Covariant Forms of the Third‐ and Fourth‐Rank Tensors such as Piezoelectric Moduli and Piezo‐Optical Coefficients

Covariant forms of the third‐rank tensor L ikl invariant under the permutation k ⇄ I and the fourth‐rank tensor L p iklm invariant under the permutations i ⇄ k and I ⇄ m are suggested for all the crystal classes. A covariant procedure of reducing the given tensor to principal axes is presented when these axes are not completely determined by the symmetry elements of the crystal. A covariant method of constructing the fourth‐rank reciprocal tensor is given.