5: Order in Solids Flashcards
in solids, atoms or molecules tend to have what kind of order? what is the structure called ?
tend to order in a closed packed structure that minimizes energy. this is called a crystal.
what are non-crystalline solids called
amorphous
describe amorphous solids
non dense, random packing
describe crystalline solids
dense, regular packing
amorphous or crystalline solids have lower energy ? why?
crystalline.
dense, regular packed structures have lower energy.
quartz is crystalline or amorphous ?
crystaline
glass is crystalline or amorphous
amorphous
amorphous solids generally have what type of bonds ? what type of material is common
directional bonds
ceramics are common
example of compounds that can be both crystalline and amorphous. why ?
sio2
crystalline quartz or amorphous glass.
solidification conditions (purity, pressure, cooling rate) decide
glass describes what
amorphous solids
single crystals can imply ___ range order
very long
poly crystals are
several crystals packed together
in solids what decides packing structure
the way that atoms can be arranged to minimize empty space
the most dense is generally the lowest energy and thus preferred
how do metallic crystal structures tend to be packed ? why ?
densely packed
reasons:
- typically only one element, so atoms have equal radius
- metallic bonding is not directional
- nearest neighbour distances tend to be small in order to lower bond energy
- electron cloud shields cores from each other
what type of material has the simplest crystal structure
metals
describe unit cell
smallest repetitive volume which contains the complete pattern of a crystal
metallic cube unit cell is characterized but that parameters
a: cube edge length (lattice constant)
R: atomic radius
atomic packing factor (APF)
APF = vol atoms in unit cell/total unit cell volume
coordination number
number of first touching neighbours in a hard sphere model (for pure metals all same radius)
name common metallic crystal structures
- simple cubic
- body entered cubic
- face centred cubic
- hexagonal close packed
review unit cell from notes for all structures
FCC vs HCP
depends on the position of the third layer of atoms.
ABCABC = fcc
(stacking planes are oriented in plane (111))
ABABAB = hcp
voids in simple cubic unit cell
two interpenetrating simple cubic
FCC voids
two types of voids: 4 octahedral (CN = 6), 8 tetrahedral (CN = 4)
(in a unit cell)
describe rock salt structure
FCC structure with all octahedral voids occupied by the other atom. the structure is symmetrical and can be described as two interpenetrating FCC structures.
compounds w rock salt structure
LiF, LiCl, NaF, NaBr, KCl, MgO
Zincblende structure
FCC structure with half of the tetrahedral voids occupied by the other atom. named after ZnS, also this is diamond structure w all atoms being C
theatrical density
polymorphism
some metals and non-metals can have more than one crystal structure
note, at any given temperature and pressure there is one thermodynamically favoured structure.
allotropy
some metals and non-metals can have more than one crystal structure, this is called polymorphism.
in elemental solids, the word allotropy is used.
example of polymorphic materials
carbon: diamond, graphite, fullerene family
tin
heating and cooling iron wire. structure changes with temp
crystal systems, # crystal lattices. theyre related to what
7 crystal systems, related to overall geometry
- a, b, c = lattice constants
14 crystal lattices, related to position of the atoms within the geometry
- bravais lattices
study crystallographic points, miller indices
bragg’s law slides
anistropic in single crystals
properties vary with direction
anisotropy polycrystals
grains are oriented
polycrystal: grains are randomly oriented
isotropic
amorphous solids
solids without a long range order or crystallinity
ex, glasses, polymers
amorphous solids occur why
when fast solidification does not allow time to organize crystal structure
appearance amorphous solids
solid with liquid like appearance
important parameter for amorphous materials
glass transition temperature (Tg)
crystalline solids before/after melting vs at melting
before/after:
- atomic vibration increases with temp
- volume expansion (+ void spaces above Tm)
at melting:
- crystal formation
- high APF (less 1st neighbours)
- sudden volume increase
theres a distinct transition
transitions with temperature in amorphous solids
- no sharp density transition from liquid to glass. at Tm it becomes a supercooled liquid, at TG it becomes glass
in amorphous solids, below Tg is there rearrangement of atoms ?
below Tg there is no further arrangement of atoms. (volume change results from reduced thermal vibrations)
thermal exp coef is then similar to the one for the crystal
if T<Tg (in polymer and ceramic)
material is hard and brittle
if T>Tg (in polymer and ceramic)
material is flexible, rubbery
view structural disorder scales
what do vacancies promote
diffusion
vacancy
vacant atomic sites in a structure
causes distortion of planes
self-interstitials
extra atoms positioned between atomic sites (less common than vacancies)
types of point defects
vacancies
self-interstitials
point defects induce _____
local strain energy
eval number vacancies in notes
effects of vacancies in a material - thermal expansion
effect can be observed by looking at the specific volume changing w temp.
changes are mainly due to thermal expansion (asymmetry in the bond potential) but there is also a small effect resulting from the increased number of vacancies with increasing temperature
ie at higher temp, the length of the material increases faster. the increasing number of vacancies with T affects the total length, but not the size a of the unit cell
effects of vacancies in a material - diffusion
vacancies promote diffusion, so they follow same trend in T.
smaller atoms diffuse quicker
dislocations
linear defect around which some atoms are misaligned
Burgers vector, b
measure of lattice distortion
edge dislocation
b is perpendicular to dislocation line
extra half-plane of atoms inserted in a crystal structure
screw dislocation
b is parallel to dislocation line
spiral planar ramp resulting from shear deformation
linear defects
one-demential defects around which atoms are misaligned
dislocation leads to plastic deformation
grain boundaries + in notes
for simple cubic, what is:
CN, #atoms/unti cell, a wrt R, APF
CN = 6
1 atoms/unit cell
a = 2R
APF = 0.52
for body entered cubic, what is:
CN, #atoms/unti cell, a wrt R, APF
CN = 8
2 atoms/unit cell
a = 4R/sqrt(3)
APF = 0.68
for face centred cubic, what is:
CN, #atoms/unti cell, a wrt R, APF
CN = 12
4 atoms/unit cell
a = 2Rsqrt(2)
APF = 0.74
for hexagonal close packed, what is:
CN, #atoms/unti cell, APF
CN = 12
6 atoms/hcp cell
APF = 0.74
