chapter 1 crystal structures. two categories of solid state materials crystalline: quartz,...
TRANSCRIPT
Two Categories of Solid State MaterialsTwo Categories of Solid State Materials
Crystalline: quartz, diamond…..
Amorphous: glass, polymer…..
Lattice Points, Lattice and Unit CellLattice Points, Lattice and Unit Cell
How to define lattice points, lattice and unit cell?
LATTICELATTICE LATTICE = An infinite array of points in space, in
which each point has identical surroundings to all others.
CRYSTAL STRUCTURE = The periodic arrangement of atoms in the crystal.
It can be described by associating with each lattice point a group of atoms called the MOTIF (BASIS)
Notes for lattice pointsNotes for lattice points
Don't mix up atoms with lattice points Lattice points are infinitesimal points in space Atoms are physical objects Lattice Points do not necessarily lie at the centre of
atoms
Unit cellUnit cell
• • A repeat unit (or motif) of the regular A repeat unit (or motif) of the regular arrangements of a crystal arrangements of a crystal
•• •• is defined as the smallest repeating unit whichis defined as the smallest repeating unit which shows the full symmetry of the crystal structure shows the full symmetry of the crystal structure
Crystal systemCrystal system is governed by unit cell shape and symmetry
Definition of symmetry elementsDefinition of symmetry elements
------------------------------------------------------------- Elements of symmetry
------------------------------------------------
Symbol Description Symmetry operations
---------------------------------------------------------------------E Identity No change
Plane of symmetry Reflection through the plane
i Center of symmetry Inversion through the center
Cn Axis of symmetry Rotation about the axis by (360/n)o
Sn Rotation-reflection Rotation about the axis by (360/n)o
axis of symmetry followed by reflection through the
plane perpendicular to the axis
---------------------------------------------------------------------
space group space group = point group + translation= point group + translation
Symmetry elements
Screw axes 21(//a), 21(//b), 41(//c)
42(//c), 31(//c) etc
Glide planes c-glide (┴ b), n-glide,
d-glide etc
Examples for 2D symmetryExamples for 2D symmetry
http://www.clarku.edu/~djoyce/wallpaper/seventeen.html
General positions of Group 14 General positions of Group 14 (P 2(P 2
11/c) [unique axis b]/c) [unique axis b]
1 x,y,z identity
2 -x,y+1/2,-z+1/2 Screw axis
3 -x,-y,-z i
4 x,-y+1/2,z+1/2 Glide plane
Multiplicity, Wyckoff Letter, Site SymmetryMultiplicity, Wyckoff Letter, Site Symmetry
4e 1 (x,y,z) (-x, ½ +y,½ -z) (-x,-y,-z) (x,½ -y, ½ +z)
2d ī (½, 0, ½) (½, ½, 0)
2c ī (0, 0, ½) (0, ½, 0)
2b ī (½, 0, 0) (½, ½, ½)
2a ī (0, 0, 0) (0, ½, ½)
General positions of Group 15 General positions of Group 15 (C 2/c) [unique axis b](C 2/c) [unique axis b]
1 x,y,z identity
2 -x,y,-z+1/2 2-fold rotation
3 -x,-y,-z inversion
4 x,-y,z+1/2 c-glide
5 x+1/2,y+1/2,z identity + c-center
6 -x+1/2,y+1/2,-z+1/2 2 + c-center
7 -x+1/2,-y+1/2,-z i + c-center
8 x+1/2,-y+1/2,z+1/2 c-glide + c-center
LatticeLattice : : the manner of repetition of atoms, ions or the manner of repetition of atoms, ions or
molecules in a crystal by an array of pointsmolecules in a crystal by an array of points
Types of latticeTypes of lattice
Primitive lattice (P) - the lattice point only at corner
Face centred lattice (F) - contains additional lattice points in the center of each face
Side centred lattice (C) - contains extra lattice points on only one pair of opposite faces
Body centred lattice (I) - contains lattice points at the corner of a cubic unit cell and body
center
14 Possible Bravais lattices 14 Possible Bravais lattices : : combination of four types of lattice and seven crystcombination of four types of lattice and seven cryst
al systemsal systems
Assignment of Miller indices to a set of Assignment of Miller indices to a set of planesplanes
1. Identify that plane which is adjacent to the one 1. Identify that plane which is adjacent to the one that passes through the origin.that passes through the origin.
2. Find the intersection of this plane on the three 2. Find the intersection of this plane on the three axes of the cell and write these intersections as axes of the cell and write these intersections as fractions of the cell edges. fractions of the cell edges.
3. Take reciprocals of these fractions.3. Take reciprocals of these fractions.
Example: fig. 10 (b) of previous pageExample: fig. 10 (b) of previous page
cut the x axis at a/2, the y axis at band the z axis at cut the x axis at a/2, the y axis at band the z axis at c/3;c/3; the reciprocals are therefore, 1/2, 1, 1/3; the reciprocals are therefore, 1/2, 1, 1/3; Miller index is ( 2 1 3 ) #Miller index is ( 2 1 3 ) #
Miller Index and other indicesMiller Index and other indices
(1 1 1), (2 1 0){1 0 0} : (1 0 0), (0 1 0), (0 0 1) ….
[2 1 0], [-3 2 3]<1 0 0> : [1 0 0], [0 1 0], [0 0 1]
Descriptions of crystal structuresDescriptions of crystal structures
The close packing approach
The space-filling polyhedron approach
Materials can be described as close Materials can be described as close packedpacked
Metal- ccp, hcp and bccAlloy- CuAu (ccp), Cu(ccp), Au(ccp)Ionic structures - NaClCovalent network structures (diamond)Molecular structures
P = sphere, O = octahedral hole, T+ / T- = tetrahedral holesP = sphere, O = octahedral hole, T+ / T- = tetrahedral holes
ABCABCABCABC.... repeat gives .... repeat gives Cubic Close-PackingCubic Close-Packing ( (CCPCCP))
Unit cell showing the full symmetry of the arrangement is Face-Centered Cubic
Cubic: a = b =c, = = = 90° 4 atoms in the unit cell: (0, 0, 0) (0, 1/2,
1/2) (1/2, 0,
1/2) (1/2,
1/2, 0)
ABABABABABAB.... repeat gives .... repeat gives Hexagonal Close-PackingHexagonal Close-Packing
((HCPHCP))
Unit cell showing the full symmetry of the arrangement is Hexagonal
Hexagonal: a = b, c = 1.63a, = = 90°, = 120° 2 atoms in the unit cell: (0, 0, 0) (2/3,
1/3, 1/2)
Face centred cubic unit cell of a Face centred cubic unit cell of a ccpccp arrangement of arrangement of spheresspheres
Hexagonal unit cell of a Hexagonal unit cell of a hcphcp arrangement of arrangement of spheresspheres
The space-filling approachThe space-filling approachCorners and edges sharingCorners and edges sharing