Structure of Crystalline Solid Chpt 3 Flashcards
What is the structure of Crystalline materials
atoms pack in periodic arrays
What type of materials are typical to have crystalline structures?
metals, many ceramics, some polymers
What’s the structure of a noncrystalline material
atoms have no periodic packing
Where do you find noncrystalline materials?
complex structures, rapid cooling
Whats another word for noncrystalline?
Amorphous
What’s the correlation between density packing and energy?
dense, regular packed (compact) structures tend to have lower bonding energy
What’s a unit cell
basic repeating unit of crystalline materials
Repeating unit cells to form entire crystal
What’s lattice
atomic arrangements in crystalline solids that can be described with respect to a network of lines
What are lattice sites
the intersections of lines of unit cells in crystalline solids
What are the three characteristics of a lattice cell
lattice parameters: how many sides
axial length: length of each axix
interaxial angle: angle between each axis
describe the relationship between unit cells and atoms
an atom can share multiple unit cells
Why do Metallic Crystal Structures have dense packing?
metallic bonding is not directional, the nearest neighbor distance tends to be small in order to lower bond energy, and typically since only one element is present, all atomic radii are the same
They have simple structures
Do metallic crystal structures have dense or non-dense packing?
Dense
What are the four structures of metallic crystals
simple cubic (SC), Body-Centered Cubic (BCC), Face-Centered Cubic (FCC), Hexagonal Closed-Packed (HCP)
What is SC and describe it
Simple Cubic, arrangement of hard spheres in each (8) corner.
each corner of the unit cell is defined by a lattice point at which an atom can be found in the crystal
each of the 8 corners of the unit cell contain an identical particle
What is APF
Atomic packing factor
APF= Vol. of atoms in unit cell / vol. of unit cell
Whats the volume of a sphere
4/3pi(r)^3
What is the APF of a SC structure
(1*4/3pi(0.5d)^3) / d^3
d = length of a side
closed-packed directions contains:
8*1/8 = 1 atom/unit cell
0.52
What is BCC?
Body-Centered Cubic
contains 9 atoms, all identical, 8 in each corner of the unit cell (they say this is the atomic centers in the unit cell), and one in the center
What’s the APF of a BCC structure?
radius is 4r because you have 1/8 circles in corners and an atom in center with radius of that circle, so diameter of 2. from one corner to the other, it’s 4r. r = (sqrt(3)/4) * a
https://www.youtube.com/watch?v=ceOOxa1A6-A
of atoms: (one atom with all 8 components in each corner and another in the center)
APF = 2 * 4/3 pi (sqrt(3)a/4)^3 / a^3
0.68
What is FCC?
Faced-Centered Cubic structure
1/8 sphere in each corner, half a sphere on each face of the cube cell
How many lattice points in a FCC structure?
14 identical points.
What elements have the FCC structure?
Cu, Al, Ag, Au
Whats the APF of FCC structure?
on one face, its diagonal = sqrt(2)a = 4r
r = sqrt(2) / 4
https://www.youtube.com/watch?v=ceOOxa1A6-A
4 atoms per unit cell
APF = 4 * 4/3 pi (sqrt(2)a / 4)^3 / a^3
0.74
What is HCP?
Hexagonal closed-packed structure
6 atoms form the regular hexagon, they surround 3 inside, 1 on top and bottom, and 2 on the corners (top and bottom)
What crystals commonly have HCP?
metallic crystals, Cd, Mg, Zn, Ti
Whats APF of HCP structure?
C is the length radius, a is length of a side of the hexagon,
Ideal ratio of two lattice parameters a, and c: c/a = 1.633
0.74 (same as FCC)
How are FCC and HCP crystal structures made
may be generated by stacking of close-packed planes
Look at infographic of all crystalline structures
What is a coordination number?
the number of nearest neighbors (of lattice points)
https://www.chemistrylearner.com/coordination-number.html
What are the coordination numbers of BCC FCC and HCP?
Body-centered cubic has 8
Face-centered cubic has 12
Hexagonal close-packed has 12
How to find the theoretical density of Crystalline Materials
density (p) = n A / V_c N_a
n = atoms per unit cell
A = atomic weight (g/mol)
V_c = volume per unit cell (cm^3/unit cell)
N_a = Avogadro’s number (6.023 x 10^23 atoms/mol)
how to write point coordinates
Px = qa
Py = rb
Pz = sc
How does the packing of atoms in FCC and HCP look like?
HCP: ABABAB
FFC: ABCABCABC
A biggest, then B then c
A inbetween b and c
closed packed planes: FCC(111) planes ABC and HCP(0001) planes AB have APF of 0.74
What are the axial angles in the hexagonal system?
90 and 120 degrees
LEARN HOW TO DO DIRECTION PLANE AND POINTS
What is Linear density? Planar density?
LD = # atoms centered on direction vector /
length of direction vector
PD = # atoms centered on a plane / A of plane
KNOW HOW TO DO LINEAR AND PLANAR DENSITY
What are single crystals?
perfect unit cells arrangment in same orientation.
Que es Polycrystalline Materials
bueno pues. estoy cansada. puta madre quiero dormirrrrrrrrrrrrrRRRRRRRRRRR
bueno bUeNO
many crystalline grains, not aligned with each other
What is the area where crystalline grains meet?
the grain boundary
Anisotropy what izzz
the directionality of properties. refers to the varience of atomic spacing with crystallographic direction.
like triclinic structures, highly anisotropic
What is isotropic
substances where the measured properties are independent of direction of measurement
What does having different directions in a crystal mean
different packing.
What is a packing characteristic of FCC
cells are more separated along the edges than along the face diagonal
KNOW HOW TO DO XRAY DIFFRACTION CRYSTALLINE IDETIFICATION AND STRUCTURE ANALYSIS
How are x-ray beams used in material science
its shot towards an object, some of it will be absorbed, transmitted and scattered
the scattering pattern gives us info on electron density distribution and therefore position of atoms
wide angle vs small angle x-ray scattering
wide angle: study small-scale structure (<1nm)
small angle: study large-scale feature (1-100nm)
