The inclusion of a tetrahedral XY4 molecule (or ion) in a
crystal is, very often, followed by a lowering of its symmetry.
In order to describe the apparent distortions of the tetrahedron,
second-rank tensors were constructed. It was shown that the
characteristic surface of such a tensor is always an ellipsoid. The
relative lengths of the axes of the ellipsoid and their position
with respect to the symmetry elements of the XY4 group can be
used to determine the effective symmetry of the molecule, as well
as the degree of its distortion. Some of the spectral properties
of the studied compounds can also be predicted. 36 S04 ions with
accurately refined structures were investigated and the results
obtained by this method were compared with the results\u27 obtained
by other-= methods. A correlation of rather high significance (r2 =
= 0.97) was found between the main components of the tensor
and the frequencies of the components of the antisymmetric
stretching vibration (V3) of the molecule
