Uniaxial Stress and Orientational Degeneracy

The splitting under stress of non-degenerate electronic and vibrational transitions of impurity centres depend almost exclusively on the symmetry of these centres through their orientational degeneracy, already defined as the 

7 It must also be noted that a stress distribution can also be generated at low temperature in crystal samples tightly fixed to a metal sample holder in vacuo in order to ensure a good thermal contact. 

Tetragonal Trigonal Rhombic I Rhombic II Monoclinic II Monoclinic I 

In cubic crystals, one can distinguish six kinds of centres whose symmetries are those of the polyhedra shown in Fig. 8.27. In this figure are also indicated the components Ay of the piezospectroscopic tensor defined in (8.15). 

In Table 8.9, these polyhedra are defined by their edges ai, a2, and 23 (along the z [001] axis) and the angles a, [3, and 7 between these edges (angle 7 taken as the one between ai and 2), and the triclinic centres have been added. 

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