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Crystal Geometry
1. Crystals
2. Lattice
3. Lattice points, lattice translations
4. Cell--Primitive & non primitive
5. Lattice parameters
6. Crystal=lattice+motif
Matter
Solid Liquid Gas
Crystalline Amorphous
Crystal?
A 3D translationally periodic arrangement of atoms in space is called a crystal.
Air, Water and Earth
A two-dimensional periodic pattern by a Dutch artist M.C. Escher
Lattice?
A 3D translationally periodic arrangement of points in space is called a lattice.
A 3D translationally periodic arrangement of atoms
Crystal
A 3D translationally periodic arrangement of points
Lattice
What is the relation between the two?
Crystal = Lattice + Motif
Motif or basis: an atom or a group of atoms associated with each lattice point
Crystal=lattice+basis
Lattice: the underlying periodicity of the crystal,
Basis: atom or group of atoms associated with each lattice points
Lattice: how to repeat
Motif: what to repeat
+
Love Pattern Love Lattice + Heart=
Formula for Love Potion?
Mix one molecule of potassium iodide with two
molecules of sulfur
KI + 2S = KISS
Space Lattice
A discrete array of points in 3-d space such that every point has identical surroundings
Lattice
Finite or infinite?
Primitivecell
Primitivecell
Nonprimitive cell
Cells A cell is a finite representation of the infinite lattice
A cell is a parallelogram (2D) or a parallelopiped (3D) with lattice points at their corners.
If the lattice points are only at the corners, the cell is primitive.
If there are lattice points in the cell other than the corners, the cell is nonprimitive.
Lattice Parameters
Lengths of the three sides of the parallelopiped : a, b and c.
The three angles between the sides: , ,
Conventiona parallel to x-axis
b parallel to y-axis
c parallel to z-axis
Angle between y and z
Angle between z and x
Angle between x and y
The six lattice parameters a, b, c, , ,
The cell of the lattice
lattice
crystal
+ Motif
In order to define translations in 3-d space, we need 3 non-coplanar vectors
Conventionally, the fundamental translation vector is taken from one lattice point to the next in the chosen direction
With the help of these three vectors, it is possible to construct a parallelopiped called a CELL
The smallest cell with lattice points at its eight corners has effectively only one lattice point in the volume of the cell.
Such a cell is called PRIMITIVE CELL
Bravais Space Lattices
Conventionally, the finite representation of space lattices is done using unit cells which show maximum possible symmetries with the smallest size.
Symmetries: 1.Translation
2. Rotation
3. Reflection
Considering
1. Maximum Symmetry, and
2. Minimum Size
Bravais concluded that there are only 14 possible Space Lattices (or Unit Cells to represent them). These belong to 7 Crystal Classes
Arrangement of lattice points in the unit cell
1. 8 Corners (P)
2. 8 Corners and 1 body centre (I)
3. 8 Corners and 6 face centres (F)
4. 8 corners and 2 centres of opposite faces (A/B/C)
Effective number of l.p.
5. Hexagonal unit cell has 12 corners of the hexagonal prism 2 centres of hexagonal faces
1. Cubic Crystals
• Simple Cubic (P)• Body Centred
Cubic (I) – BCC• Face Centred
Cubic (F) - FCC
2. Tetragonal Crystals
• Simple Tetragonal• Body Centred
Tetragonal
3. Orthorhombic Crystals
• Simple Orthorhombic
• Body Centred Orthorhombic
• Face Centred Orthorhombic
• End Centred Orthorhombic
4. Hexagonal Crystals
• Simple Hexagonal or most commonly HEXAGONAL
5. Rhombohedral Crystals
• Rhombohedral (simple)
6. Monoclinic Crystals
• Simple Monoclinic• End Centred
Monoclinic (A/B)
7. Triclinic Crystals
• Triclinic (simple)
Crystal Structure
Space Lattice + Basis (or Motif)Basis consists of a group of atoms located
at every lattice point in an identical fashionTo define it, we need to specify• Number of atoms and their kind• Internuclear spacings• Orientation in space
Atoms are assumed to be hard spheres