Ch 3 - The Structure of Crystalline Solids Flashcards
What is the correlation between atomic packing and energy?
dense, ordered packed structures tend to have lower energies
How are crystalline materials packed?
- atoms pack in periodic, 3D arrays
- typical of metals, many ceramics & some polymers
How are noncrystalline materials packed?
- atoms have no periodic packing
- occurs for complex structures & rapid cooling
What is another name for noncrystalline?
amorphous
What are the reasons for dense packing (4)?
- typically only one element 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
- e- cloud shields cores from each other
Describe the Simple Cubic structure (SC)
- rare due to low packing density
- close packed directions are cube edges
What is the simple cubic’s coordination number?
6
Describe the Body Centered Cubic structure (BCC)
- one atom at each corner plus one in the center
- atoms touch each other along cube diagonals
What is BCC’s coordination number?
8
How many atoms per unit cell in SC?
1
How many atoms per unit cell in BCC?
2
Describe the Face Centered Cubic structure (FCC)
- one atom on each corner
- one at the center of each face
- atoms touch each other along face diagonals
What is FCC’s coordination number?
12
How many atoms per unit cell in FCC?
4
Describe FCC’s stacking sequence
ABCABCABC….with 2D projection
Describe the Hexagonal Close-Packed structure (HCP)
- ABAB stacking sequence with 2D projection
- hexagonal instead of cubic
What is the coordination number for HCP?
12
How many atoms per unit cell in HCP?
6
Describe the tetragonal bravais lattice
- base is a square
- z axis not equal to side lengths
- all internal angles are 90 degrees
Describe the rhombohedral (trigonal) bravais lattice
- all sides equal
- all angles equal but not right angles
Describe the orthorhombic bravais lattice
- all sides unequal
- all angles equal to 90 degrees
Describe the monoclinic bravais lattice
- all sides unequal
- two right angles, third is not
Describe the triclinic bravais lattice
- all sides unequal
- all angles unequal
- no right angles
What is a unit cell?
smallest repetitive volume which contains the complete lattice pattern of a crystal
What is the algorithm to determine crystallographic directions?
(1) position vector to go through origin
(2) read off projections in terms of unit cell dimensions
(3) adjust to smallest integer values
(4) enclose in square brackets w/o commas
What is the linear density of atoms?
number of atoms in a given direction over the unit length of that direction vector
What is the algorithm for crystallographic planes?
(1) pick origin that does not go through plane
(2) read off intercepts of plane with axes in terms of unit cell dimensions
(3) take reciprocals of intercepts
(4) reduce to smallest integer values
(5) enclose in parentheses w/o commas
What is planar density?
number of atoms in a specific plane over the area of said plane
