Second harmonic generation and symmetry of SrS2O6 center dot 4 H2O

We show that although the crystalline structure of strontium dithionate tet rahydrate can be well described in either P6(4) and P3(1) space groups (or respectively, P6(2) and P3(2)) and electro-optic measurements have indicate d the 32 point group, optical second harmonic measurements performed on an oriented crystal are only compatible with point group 3.