Post on 14-Apr-2018
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CRISTALOGRAFIA
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Que es Cristalografa
La ciencia que estudia el ordenamiento delos tomos en las molculas.
La ciencia que estudia la distribucin yorden de las molculas entre s
Implicaciones de estos tipos deordenamientos en otras areas de laciencias, desde las biociencias a loscampos de la nanotecnologa
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Example; H2O system
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CdS
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Orden Interno: PERIODICIDAD
TRASLACION VECTOR DE TRASLACION direccin y mdulo
TRASLACIONES FUNDAMENTALES
Traslaciones en 3 direcciones de espacioa,b,c
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Redes planasEn este caso la red viene definida por dos traslaciones (a y b) y el ngulo que
forman entre ellas (a). La celda unidad es un paralelogramo. En el plano soloexisten 5 posibles tipos de redes:
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Lattice Requirements
Environment about all lattice pointsmust be identical
Unit cell must fill all space, with noholes
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41!e can 'ic) *+t t-e smallest 1e'eating +nit344
LATTICE EXAMPLE
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!e can 'ic) *+t t-e smallest 1e'eating +nit344
!e call t-is t-e 567T 9E;;33344
UNIT CELL
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!e call t-is t-e 567T 9E;;33344
T-e +nit cell d1a=n -e1e is a sim'le c+bic cell
UNIT CELL
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The definition of the unit cellfrequentlyincorrect!
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Crystal System # Parameters*
Triclinic 6 abc; Monoclinic 4 abc; = = 90 90 Orthorhombic 3 abc; = = = 90 Tetragonal 2 a = bc; = = = 90 Trigonal
hexagonal 2 a = bc; = = 90o= 120 rhombohedral 2 a = b = c; = = 90
Hexagonal 2 a = bc; = = 90 = 120 Cubic 1 a = b = c; = = = 90
Unit Cells and Crystal Systems
A summary of the different crystal systems shows how the
number of variables in the unit cell parameters decreases
with increasing lattice symmetry.
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Unit Cells and Crystal Systems
There are seven different classes of unit cells that, each
defined by different limiting conditions on the unit cell
parameters (a, b, c, , , ). The most general system iscalled Triclinic in which none of the distances and angles
are restricted to have any particular value
Triclinic
abc,
Monoclinic
abc, = = 90
(by convention b is the unique axis)
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Unit Cells and Crystal Systems
Orthorhombic
abc, = = = 90
Tetragonal
a = bc, = = = 90
(by definition c is the unique axis)
As more of the angles and distances are restricted, the box
becomes more symmetric.
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Unit Cells and Crystal Systems
Hexagonal or Trigonal
a = bc, = = 90 = 120
(by definition c is the unique axis)
Rhombohedral
a = b = c, = = 90
Note that for the hexagonal or trigonal systems, three unit
cells are necessary to see the symmetry of the system. The
choice of trigonal or hexagonal is dependent on the contentsof the cell (more on this later).
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Cubic
a = b = c, = = = 90
Unit Cells and Crystal Systems
The most symmetric boxes are cubic and have only one
variable parameter.
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7 unit cell shapes
Cbico a=b=c ===90 Tetragonal a=bc ===90 Ortorrombico abc ===90 Monoclnico abc ==90, 90 Triclnico abc 90 Hexagonal a=bc==90, =120 Rombohedral a=b=c ==90Think about the shapes that these define - look at the
models provided.
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Celdas Unitarias
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