + (8 corners x 1/8)

 Atomic Packing Factor: BCC 3 a 

a  2 a 

R  atoms

Close-packed directions: length = 4R = 3 a 

a  4

volume

p ( 3 a /4 ) 3 2 unit cell atom 3  APF = volume  Adapted from 3 a  Fig. 3.2(a), Callister 7e. unit cell • APF for a body-centered cubic structure = 0.68

Face Centered Cubic Structure (FCC) • Atoms touch each other along face diagonals . --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.

ex: Al, Cu, Au, Pb, Ni, Pt, Ag

• Coordination # = 12

 Adapted from Fig. 3.1, Callister 7e.

4 atoms/unit cell: (6 face x ½) + (8 corners x 1/8) (Courtesy P.M. Anderson)

 Atomic Packing Factor: FCC • APF for a face-centered cubic structure = 0.74 The maximum achievable APF! Close-packed directions: length = 4R = 2 a 

2 a 

(a = 22*R)

Unit cell contains: 6 x 1/2 + 8 x 1/8 = 4 atoms/unit cell

a   Adapted from Fig. 3.1(a), Callister 7e.

atoms

4

4 unit cell 3  APF =

p ( 2a /4 ) 3 a 3

volume atom volume unit cell

Hexagonal Close-Packed Structure (HCP) ex: Cd, Mg, Ti, Zn • ABAB... Stacking Sequence • 3D Projection

• 2D Projection  A sites

c 

Top layer 

B sites

Middle layer 

 A sites a 

Bottom layer 

 Adapted from Fig. 3.3(a), Callister 7e.

• Coordination # = 12

6 atoms/unit cell • APF = 0.74

• c /a = 1.633 (ideal)

We find that both FCC & HCP are highest density packing schemes (APF = .74) – this illustration shows their  differences as the closest packed planes are “built-up”

Theoretical Density, r Density = r =

r =

where

Mass of  Atoms in Unit Cell Total Volume of Unit Cell

n A V C N  A

n = number of atoms/unit cell A = atomic weight V C  = Volume of unit cell = a 3 for cubic N  A = Avogadro‟s number  = 6.023 x 1023 atoms/mol

Theoretical Density, r •

R 

Ex: Cr (BCC)

A = 52.00 g/mol R = 0.125 nm n = 2  a = 4R /3 = 0.2887 nm a 

atoms unit cell

r= volume unit cell

2 52.00

a 3 6.023 x 1023

rtheoretical = 7.18 g/cm3 g = 7.19 g/cm3 mol ractual atoms mol

Locations in Lattices: Point Coordinates z 

Point coordinates for unit cell center are

111

c 

a /2, b /2, c /2 a 

000

x 

y 

b  

z 

½½½

2c 

Point coordinates for unit cell (body diagonal) corner are 111

 



b  b 

y 

Translation: integer multiple of  lattice constants  identical position in another unit cell 

Crystallographic Directions z 

 Algorithm

y  x 

1. Vector is repositioned (if necessary) to pass through the Unit Cell origin . 2. Read off line projections (to principal axes of U.C.) in terms of unit cell dimensions a , b , and c  3. Adjust to smallest integer values 4. Enclose in square brackets, no commas 

[uvw ]

ex: 1, 0, ½ => 2, 0, 1 => [ 201 ] -1, 1, 1 => [ 111 ]

where „overbar‟ represents a negative index

families of directions < uvw >

What is this Direction ?????

Projections: Projections in terms of a,b and c: Reduction: Enclosure [brackets]

 x

 y

z

a/2 1/2 1

b 1 2

0c 0

[120]

0

Linear Density – Density – considers equivalance and is important in Slip Number of atoms  Linear Density of Atoms  LD = Unit length of direction vector  [110] ex: linear density of Al in [110] [110] direction a = 0.405 nm

a 

# atoms

2 = = 3.5 nm -1 LD length

2a 

# atoms CENTERED on the direction of interest! Length is of the direction of interest within the Unit Cell

Determining Angles Between Crystallographic Direction:

 -1   = Cos   

 u1u2  v1v2  w1w2  2 2 2 2 2 2  u v w u v w  1  1  1    2  2  2  

Where ui‟s , vi‟s & wi‟s are the “Miller Indices” of the directions in question  – also (for information) If a direction has the same Miller Indices as a plane, it is NORMAL to that plane

HCP Crystallographic Directions z 

 Algorithm

a 2

-

a 3

1. Vector Vector repositioned (if necessary) to pass through origin. 2. Read off projections in terms of unit cell dimensions a 1, a 2, a 3, or c  3. Adjust Adjust to smallest integer values 4. Enclose in square brackets, no commas [uvtw ]

a 2

a 1

ex:

½, ½, -1, 0

-a 3

a 2

 Adapted from Fig. 3.8(a), Callister 7e.

2

=>

[ 1120 ]

a 3

dashed red lines indicate projections onto a 1 and a 2 axes

a 1 2

a 1

HCP Crystallographic Directions 

Hexagonal Crystals 

4 parameter Miller-Bravais lattice coordinates are related to the direction indices (i.e., u 'v 'w ') in the „3 space‟ Bravais lattice as follows. z 

[ u 'v 'w ' ]  [ uvtw ]

u  = a 2

v  = -

a 3 a 1 Fig. 3.8(a), Callister 7e.

1 3 1

( 2 u ' - v ') ( 2 v ' - u ')

3 t  = - (u  +v )

w = w '

Computing HCP Miller- Bravais Directional Indices (an alternative way): We confine ourselves to the bravais parallelopiped in the hexagon: a 1a2-Z and determine: (u‟,v‟w‟)

z 

Here: [1 1 0] - so now apply the models to create M-B Indices a 2

-

a 3 a 1

u

= 1

v

= 1

t

= - u  v  = -

w

=

3 3

w'

 2u '- v '  =

1

 2v '- u '  =

1

3 3

 2  1 - 1 =

1

 2  1 - 1 =

1

3 3

1 1

 1 3  1 3  = - 2 3  -2

=0

M-B Indices: [1120]

Defining Crystallographic Planes 



Miller Indices: Reciprocals of the (three) axial intercepts for a plane, cleared of fractions & common multiples. All parallel planes have same Miller indices.  Algorithm (in cubic lattices this is direct) 1. Read off intercepts of plane with axes in terms of a , b , c  2. Take reciprocals of intercepts 3. Reduce to smallest integer values 4. Enclose in parentheses, no commas i.e., (hkl )  families {hkl}

Crystallographic Planes -- families

Crystallographic Planes

3.

Reduction

a  1 1/1 1 1

4.

Miller Indices

(110)

example 1. Intercepts

2.

Reciprocals

3.

Reduction

a  1/2 1/½ 2 2

4.

Miller Indices

(100)

example 1. Intercepts 2. Reciprocals

b  1 1/1 1 1

z 

c 

c 

 1/ 0 0

b 

a  x 

b 

0 0

z 

c 

  1/ 1/

y 

c 

0 0

a  x 

b 

y 

Crystallographic Planes z  example 1. Intercepts 2. Reciprocals 3.

Reduction

4.

Miller Indices

a 1/2 1/½ 2 6

b 1 1/1 1 3

(634)

c  c  3/4  1/¾ 4/3  4 a  x 



b 

Family of Planes {hkl } Ex: {100} = (100), (010), (001), (100), (010), (001)

y 

Determine the Miller indices for the plane shown in the sketch

 x

Intercepts Intercept in terms of lattice parameters Reciprocals Reductions Enclosure

a 0

y

 z

-b -1 -1

c/2 1/2 2

N/A

(012)

Crystallographic Planes (HCP) 

In hexagonal unit cells the same idea is used z 

example 1. Intercepts 2. Reciprocals

3.

Reduction

a 1 1 1 1 1

a 2

 1/ 0

0

a 3 -1 -1 -1 -1

c  1 1 1 1

a 2

a 3

4.

Miller-Bravais Indices

(1011)

a 1  Adapted from Fig. 3.8(a), Callister 7e.

Crystallographic Planes 



We want to examine the atomic packing of  crystallographic planes – those with the same packing are equivalent and part of  families Iron foil can be used as a catalyst. The atomic packing of the exposed planes is important. Draw (100) and (111) crystallographic planes for Fe. b) Calculate the planar density for each of these planes. a)

Planar Density of (100) Iron Solution: At T < 912C iron has the BCC structure. 2D repeat unit

(100)

4 3 = a  R  3

Radius of iron R = 0.1241 nm atoms 2D repeat unit

Planar Density = area 2D repeat unit

1

a 2

1

=

4 3 3

R 

atoms atoms 19 = 1.2 x 10 2 = 12.1 2 nm m2

 Atoms: wholly contained and centered in/on plane within U.C., area of plane in U.C.

Planar Density of (111) Iron Solution (cont): (111) plane

1/2 atom centered on plane/ unit cell

2 a 

atoms in plane atoms above plane atoms below plane

h =

3 2

a 

 Area 2D Unit: ½ hb = ½*[(3/2)a][(2)a]=1/2(3)a2=8R2/(3)

atoms 2D repeat unit

3*1/6

Planar Density = area 2D repeat unit

8 R

2

3

= 7.0

atoms = 2 nm

0.70 x 1019

atoms m2

 Adding Ionic Complexities

LOOKING AT THE CERAMIC UNIT CELLS (CH 12)

Cesium chloride (CsCl) unit cell showing (a) ion positions and the two  ions per lattice point and (b) full-size ions. Note that the Cs +−Cl − pair  associated with a given lattice point is not a molecule because the ionic  bonding is nondirectional and because a given Cs + is equally bonded to  eight adjacent Cl −, and vice versa. [Part (b) courtesy of Accelrys, Inc.] 

Sodium chloride (NaCl) structure showing (a) ion  positions in a unit cell, (b) full-size ions, and (c) many adjacent unit cells. [Parts (b) and (c) courtesy of Accelrys, Inc.] 

Fluorite (CaF 2 ) unit cell showing (a) ion positions and (b) full-size ions. [Part (b) courtesy of Accelrys, Inc.] 

4-

SiO 4

4-

SiO 4

Polymorphism: Also in Metals 

Two or more distinct crystal structures for the same material (allotropy/polymorphism) iron system: titanium liquid  (HCP), (BCC)-Ti 1538ºC carbon:

BCC

-Fe

1394ºC

diamond, graphite FCC

-Fe

912ºC BCC

-Fe

The corundum (Al 2O 3 ) unit cell is shown superimposed on the repeated  stacking of layers of close-packed O 2− ions. The Al 3+ ions fill two-thirds  of the small (octahedral) interstices between adjacent layers.

Exploded view of the kaolinite unit cell, 2(OH) 4Al 2Si 2O 5. (From F. H. Norton, Elements of Ceramics , 2nd ed., Addison-Wesley Publishing  Co., Inc., Reading, MA, 1974.)

Transmission electron  micrograph of the structure  of clay platelets. This  microscopic-scale structure  is a manifestation of the  layered crystal structure  shown in the previous slide. (Courtesy of I. A. Aksay.)

( a) An exploded view of the graphite (C) unit cell. (From F. H. Norton, Elements of Ceramics, 2nd ed., Addison-Wesley Publishing Co., Inc., Reading, MA, 1974.) (b) A schematic of the nature of graphite’s layered structure. (From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd  ed., John Wiley & Sons, Inc., NY, 1976.)

(a) C 60 molecule, or  buckyball. (b) Cylindrical  array of hexagonal rings of  carbon atoms, or  buckytube. (Courtesy of  Accelrys, Inc.)

Arrangement of polymeric chains in the unit cell of polyethylene. The  dark spheres are carbon atoms, and the light spheres are hydrogen  atoms. The unit-cell dimensions are 0.255 nm  × 0.494 nm × 0.741 nm. (Courtesy of Accelrys, Inc.)

Weaving-like pattern of folded polymeric chains that occurs in thin  crystal platelets of polyethylene. (From D. J. Williams, Polymer Science and Engineering, Prentice Hall, Inc., Englewood Cliffs, NJ, 1971.)

Diamond cubic unit cell showing (a) atom positions. There are two  atoms per lattice point (note the outlined example). Each atom is  tetrahedrally coordinated. (b) The actual packing of full-size atoms  associated with the unit cell. [Part (b) courtesy of Accelrys, Inc.] 

Zinc blende (ZnS) unit cell  showing (a) ion positions. There  are two ions per lattice point (note  the outlined example). Compare  this structure with the diamond  cubic structure (Figure 3.20a). (b) The actual packing of full-size  ions associated with the unit cell. [Part (b) courtesy of Accelrys, Inc.] 

Densities of Material Classes

In general rmetals > rceramics > rpolymers

30

Why? Metals have... • close-packing (metallic bonding) • often large atomic masses

Ceramics have... • less dense packing • often lighter elements

Polymers have...

   )

   3

• intermediate values

20

Platinum Gold, W Tantalum

10

Silver, Mo Cu,Ni Steels Tin, Zinc

5 4

  m   c 3    /   g    (

• low packing density (often amorphous) • lighter elements (C,H,O)

Composites have...

Metals/  Alloys

2       r 1

Titanium  Aluminum Magnesium

Graphite/ Ceramics/ Semicond

Polymers

Composites/ fibers

*GFRE, CFRE, & AFRE are Glass, Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on 60% volume fraction of aligned fibers in an epoxy matrix). Zirconia  Al oxide Diamond Si nitride Glass -soda Concrete Silicon Graphite

Glass fibers PTFE Silicone PVC PET PC HDPE, PS PP, LDPE

0.5 0.4

GFRE* Carbon fibers CFRE*  Aramid fibers  AFRE*

Wood

0.3 Data from Table B1, Callister 7e.

Crystals as Building Blocks • Some engineering applications require single crystals: --diamond single crystals for abrasives (Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.)

--turbine blades Fig. 8.33(c), Callister 7e. (Fig. 8.33(c) courtesy of Pratt and Whitney).

• Properties of crystalline materials often related to crystal structure. --Ex: Quartz fractures more easily along some crystal planes than others.

(Courtesy P.M. Anderson)

Polycrystals

 Anisotropic

• Most engineering materials are polycrystals.

 Adapted from Fig. K, color inset pages of  Callister 5e . (Fig. K is courtesy of  Paul E. Danielson, Teledyne Wah Chang  Albany)

1 mm

• Nb-Hf-W plate with an electron beam weld. • Each "grain" is a single crystal. • If grains are randomly oriented, overall component properties are not directional.

• Grain sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers).

Isotropic

Single vs Polycrystals • Single Crystals

E (diagonal) = 273 GPa

-Properties vary with direction: anisotropic.

Source of data is R.W. Hertzberg, Deformation  and Fracture Mechanics  of Engineering  Materials , 3rd ed., John Wiley and Sons, 1989.

-Example: the modulus of elasticity (E) in BCC iron:

• Polycrystals -Properties may/may not vary with direction. -If grains are randomly oriented: isotropic. (Epoly iron = 210 GPa)

-If grains are textured, anisotropic.

E (edge) = 125 GPa

200 mm courtesy of L.C. Smith and C. Brady, the National Bureau of  Standards, Washington, DC [now the National Institute of Standards and Technology, Gaithersburg, MD].

Effects of Anisotropy:

X-Ray Diffraction



 

Diffraction gratings must have spacings comparable to the wavelength of diffracted radiation. Can‟t resolve spacings   Spacing is the distance between parallel planes of  atoms.

Figure 3.32 Relationship of the Bragg angle (θ) and the experimentally measured diffraction angle (2θ).

X-ray intensity (from detector)

n d = 2 sinc

 c

X-Rays to Determine Crystal Structure • Incoming X-rays diffract from crystal planes.

extra distance traveled by wave “2”



Measurement of  critical angle, c, allows computation of  planar spacing, d . For Cubic Crystals: d hkl



 Adapted from Fig. 3.19, Callister 7e .

spacing d  between planes

X-ray intensity (from detector)

d =

2

 k2  l2

h, k, l are Miller Indices

n 2 sin c



a

= h



reflections must be in phase for  a detectable signal!

c

Figure 3.34 (a) An x-ray diffractometer. (Courtesy of  Scintag, Inc.) (b) A schematic of the experiment.

X-Ray Diffraction Pattern z  c 

   )   e   v    i    t   a    l   e   r    (   y    t    i   s   n   e    t   n    I

a  x 

z  c  b 

y (110)

a  x 

z  c  y 

b 

a  x  (211)

b 

(200)

Diffraction angle

2

Diffraction pattern for polycrystalline -iron (BCC)  Adapted from Fig. 3.20, Callister 5e.

y 

Diffraction in Cubic Crystals:

SUMMARY

• Atoms may assemble into crystalline or  amorphous structures. • Common metallic crystal structures are FCC, BCC, and HCP. Coordination number and atomic packing factor  are the same for both FCC and HCP crystal structures. • We can predict the density of a material, provided we know the atomic weight, atomic radius, and crystal geometry (e.g., FCC, BCC, HCP). • Crystallographic points, directions and planes are specified in terms of indexing schemes. Crystallographic directions and planes are related to atomic linear densities and planar densities.