Crystallography And Diffraction Flashcards
How can we observe crystals?
Regular form of the faces of mineral specimens.
Scattering of light by periodic objects (eg opals).
Diffraction experiments.
High resolution electron microscope images.
What makes a material crystalline?
Ordered arrangement of atoms.
Atoms stack together to form regular networks.
Atomic arrangement is often reflected in the macroscopic geometry.
What makes a material amorphous?
Random arrangement of atoms.
What makes a material polycrystalline?
It is made up of many small crystals.
What is a lattice?
An infinite array of points in space in which the environment of every point is identical.
What is a motif?
A repeating unit of pattern (eg the arrangement of atoms that is placed on each lattice point).
What is a mesh?
The arrangement of lines that joins the lattice points.
What is a unit mesh?
One unit which, when repeated, makes up the mesh.
Where can the motif be placed on the lattice?
The motif can be placed anywhere in relation to the lattice point as long as the same position is chosen for every motif.
Can there be more than one unit mesh for a lattice?
Yes, it is possible, however we generally use the primitive mesh.
What is the primitive mesh?
The unit mesh with only one lattice point.
What is a centred mesh?
A square mesh with a lattice point in the middle.
How can lattice points be reached on a lattice?
With a combination of lattice vectors.
What method can we use to project 3D to 2D?
We can use crystal plans or crystal projections.
How do we draw crystal projections?
We view the cell in a set plane and label elevations for each point not at 1 or 0.
What is the most efficient way to pack in 2D?
Hexagonal packing (91%).
What are the most efficient ways to pack in 3D?
HCP and FCC (74%).
What interstices do both HCP and FCC possess?
Octahedral and Tetrahedral interstices?
What are interstices?
Small spaces within a lattice that may be able to accommodate another atom.
Example of an interstice:
Na+ in a FCC Cl- lattice inside the octahedral interstice.
What symmetry elements are there in 2D?
Rotational and Mirror.
If when does a body have n-fold symmetry?
When the final and initial forms of the body are identical when it is rotated 360º/n around the axis of rotation.
How does mirror symmetry work?
When an object is reflected along a mirror plane and its image in that plane is identical.
Pure rotation operations generate what?
4 2D plane point groups (2-fold, 3-fold, 4-fold 6-fold).