Effect of L-histidine hydrochloride monohydrate (LHICL) doping in potassium dihydrogen phosphate (KDP) on single crystal growth, structural, optical, electrical, and mechanical traits

Potassium dihydrogen phosphate (KH₂PO₄; KDP) is a well known non-linear optical (NLO) material for vast number of applications in optical device fabrication. Most of the amino acid group materials exhibit NLO property. In a view to get the probable improvement in the NLO properties with good growth rate, structural, optical, electrical, and mechanical behavior, a suitable dopant L-Histidine hydrochloride monohydrate (LHICL) at different concentration levels added to the saturated solutions of KH₂PO₄ (KDP) was investigated. The bulk single crystals of pure and LHICL-doped KDP crystals were grown by slow evaporation solution growth technique. The growth rate was found to be increased along (100) plane with increasing the dopant concentration. The grown crystals characterized by powder X-ray diffraction showed interesting results in doped crystals with increased intensity of a few peaks due to relative increase in the number of crystallites which are aligned along the growth direction. Fourier transform infrared spectroscopy (FT-IR) confirms the dopant presence in the host compound. High-resolution X-ray Diffraction (HRXRD) curves for both undoped and doped KDP crystals showed quite good crystalline perfection. In case of doped crystals, a slight increased scattered intensity on the positive side of diffraction peak of the high-resolution diffraction curves was observed depicting the incorporation of the dopants in the interstitial positions of the lattice. At higher concentrations, the internal stress generated by the dopants led to the formation of a very low angle boundary. This is followed by an interesting finding that the quality of the main crystal block enhanced by the segregation of dopants at the grain boundaries. UV–Vis–NIR spectroscopic studies revealed that there is no appreciable change in the lower cut-off wavelength ~ 275 nm due to doping and the band gap energy was increased from 2.8 to 3.4 eV with increasing doping concentration. However, doping led to enhanced optical transmittance in KDP crystal. The photoluminescence spectrum revealed different defects observed due to doping in KDP specimen. The hardness of the grown crystals was measured by Vicker’s hardness measurements and observed an enhancement of hardness due to doping. The frequency dependent of dielectric constant (ε_r), dielectric loss (tan δ), a.c. resistivity, and a.c. conductivity were studied for pure KDP and doped KDP crystals. The Nyquist plots drawn for pure and doped KDP crystals revealed good crystalline perfection without any major grain boundaries, in line with the HRXRD results. The observed relative SHG measurements revealed some enhancement with doping in KDP crystals.