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2.1 Basics of Structures

Fractional coordinates

• Rules for marking the position of an atom in a unit cell:

• fractional coordinates are related to directions

• possible values for x, y, z: [0; 1]

• atoms are generated by symmetry elements

• negative values: add 1.0, values > 1.0: substract 1.0 (or multiples)

• Example: Sphalerite (Zincblende) factional

coordinates

• Equivalent points are represented by one triplet only

• equivalent by translation

• equivalent by other symmetry elements, later