Positioning and orienting the sample

1 Usually, in this case, only two reference directions are available, which are those that define the surface, whereas, of course, if the substrate is a single crystal, the frame of reference is comprised of three directions. Note, however, that if the substrate is amorphous or polycrystalline, the film is rarely epitaxial and therefore knowing the orientation with respect to two directions is usually enough. 

We will consider five different orthonormal sets of coordinates  

Rotation axis to Rotation axis ( ) normal to the axis i 

Rotation axis Xi Rotation axis X2 normal to the axis X2 

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