Cristalografia Modulo 1- 2012-2ppt

7/27/2019 Cristalografia Modulo 1- 2012-2ppt

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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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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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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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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


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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Documents

Cristalografia Modulo 1- 2012-2ppt