On the second harmonic generation activity in centrosymmetric crystals

The observation of second harmonic generation (SHG) signals, which is forbidden in centrosymmetric crystals, should necessitate clarification of their causes. The authors of a recent paper (Sivaraman et al Molecular crystals & Liquid crystals 656 (2017) 153-168) report to have grown a so-called crystal of diglycine zinc dipicrate which, in fact, is tetraaquabis(glycine)zinc(II) dipicrate dihydrate. This compound despite being centrosymmetric, was reported to exhibit a strong second harmonic response viz. 1.07 times of potassium dihydrogen phosphate. Based on a reinvestigation, we prove that the observation of SHG is due to dehydration of the crystal under laser irradiation.