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Crystals > Lecture 2 > Flashcards

Lecture 2 Flashcards

1
Q

What did Robert Hooke suggest the different shapes of crystals was due to?

A

The packing together of spheres

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1
Q

What is the difference in hexagonal versus square atomic layers

A

hexagonal have the spheres sitting in the diagonal space and squares are parallel with each other

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2
Q

What is an interstice

A

the space between the spheres

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3
Q

What does the atomic packing diagram of close-packed hexagonal layers look like?

A
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3
Q

What is the simplest way of stacking layers of atoms for close-packed hexagonal? What is the result?

A

place the atoms center directly above one another.

The result of this is the simple hexagonal structure (SH)

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4
Q

Are there pure elements in simple hexagonal structure? why?

A

no because the atoms in the second layer occupy the “hollows” or interstices between the atoms from the layer below.

This minimizes the total energy of the system (so it’s the most stable configuration)

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5
Q

where do atoms in the second layer of the close-packed hexagonal layer of atoms exist?

A

the interstices between the atoms from the layer below to minimize the total energy of the system to the most stable configuration

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6
Q

For close-packed hexagonal (not SH) layers of atoms, where are the options for the 3rd layer?

A

“A” position, or ABABABAB…
“C” position, or ABCABCABC…

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7
Q

If the atoms on the 3rd layer of a hexagonal-close packed go directly above the atoms on the first layer, what are the characteristics of this crystal?

A

hexagonal-close packed

ABABAB stacking

2-layer repeat

open channels

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8
Q

if the atoms in the 3rd layer of a hexagonal close packed go directly above the unoccupied interstices (C sites), what are the characteristics of this crystal?

A

cubic close-packed (CCP)

ABCABCABC

3-layer repeat

no open channels

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9
Q

What is the difference between the top view versus side view of 2nd layer HCP and CCP

A

the same

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10
Q

What is the difference between the top view versus the side view of the 3rd layer of HCP and CCP

A

HCP– above A

CCP- interstices

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11
Q

Why is a structure made of a three-layer stacking sequence of hexagonal layers called cubic-close packed

A

the shape and symmetry of the unit cell

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12
Q

What is a unit cell

A

the smallest group of atoms that can be repeated in three dimensions to form the entire crystal lattice

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13
Q

what’s another name for CCP

A

FCC

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14
Q

Describe the unit cell with CCP structure

A

close-packed layers lie perpendicular to the body diagonal of the cube (ABC in figure)

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15
Q

how many close-packed layers are there in CCP

A

4 different body diagonals in a cube

atoms in contact along the diagonals

**think the ABC is close-packed in the diagonals on 2 different plane directions

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16
Q

how many atoms are in the CCP unit cell

A

4 atoms

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17
Q

how do you take into account half atoms in calculating atoms per unit cell

A

count half to this atom, because the other half goes to a different atom

18
Q

What is the coordination number of CCP

A

12

19
Q

what is a coordination number

A

number of nearest neighbors

20
Q

what is the relationship between a (lattice parameter) and atomic radius r in CCP

A
21
Q

what does the variable “a” stand for

A

the lattice parameter, proportional to the distance between atoms

22
Q

what is the atomic radius r

A

the radius of an atom within the unit cell, usually in relation to the lattice parameter a

23
Q

what are the different unit cells of the ccp structure

A

cubic (conventional) cell

rhombohedral cell

24
Q

how is the rhombohedral unit cell of ccp built

A

stacking triangular layers of three atoms instead of six

25
Q

why is the rhombohedral unit cell for ccp less useful

A

it doesn’t show symmetry

26
Q

for the unit cell of the HCP structure, how are atoms in contact?

A

atoms are in contact along the side faces of hexagon and in between the A and B atoms

27
Q

how do HCP, CCP, and rhombohedral relate

A

same AB layers,

HCP and CCP change from the position of the 3rd layer

HCP ABABA, CCP ABCABC

rhombohedral is another version of the ABCABCABC (CCP) pattern but built by stacking layers of three atoms not 6

28
Q

how many atoms are in the HCP unit cell

A

6

29
Q

what is the HCP coordination number

A

12

30
Q

what is the relation between the a and r in HCP

A

a = 2r

31
Q

What are the differences between HCP and CCP unit cells

A

HCP
coordination number = 12
atoms/unit cell = 6
a = 2r

CCP
coordination number = 12
atoms/unit cell = 4
a = 2sqrt(2) * r

32
Q

why is it important to identify the close-packed planes/directions?

A

close packed planes have high catalytic activity

deformation in crystalline materials happen primarily via sip on close-packed planes along close-packed direction

33
Q

why are close packed planes importan

A

highest catalytic activity

34
Q

where does deformation in crystalline materials happen primarily

A

via slip on close-packed planes along close-packed directions

35
Q

what does close-packed mean

A

most tightly packed (high density of spheres), most efficient packing

36
Q

are there gaps in between atoms in close-packed HCP and CCP?

A

yes, called interstices

37
Q

what is another name for packing density

A

atomic packing factor

38
Q

how do you calculate the packing density

A

PD = (vol occupied by atoms)/(total volume of unit cell)

vol of atom = 4/3 pi r^3
# atoms inside unit cell

example for FCC

39
Q

what is the packing efficiency of close-packed structures?

A

74%

40
Q

what is the packing density of CCP/FCC

A

0.74

41
Q

what is planar density

A

density of atoms on a particular crystal plane

42
Q

how do you calculate planar density

A

PD = (# atoms on a plane * area occupied) / (area of the plane)

43
Q

do the different crystallographic planes of a unit cell have the same or different surface areas

A

different

44
Q

what are three examples of properties that might be different depending on the direction a plane of a unit cell is measured in

A

hardness, resistivity, catalytic activity

45
Q

What do the (100) (110) and (111) planes look like

A

Crystals (11 decks)

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