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

Abstract

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 bis-hydrogen 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.