4 Crystals

Question 1

what is a crystal?

Answer 1

solid with long range order
- infinite array atoms/molecules that extend in 3d

Question 2

what are the 3 crystal forms, describe them

Answer 2

• polymorph: similar substance, have diff arrangements atoms and unit cell dimensions
• pseudo polymorphs: diff chemical content (solvent, host-guest atoms)
• allotropes: polymorphs with only one element

Question 3

3 allotropes of P

Answer 3

-white (P4), molecular solid, very reactive
- red (P8)
- black, network covalent

Question 4

what is a unit cell

Answer 4

smallest potion of structure that can be repeated by translation (along unit cell) and show full symmetry

Question 5

How many crystal classes are there? name them

Answer 5

7
cubic (all angles/ edges same)
Tetragonal
orthorhombic
trigonal
Hexagonal
Monoclinic
Triclinic (all angles/ edges different)

Question 6

How to draw 3D-2D projections?

Answer 6

bottom atoms 0
top =1

Question 7

what is a lattice?

Answer 7

array equiv pt in 3D
- gives info translational sym of material
- doesn’t give info about actual atom position space

Question 8

what are the 4 types of Bravais lattices

Answer 8

• primitive/simple (atom each corner)
• face centered (primitive and one in center of face)
• body centered (primitive and atom in middle)
  -side centered (primitive and atom in middle each side)

Question 9

how many possible systems are there in Bravais lattices

Answer 9

14

Question 10

what type of lattices does cubic have

Answer 10

Simple
face
body

Question 11

what are Millar indices?

Answer 11

(hkl) witch intersects unit cell at (a,b,c): (1/h, 1/k, 1/l)
if h,k, or l, =0 plane is parallel to a,b,orc

bar over indicies rep negative, move axis over

Question 12

what does {100} rep

Answer 12

family (100), (010), (001) ect
6 total

Question 13

what do these rep?
- (a,b,c)
- [abc]
- (hkl)
- {hkl}

Answer 13

• (a,b,c): point
• (hkl): plane
• {hkl}: family of planes

Question 14

what is d-spacing
how to calculate

Answer 14

distance b/w parallel lattice planes
- can be used to determine lattice constant

1/d^2 = (h^2+k^2+l^2)/a^2

Question 15

what are the 2 types of packing

Answer 15

-simple
-closed (more efficient)

Question 16

types of closed pack

Answer 16

-hexagonal: ABA
-cubic: ABC

Question 17

how to count atoms in cell

Answer 17

divide by fraction actually in the cell

Question 18

what is the diff b/w fcc and ccp?

Answer 18

same but
ccp - cubic closed packed: is a packing type

fcc - face centered cubic: is a lattice type

Question 19

density=?
depends on

Answer 19

(# atoms/cell)(mass atoms)/Volume unit cell

-unit cell dimensions (d-space), crystal structure, atomic mass

Question 20

what are holes

Answer 20

small hole that has a diff smaller atom (usually cation)

cubic, tetrahedral, octahedral

Question 21

how many holes are in fcc?

Answer 21

• 8 tetrahedral
• 4 octahedral
• not all has to be occupied

Question 22

what is radius ratio?

Answer 22

used to predict type hole cation could occupy

r+/r-

most stable if large enough minimize repulsion b/w neighboring anions and small enough for enough cation/anion contact

Question 23

Packing efficiency for cubic

Answer 23

= (# atoms/unit cell)(V atom)/ V unit cell *100

measures how tightly packed the crystal is, least empty space
-closed pack has highest

Question 24

what is the V of a Sphere

Answer 24

4/3pir^3

Question 25

when would a crystal be 100% perfect

Answer 25

0K

Question 26

what are all the types of defects (9)

Answer 26

• Piont: schottky, frenkel, F-center, non stoichiometric
• Extended: shear, stacking, grain boundaries
• Dislocation: edge, screw

Question 27

what is a Schottky defect

Answer 27

• piont
• common in alky halides
• pair vacant (cation and anion)
• to balance charge 2 extra atoms at surface

Question 28

Frenkel defect

Answer 28

• point
  -common in TM halides
• atom displaced to empty internal site

Question 29

F-center

Answer 29

-piont
-e trapped in anion vacency
- made by alkali halides in metal vapour, cation surface cause anions migrate and e fill space
- e like particle in box, emits light

Na to Na+ and e-

Question 30

non-stoichiometric defects

Answer 30

-point
- replace with diff atom, if charge not the same then some atoms leave to balance charges

NaCl and replace Na+ with Ca2+

Question 31

Crystallographic shear

Answer 31

• extended
• ex WO3 when W6+ to W5+ structure collapses and W shares edges not corners

Question 32

Staking faults

Answer 32

-extended

found in hcp or ccp with layers out of order

ABABAB(B)ABAB

Question 33

antiphase boundary

Answer 33

-extended
-lateral displacement 2 parts same crystal
- get antiphase domains

• shift one over

Question 34

grain boundaries

Answer 34

• extended
• structural mismatch b/w poly crystalline domains
• can be diff less 1 degrees

-growth direction don’t line up

Question 35

edge dislocation

Answer 35

-dislocation
- common in metals, responsible of weakness
-plane only extends part way

• don’t line up properly

Question 36

slip plane

Answer 36

-edge dislocation
-planes slip past
- buger vector perpendicular defect
-makes material stronger
-slip steps from pile up half steps at end

Question 37

what is a burger vector

Answer 37

mag dislocation by unit cell length and type dislocation

Question 38

Screw dislocation

Answer 38

• dislocation
• Burgers parallel defect
  -like the crystal had been twisted