Comments on “Growth, XRD, optical, thermal, mechanical, electrical and NLO studies on mixture of potassium hydrogen phthalate and l‑tartaric acid crystal for NLO applications”

The authors of the title paper (Appl Phys A 127 496 (2021)) claim to have synthesized a “mixture of potassium hydrogen phthalate and l‑tartaric acid” crystal by a slow evaporation of an aqueous solution containing equimolar amounts of potassium hydrogen phthalate (KHP) and l‑tartaric acid. In this comment, many points of criticism, concerning the synthesis of the title crystal and its characterization, are highlighted to show that a so-called “mixture of potassium hydrogen phthalate and l‑tartaric acid” crystal is, in fact, potassium hydrogen phthalate, (C₈H₅O₄K). Due to no chemical reaction between KHP and l‑tartaric acid in aqueous solution, fractional crystallization of the less soluble KHP occurs.