Lecture 11

Question 1

What are the three different types of symmetry elements in 3D?

Answer 1

point: centers of symmetry, mirror planes, and inversion axes

translational: glide planes and screw axes

Question 2

How many different space groups are there for 3d?

Answer 2

230

Question 3

How are bravais lattices grouped?

Answer 3

through their point group symmetries into seven crystal systems

Question 4

Do P F I cubic lattices have the same point group symmetry?

Answer 4

yes

Question 5

What is a crystal habit?

Answer 5

external shape

Question 6

What are the centrosymmetric groups?

Answer 6

Question 7

What does centrosymmetric mean?

Answer 7

the crystal posses an inversion center

Question 8

What two shapes have the same point group symmetry? How is that determined?

Answer 8

cube and octahedron

you go through all the options (ex 4-fold 3-fold 2-fold) if they have the same # of each they’re the same

Question 9

Does a tetrahedron have more or less symmetry than a cube? octahedron?

Answer 9

less, both are more

Question 10

What is the characteristic of the cubic crystal? What are three crystal shapes in this category?

Answer 10

four triads

cube, octahedral, tetrad

Question 11

What does it mean when two crystals belong to the same crystal class?

Answer 11

they have the same point group symmetry (ex cube and octahedron)– not necessarily the same shape

Question 12

In 3D, what is a point group called?

Answer 12

a crystal class

Question 13

How many different 3D crystal classes are there? 2d point groups?

Answer 13

3D- 32
2D- 10

Question 14

What is the notation for an identity? inversion center? mirror plane? rotation axis n-fold? rotoinversion?

Answer 14

Question 15

what does an 1 hat inversion look like? what is it also called?

Answer 15

inversion center

Question 16

What does a 2 hat (m) look like? Also called?

Answer 16

2-fold rotoinversion

inversion diad

Question 17

What does a 4 hat look like? also called?

Answer 17

4-fold rotoinversion

inversion tetrad

Question 18

what does a 3 hat look like? also called?

Answer 18

3 fold rotoinversion

inversion triad

Question 19

What does it for crystals to be in the same class?

Answer 19

their outer shapes have the same point group symmetries

Question 20

What does point group symbol notation indicate?

Answer 20

the main symmetry elements present (not all, but most important)

Question 21

what is the minimum symmetry for the cubic system? how do you identify with the symmetry notation?

Answer 21

4 triads

3 bar is the second number written or 3 for non-centro

Question 22

what is the minimum symmetry for the tetragonal system? which ones can it be? how do you identify?

Answer 22

1 inversion tetrad

Primitive, bcc (I)

4 is the first one listed in the notation

Question 23

what is the minimum for orthorhombic? what is the centro symmetric notation? non?

Answer 23

3 diads

PICF

centro; mmm
non centro: 222, mm2^P

Question 24

what is the minimum symmetry for trigonal?

Answer 24

1 rotation triad

trigonal=rhombohedral

R

Question 25

what is the minimum symmetry for hexagonal?

Answer 25

1 rotation hexad (6 in the phrase)

Question 26

By looking at the point symmetry notation, how do you kow if it's centrosymmetric?

Answer 26

if it's only one number and m's

Question 27

what is the minimum qualification for monoclinic? triclinic?

Answer 27

1 inversion diad none

Question 28

What does the order that the symmetry elements appear in the point group symbol depend on?

Answer 28

the crystal system

Question 29

what does /m mean? mm?

Answer 29

the mirror plane is perpendicular to the rotation axis mm: mirror planes parallel to rotation axis

Question 30

what is the most common crystal class?

Answer 30

60% correspond to 2/m and m

Question 31

what is the most symmetric class?

Answer 31

m3m- cubic

Question 32

what does the addition of glide planes and screw axes make it called?

Answer 32

space groups

Question 33

How many Bravais lattices are there in 3D space

Answer 33

14

Question 34

What is body-centered in bravais notation

Answer 34

I

Question 35

what is face-center in bravais notation

Answer 35

F

Question 36

what is base-center in bravais notation

Answer 36

C

Question 37

What are the 14 bravais lattices for 3D space?

Answer 37

the 14 lattices are in 7 crystal systems

Question 38

What are the 7 crystal systems for the 14 bravais lattices?

Answer 38

1. Cubic (3): simple cubic, BCC, FCC 2. Orthorhombic (4): simple ortho, body-centered ortho, base-centered ortho, face-centered ortho 3. tetragonal (2): simple tetrag, body-centered tetrag 4. trigonal (1): rhombohedral 5. hexagonal (1): hexagonal 6. monoclinic (2): simple mono, base-centered mono 7. triclinic (1): triclinic

Question 39

How are bravais lattices grouped?

Answer 39

by their point group symmetries into the seven crystal systems

Question 40

What are the different centrosymmetric crystal groups?

Answer 40

Question 41

What is the qualification to belong in the cubic crystal sysem?

Answer 41

four 3-fold axes

Question 42

What 3 3D shapes belong in the cubic system?

Answer 42

Question 43

What is the difference between crystal system and crystal class?

Answer 43

system-- same qualifier class- all the same point group symmetry

Question 44

In 2D, how many different crystallographic point groups are there? How many 3D point groups are there?

Answer 44

10- point groups 32- crystal classes

Question 45

What does inversion look like?

Answer 45

Question 46

What does inversion diad look like?

Answer 46

Question 47

What does inversion tetrad look like

Answer 47

Question 48

inversion triad

Answer 48

Question 49

what is the minimum symmetry for cubic, tetragonal, orthorhombic, trigonal, hexagonal, monoclinic and triclinic?

Answer 49

Question 50

How do you know the nomanclature order for different crystal classes?

Answer 50

the viewing directions listed on the chart

Question 51

what is the least common crystal class?

Answer 51

m3m