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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
Adapted from Fig. 3.1, Callister & Rethwisch 3e.
• 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)
METALLIC CRYSTAL STRUCTURES
METALLIC CRYSTAL STRUCTURES
• 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-
CERAMIC CRYSTAL STRUCTURES
CaF2 :Ca 2+
cation
F-
F-
anions+
Crystal structure in crystalline ceramics is influenced by
2. Relative sizes of cations, rc, and anions, ra
CERAMIC CRYSTAL STRUCTURES
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
CERAMIC CRYSTAL STRUCTURES
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)
CERAMIC CRYSTAL STRUCTURES
rNa = 0.102 nm
rNa/rCl = 0.564
cations (Na+) prefer octahedral sites
Adapted from Fig. 3.5, Callister & Rethwisch 3e.
rCl = 0.181 nm
AX-Type Crystal Structures
2. Cesium Chloride Structure (CsCl)
CERAMIC CRYSTAL STRUCTURES
939.0181.0
170.0
Cl
Cs
r
r
Since 0.732 < 0.939 < 1.0, cubic sites preferred
AX-Type Crystal Structures
3. Zinc Blende/Sphalerite (ZnS)
CERAMIC CRYSTAL STRUCTURES
CERAMIC CRYSTAL STRUCTURES
AmXp-Type Crystal Structures
m and/or p ≠ 1
1. Fluorite (CaF2)
• Cations in cubic sites
CERAMIC CRYSTAL STRUCTURES
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