Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett Research paper Studies on structural, optical, homo-lumo and mechanical properties of piperazinium p-hydroxybenzoate monohydrate single crystal for nonlinear optical applications G. Parvathy a , R. Kaliammal a , K. Velsankar a , M. Krishna Kumar b , K. Sankaranarayanan a , S. Sudhahar a, ⁎ a Department of Physics, Alagappa University, Karaikudi 630003, India b Department of Physics, Kalasalingam Academy of Research and Education, Krishnankoil 626 126, India HIGHLIGHTS • Organic single crystal of piperazinium p-hydroxybenzoate (PPH) was grown by slow evaporation technique. • PPH Crystal belongs to monoclinic crystal system with space group P2 1 /c. • The difference between HOMO-LUMO energy gap value is −0.01042 a.u. • SHG efficiency of PPH is 1.5 times that of KDP crystal. • The grown PPH belongs to the soft kind of materials. ARTICLE INFO Keywords: Organic crystals Structural analysis Density Functional Theory (DFT) Optical properties Mechanical properties ABSTRACT An organic single crystal of piperazinium p-hydroxybenzoate monohydrate (PPH) was grown by solution growth technique. It belongs to monoclinic system with space group P2 1 /c. Density functional theory (DFT) computa- tions were used by B3LYP/6–311++G (d,P) basis set to optimize molecular geometry and HOMO-LUMO en- ergies. The infrared spectrum confirms the functional groups present in the grown crystal. The UV–Visible, PL and Thermal studies were also analyzed. Hardness property was measured by Vicker's microhardness tester. The chemical etching studies confirms that PPH crystal possesses a layer growth pattern. Second harmonic gen- eration efficiency of PPH crystal has 1.5 times larger than KDP crystal. 1. Introduction An organic nonlinear optical crystalline materials (NLO) have at- tracted the acute research interest due to its lively applications in the field of fast response, high density data processing, storage, tele- communication, colour displays, optical interconnect oscillator, am- plifier, laser scanner [1–3]. Second order NLO crystals are largely used to convert the frequency of coherent laser sources, optical computing. The efficiency of the conversion of NLO crystal is to be amended and it is searched by scholars around-the-clock [4,5] Owing to an inter-mo- lecular h-bonding amongst electron donor and acceptor groups (π conjugated systems) in the organic NLO crystal parades high molecular hyperpolarizability and it affords great nonlinear optical competence, high packing densities, great photochemical strengths [6,7]. The p- hydroxybenzoic acid yields mono-phenolic structure of derivatives also it holds two (COOH, OH) different proton donor assemblies [8,9]. Hetero-cyclic diamine piperazine (1,4-hexahydropyrazine) have six membered ring and it has two –NH groups in equilateral positions. It leads to a crystallographic inversion center and complex vibrational interactions [10]. It is attempted to grow single crystals of piperazinium p-hydroxybenzoate (PPH) and its physical, chemical results about the crystal system, molecular geometry, HOMO-LUMO, crystal planes, vi- brational behavior of the complex, optical response, hardness, etching response, and nonlinear optical studies are presented in this work. 2. Experimental 2.1. Materials preparation and crystal growth An equimolar amount of piperazine and p-hydroxybenzoic acid was https://doi.org/10.1016/j.cplett.2020.137934 Received 1 May 2020; Received in revised form 24 August 2020; Accepted 25 August 2020 ⁎ Corresponding author. E-mail address: sudhahars@alagappauniversity.ac.in (S. Sudhahar). Chemical Physics Letters 758 (2020) 137934 Available online 29 August 2020 0009-2614/ © 2020 Elsevier B.V. All rights reserved. T