Comments on the paper: "Studies on the third order nonlinear optical properties of a novel o-phenylenediaminium p-toluenesulfonate single crystal"

The authors of the title paper (Materials Letters 247 (2019) 25-28) report to have grown dark brown single crystals of o-phenylenediaminium p-toluenesulfonate (PTOP) by slow evaporation of an aqueous solution containing o-phenylenediamine and p-toluenesulfonic acid. In this letter, many points of criticism, concerning the crystal growth and color of this so-called PTOP crystal are discussed to show that the dark brown color is due to extensive surficial oxidation. The brown color of o-phenylenediaminium bishydrogen phosphonate (P2HP) crystal reported by the same group (Materials Letters, 209 (2017) 167-170) is explained due to surface decay. Because of surface degradation, the grown crystals are impure and the reported optical properties are questionable. (C) 2020 Elsevier B.V. All rights reserved.