L16 - SUMMARY FROM LAST CLASS Crystalline compounds Closed...

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Dr. P. Lucas U of A MSE 110 SUMMARY FROM LAST CLASS Crystalline compounds Closed packed structures FCC and HCP have one octahedral and two tetrahedral site for each atoms. Many structure types are derived by filling partially or totally the octahedral sites and tetrahedral sites of an FCC or HCP lattice. These structure types can be adopted by many different compounds. FCC : NaCl (all octa), ZnS blende (half tetra), CaF 2 (all tetra), Li 3 Bi (all octa, all tetra) HCP : NiAs (all octa), CdI 2 (half octa), ZnS Wurtzite (half tetra) NaCl and ZnS Blende can also be described as two interpenetrating FCC lattices while ZnS Wurtzite is described as two interpenetrating HCP lattices. Nearest neighbors generate a geometrical environment around an atom which is defined as the coordination polyhedra . Crystalline structures are commonly described in terms of coordination polyhedra: ZnS Blende and ZnS Wurtzite are based on layers of corner sharing tetrahedra while NiAs can be described in terms of edge and face sharing octahedra. The CsCl structure type is described as two intertwined cubic lattices.
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Dr. P. Lucas U of A MSE 110 Complex structures: This is achieved by identifying the smallest repeatable unit “the unit cell” and specifying the coordinates of each atom in this cell. CRYSTALLINE STRUCTURE It is then necessary to use a formal method to describe each structure. The unit cell defined as the smallest repeatable units of the structures can include dozens of atoms and can be quite complex Complex structures can often be related to basic structure types with distorted, missing or additional structural element (coordination polyhedra). There are thousands of known structures and new ones are discovered regularly. It can be shown that any crystal can be described using one of only 14 types of unit cell geometry with different shape and repeatable atomic positions.
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Dr. P. Lucas U of A MSE 110 Space Lattices: The lattice points correspond to the origin of that coordinate system and do not necessarily correspond to the position of an atom. CRYSTALLINE LATTICES
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This note was uploaded on 11/19/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at University of Arizona- Tucson.

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L16 - SUMMARY FROM LAST CLASS Crystalline compounds Closed...

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