crystal structures june 23

SCIMATP

CRYSTAL STRUCTURES

X Ray Diffraction

X Ray Diffraction

• density of electrons within the crystals is obtained form the measurement of the angles and intensities– the mean positions of the atoms in the crystal– their chemical bonds– their disorder and various other information.

RECALL

• Coordination # = 6 (# nearest neighbors)

RECALL

• Coordination # = 8

Adapted from Fig. 3.2, Callister & Rethwisch 3e.

RECALL

• Coordination # = 12


• Atomic bonding in this group is metallic and thus non-directional

– minimal restriction to the number and position of nearest neighbor atoms

– Leads to relatively large numbers of nearest neighbors and dense atomic packing

METALLIC CRYSTAL STRUCTURES

1. SIMPLE CUBIC – Rare (Po)

2. BODY-CENTERED CUBIC (BCC)

3. FACE-CENTERED CUBIC (FCC)

4. HEXAGONAL CLOSE-PACKED (HCP)


• more complex since they are composed of different elements

• the bonding in ceramics may range from purely ionic (nondirectional) to totally covalent (directional)

CERAMIC CRYSTAL STRUCTURES

Crystal structure in crystalline ceramics is influenced by1. Magnitude of electric charge

• Cation, +• Anion, -• Charge Neutrality: Net charge in the ceramic

structure should be zero» Calcium Fluoride» Ca 2+, F1-


CaF2 :Ca 2+

cation

F-

F-

anions+

Crystal structure in crystalline ceramics is influenced by

2. Relative sizes of cations, rc, and anions, ra


1a

c

r

r

rcationranion

Coord #

< .155 .155-.225 .225-.414 .414-.732 .732-1.0

ZnS (zincblende)

NaCl (sodium chloride)

CsCl (cesium chloride)

2 3 4 6 8


To form a stable structure, how many anions can surround around a cation?

http://www.chem.ubc.ca/courseware/121/tutorials/exp7A/tetrahedral_hole.gif http://www.math.twsu.edu/history/Images/tetrahedron2.gif

http://www.chem.ubc.ca/courseware/121/tutorials/exp7A/octahedral_hole.gif http://www.luzdegaia.org/alertas/diversos/octahedron-1.gif

AX-Type Crystal Structures

1. Rocksalt strcuture (NaCl)


rNa = 0.102 nm

rNa/rCl = 0.564

cations (Na+) prefer octahedral sites


rCl = 0.181 nm


2. Cesium Chloride Structure (CsCl)


939.0181.0

170.0

Cl

Cs

r

r

Since 0.732 < 0.939 < 1.0, cubic sites preferred


3. Zinc Blende/Sphalerite (ZnS)



AmXp-Type Crystal Structures

m and/or p ≠ 1

1. Fluorite (CaF2)

• Cations in cubic sites


AmBnXp-Type Crystal Structures

ceramic compounds with two types of cations

1. Perovskite Crystal Structure

• Ex: Barium Titanate

• FCC

IONIC LATTICES• Watch video Clip

– Take note of the following terms:• Octahedral Holes• Tetrahedral Holes

Interstitial sites – locations between the “normal”

atoms or ions in a crystal into which another – usually different – atom or ion is placed

• Fluorite (CaF2)

• Hallite (NaCl)• Sphalerite/Zincblende (ZnS)

COVALENT STRUCTURES

• strength of bonds throughout the crystal is non uniform

• Crystals of low symmetry and complex structure

Diamond Structure

• No polymer is completely crystalline– a single polymer chain may be partly in a crystalline

lamella, and partly in the amorphous state

Is this a good thing?– If you are making PLASTICS, yes

• Crystallinity makes a material strong, but it also makes it brittle

• A completely crystalline polymer would be too brittle to be used as plastic

• The amorphous regions give a polymer toughness, that is, the ability to bend without breaking.

POLYMERIC CRYSTAL STRUCTURES

DENSITIES OF MATERIAL CLASSES• ρmetals > ρceramics >ρpolymers

Why?Metals have...

• close-packing (metallic bonding)• large atomic mass

Ceramics have...• less dense packing (covalent

bonding)• often lighter elements

Polymers have...• poor packing (often amorphous)• lighter elements (C,H,O)

Composites have...• intermediate values

(g

/cm

3)

Graphite/ Ceramics/ Semicond

Metals/ Alloys

Composites/ fibersPolymers

1

2

20

30Based on data in Table B1, Callister *GFRE, CFRE, & AFRE are Glass,

Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on 60% volume fraction of aligned fibers

in an epoxy matrix). 10

3 4 5

0.3 0.4 0.5

Magnesium

Aluminum

Steels

Titanium

Cu,Ni

Tin, Zinc

Silver, Mo

Tantalum Gold, W Platinum

Graphite Silicon

Glass -soda Concrete

Si nitride Diamond Al oxide

Zirconia

HDPE, PS PP, LDPE

PC

PTFE

PET PVC Silicone

Wood

AFRE *

CFRE *

GFRE*

Glass fibers

Carbon fibers

Aramid fibers

References

• http://users.encs.concordia.ca/~woodadam/MECH221/Course_Notes/Crystal%20structure%20and%20properties.pdf

• cmsweb1.loudoun.k12.va.us/.../lib/.../Atoms_to_Minerals.ppt • www.ims.uconn.edu/~alpay/Group_Page/.../StructureOfSolids.ppt • http://www.yourgemologist.com/crystalsystems.html• http://en.wikipedia.org/wiki/Crystal_system• CRYSTAL STRUCTURES Lecture. PowerPoint Presentation by Michelle

Natividad (2009)• Lesson 5. Powerpoint Presentation by Gwendolyne Pascua (2007)• http://ari.cankaya.edu.tr/~ebiber/ie114/Week-3.ppt• http://www.ims.uconn.edu/~alpay/Group_Page/Courses/MMAT

%20201/StructureOfSolids.ppt• http://www.che.yuntech.edu.tw/teacher/lincw/%E9%AB%98%E7%AD

%89%E9%AB%98%E5%88%86%E5%AD%90%E7%89%A9%E6%80%A7/CHAPTER_3_-_Morphology_and_Order_in_Crystalline_Polymers.pdf

• http://bgu.uniclass.co.il/mate09/uploader.php?file=Chapter_03.ppt&id=2

http://users.encs.concordia.ca/~woodadam/MECH221/Course_Notes/Crystal%20structure%20and%20properties.pdf

http://users.encs.concordia.ca/~woodadam/MECH221/Course_Notes/Crystal%20structure%20and%20properties.pdf

crystal structures june 23

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