An Interesting Correlation between Crystalline Perfection and Second Harmonic Generation Efficiency on KCl- and Oxalic Acid-Doped ADP Crystals

The effect of dopants (over a concentration range from 1 to 10 mol %), namely KCl and oxalic acid (C 2 H 2 O 4 ·2H 2 O, Oxa), on the growth process, crystalline perfection, and nonlinear optical (NLO) properties of ammonium dihydrogen phosphate (NH 4 H 2 PO 4, ADP) single crystals grown by a slow evaporation solution growth technique has been investigated. The high-resolution X-ray diffraction (HRXRD) studies used to evaluate crystalline perfection reveal some interesting features on the ability of accommodating the dopants by the crystalline matrix. The remarkable and systematic increase in the broadness and asymmetry of the diffraction curves as the concentration increases clearly indicates that the dopants predominantly occupied the interstitial positions in the crystalline matrix. Due to the larger size of the Oxa molecule, the crystalline matrix could not accommodate these dopants, which led to the formation of a very low angle (tilt angle <1′) internal structural grain boundaries at high concentrations. The relative second harmonic generation (SHG) efficiency measurements revealed that both KCl and Oxa dopants enhance the SHG efficiency. However, at higher concentrations of Oxa, SHG efficiency of ADP is not increased but rather decreased from its undoped condition. This may be due to deterioration in the crystalline perfection as observed by HRXRD. Powder XRD and FT-IR spectral analyses confirm the slight distortion of the structure of the crystal in the presence of a high concentration of dopants (10 mol %). UV–vis study shows that the transparency is not affected much by the dopants. The surface morphology of the as-grown specimens, which is changed with the nature and concentration of dopants, was studied by scanning electron microscopy. Presence of dopants was confirmed by energy-dispersive spectrometry.