Abstract:
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 and 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.
