Crystallography 101

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Define lattice
an infinite array of points in space in which the environment of each point is identical.
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What makes a structure 'amorphous'?
It has 'long' range order.
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What makes a structure 'crystalline'?
It has 'short' range order
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What is a mesh?
An arrangement of lines which joins the lattice points.
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Describe a 'unit' mesh
One unit which when repeated makes up the mesh.
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Name the two types of meshes
1) Primitive: smallest area that can be achieved by connecting the lattice points;2) Non-Primitive: A larger area created by emitting some lattice points.
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How can we define a lattice in 2D?
Using two vectors and an angle.
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What is an ideal crystal?
An infinite periodic repetition of identical structural units in space
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Define 'motif'
The repeating unit of the pattern (the arrangement of atoms in molecules)
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What is a crystal structure?
Lattice + motif --> Crystal Structure
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How can a lattice be identified?
The lattice must be invariant under transition: It can be translated perfectly.
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What is a 2D unit cell?
A repeatable unit of the crystal which when stacked together with pure transnational repetition produces a whole crystal. Lattice points do not need to be the same atom but the unit size must be the same
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What is a 'space' lattice ?
An imaginary network of lines, lattice points =intersection.
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How many atoms in a simple cubic unit cell?
1 ] If an atom is on the edge of a unit cell then it is also shared by 8 other unit cells (they are stacked on each other) so only 1/8 of thee atom is occupies 1 unit cell. There are 8 edges so 1/8 *8 = 1 atom per unit cell.
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What are 'fractional coordinates and what are they used for ?
They represent heights and are needed to accurately represent 3D structures through 2D projections (plan view). The plan view is easily drawn as the crystal structure is invariant so identical points do not need to be repeated.
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Give the equation for the volume of the primitive unit cell
V= a_1 。(a_2 *a_3) → where a_1,a_2 and a_3 are primitive translation vectors
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What is a lattice vector and how can it be represented?
Give the direction of a lattice. Represented by [ ] . R =Ua_ +Vb_ (for negative values of U or V a bar is put on top of the letter)
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Explain lattice direction symmetry
It represents a group of lattice vectors that show symmetry. Shown by < >
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Other cards in this set

Card 2

Front

What makes a structure 'amorphous'?

Back

It has 'long' range order.

Card 3

Front

What makes a structure 'crystalline'?

Back

Preview of the front of card 3

Card 4

Front

What is a mesh?

Back

Preview of the front of card 4

Card 5

Front

Describe a 'unit' mesh

Back

Preview of the front of card 5
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