\n
3.<\/strong> RESULTS AND DISCUSSION<\/strong><\/p>\nSECTION 3<\/strong>.1<\/strong><\/p>\nSynthesis, growth, structure and characterization of potassium lithium hydrogen phthalate mixed crystals<\/strong>*<\/p>\nIn the present work, we report the growth and structure of a new mixed crystal C16<\/sub>H16<\/sub>KLiO11<\/sub> (PLHP), which crystallizes in a non-centrosymmetric space group P<\/em>21<\/sub> and SHG-active. The grown crystals were subjected to various characterization studies which are briefly described below. Here it is established that by synthesising the mixed crystal in a different route with a controlled concentration of additive, one can sustain nonlinearity at the macro level by allowing the specimen to crystallise in a polar space group. The main objective of the investigation is to design a noncentrosymmetric structure by attempting a different route of synthesis, leading to NLO activity. Steering to noncentrosymmetry from centrosymmetry is made possible by changing the growth conditions.<\/p>\n3.1.1.<\/strong> Crystal growth<\/strong><\/p>\nThe mixed crystal PLHP was synthesized from an aqueous solution containing equimolar quantities of AR grade potassium hydrogen phthalate (KHP) and lithium carbonate (Li2<\/sub>CO3<\/sub>) in slightly<\/p>\n<\/p>\n
acidic conditions using de-ionized water. After successive recrystallization, the mixed crystals were grown by the slow evaporation solution growth technique. The crystallization took place within 20-25 d and the crystals were harvested. Photographs of as- grown crystals are shown in Fig. 3.1.1.<\/strong><\/p>\n<\/p>\n
Fig. 3.1.1. Photographs of mixed crystal PLHP<\/strong><\/p>\n 3.1.2. FT-IR<\/strong><\/p>\nThe FT-IR spectrum of the as-grown specimen is shown in Fig.3.1.2.<\/strong> An absorption band in the region 500-900 cm-1<\/sup> corresponds to the C-H out of plane deformations of aromatic ring. The C=O stretching frequency appeared at 1670 cm-1<\/sup>. The characteristic vibrational patterns of KHP104<\/sup><\/strong>, lithium hydrogen phthalate (LiHP) 22<\/sup><\/strong> and PLHP are given in Table 3.1.2<\/strong>. A slight shift of some of the characteristic vibrational frequencies could be due to the stress development because of Li incorporation.<\/p>\n Fig. 3.1.2.<\/strong> FT\u2013IR spectrum of mixed crystal PLHP<\/strong><\/p>\nTable 3.1.2. FT-IR frequencies of some acid phthalate crystals (cm<\/strong>-1<\/sup><\/strong>)<\/strong><\/p>\n\n\n\n\n Frequencies<\/strong><\/p>\n<\/td>\n\n KHP<\/strong>a<\/sup><\/strong><\/p>\n<\/td>\n\n LiHP<\/strong>b<\/sup><\/strong><\/p>\n<\/td>\n\n PLHP<\/strong>c<\/sup><\/strong><\/p>\n<\/td>\n<\/tr>\n\n\n \u03bd<\/em>as<\/sub> (O-H-O)<\/p>\n<\/td>\n\n 1090<\/p>\n<\/td>\n | \n 1072<\/p>\n<\/td>\n | \n 1089<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03bd<\/em>s<\/sub> (O-H-O)<\/p>\n<\/td>\n\n 1144<\/p>\n<\/td>\n | \n 1172<\/p>\n<\/td>\n | \n 1147<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03bd<\/em>as<\/sub> (O-C=O)<\/p>\n<\/td>\n\n 1445<\/p>\n<\/td>\n | \n 1401<\/p>\n<\/td>\n | \n 1479<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03bd<\/em>s<\/sub> (O-C-O)<\/p>\n<\/td>\n\n 1565<\/p>\n<\/td>\n | \n 1552<\/p>\n<\/td>\n | \n 1531<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03bd<\/em>s<\/sub> C=O<\/p>\n<\/td>\n\n 1675<\/p>\n<\/td>\n | \n 1685<\/p>\n<\/td>\n | \n 1670<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03bd<\/em>s<\/sub> O=H<\/p>\n<\/td>\n\n 3470<\/p>\n<\/td>\n | \n 3391<\/p>\n<\/td>\n | \n 3537<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n a<\/sup>Ref 105 b<\/sup>Ref 22 c<\/sup> Present study<\/p>\n3.1.3. TGA\/DTA<\/strong><\/p>\nThermal studies reveal the purity of the material. The TGA curve shows a single stage weight loss at \u00ef\u0081\u00be150o<\/sup> C due to loss of water molecule. In DTA, the broad endothermic peak at 420\u00b0<\/sup>C, is due<\/p>\nto decomposition. The residual mass observed from thermogram at 600\u00b0<\/sup>C is ~50%. (Fig.<\/strong> 3.1.3<\/strong>).<\/p>\n<\/p>\n 3.1.4.<\/strong> SEM \/ EDS<\/strong><\/p>\nThe SEM micrographs give information about the surface morphology and it is used to check the imperfections105<\/sup><\/strong>. The SEM pictures of PLHP at different magnifications are shown in Fig. 3.1.4.1<\/strong>. It shows highest surface roughness in a plate like structure, due to defect centers and crystal voids. The presence of Li and K in the PLHP crystal lattice is confirmed by energy dispersive spectroscopy (EDS) (Fig. 3.1.4.2<\/strong>).<\/p>\n<\/p>\n <\/p>\n Fig. 3.1.4.2. EDS spectrum of PLHP<\/strong><\/p>\n3.1.5.<\/strong> AAS and CHN analysis<\/strong><\/p>\nAtomic absorption spectroscopic studies were carried out to quantify Li (20.6 ppm) and K (21. ppm ) in the sample. Also, CHN elemental analysis was performed to estimate the quantity of carbon and hydrogen present in PLHP. The elemental composition found was: C 42.93%, H 3.29%. The calculated composition was: C 44.63%, H 3.7%.<\/p>\n 3.1.6.<\/strong> UV-visible<\/strong><\/p>\nThe UV-visible spectrum of the mixed crystal PLHP reveals high transmittance in the visible region and the lower cut-off wave length is observed at ~300 nm. Incorporation of foreign metal ion into the KHP crystal lattice does not destroy the optical transmission of potassium hydrogen phthalate. The concentration of an absorbing species can be determined using the Kubelka-Munk equation106<\/sup><\/strong> correlating reflectance and concentration,<\/p>\nF(R)<\/em> = (1-R<\/em>)2<\/sup> \/ 2R =<\/em> \u03b1 \/ s=Ac \/ s<\/em><\/p>\nwhere F(R<\/em>) is Kubelka-Munk function, R<\/em> is the reflectance of the crystal, \u03b1 is absorption coefficient, s<\/em> is scattering coefficient, A<\/em> is absorbance and c<\/em> is concentration of the absorbing species. The direct band-gap energy of the specimen is estimated as 4.05 eV, from the Tauc plot [F(R<\/em>)h\u03bd<\/em>]2<\/sup> versus<\/em> h\u03bd<\/em> (eV) (Fig. 3.1.6).<\/strong><\/p>\n<\/p>\n <\/p>\n <\/p>\n Fig. 3.1.6. Tauc plot (Direct Band gap energy)<\/strong><\/p>\n3.1.7<\/strong> X-ray diffraction analysis<\/strong><\/p>\nThe powder XRD pattern of PLHP shows that the sample is of a single phase without a detectable impurity. Narrow peaks indicate the good crystallinity of the material. At room temperature all the observed reflections were indexed. The indexed powder XRD pattern is shown in Fig. 3<\/strong>.1.7.1.<\/strong> Peak positions in powder XRD match with simulated XRD patterns from single crystal X-ray diffraction. The relative intensity variations could be due to the preferred orientation of the sample used for diffractogram measurement. Also, the mosaic spread of powder and single crystal patterns may differ, resulting in intensity variations. The structure of PLHP is elucidated and the ORTEP<\/em> is given as Fig. 3<\/strong>.1.7.2<\/strong>.<\/strong> Three-dimensional view of intramolecular hydrogen bonding interactions is displayed in Fig. 3.1.7.3<\/strong>. The chemical formula C16<\/sub>H16<\/sub>KLiO11<\/sub> confirms the presence of K and Li in the crystalline matrix, well supported by energy dispersive X-ray spectroscopy (EDS) and atomic absorption spectroscopy (AAS). The specimen crystallizes in the monoclinic crystal system with the noncentrosymmetric space group P2<\/em>1<\/sub><\/em>. The crystallographic parameters of PLHP, KHP, LiKP and LiHP are listed in Table 3<\/strong>.1.7.1<\/strong>.<\/p>\n<\/p>\n <\/p>\n Fig.<\/strong>3.1.7.1.<\/strong> Experimental (red) and simulated (blue) powder XRD patterns <\/strong><\/p>\n<\/p>\n Fig.<\/strong>3.1.7.2.<\/strong> ORTEP<\/em><\/strong> of PLHP<\/strong><\/p>\n<\/p>\n Fig.<\/strong>3.1.7.3.<\/strong> Three \u2013 dimensional view of intramolecular hydrogen bonding interactions (OH\u00e2\u02c6\u2122\u00e2\u02c6\u2122\u00e2\u02c6\u2122O)<\/strong><\/p>\nTable<\/strong> 3.1.7.1<\/strong>. Crystal data of<\/strong> LiHP, KHP, LiKP and PLHP c<\/strong>rystals<\/strong><\/p>\n\n\n\n\u00a0<\/td>\n | \n LiHP<\/strong><\/p>\n<\/td>\n\n KHP<\/strong><\/p>\n<\/td>\n\n LiKP<\/strong><\/p>\n<\/td>\n\n PLHP<\/strong><\/p>\n<\/td>\n<\/tr>\n\n\n Chemical formula<\/p>\n<\/td>\n | \n LiH(C8<\/sub>H4<\/sub>O4<\/sub>2H2<\/sub>O)<\/p>\n<\/td>\n\n KHC8<\/sub>H4<\/sub>O4<\/sub><\/p>\n<\/td>\n\n C16<\/sub>H12<\/sub>KLi3<\/sub>O11<\/sub><\/p>\n<\/td>\n\n C16<\/sub>H16<\/sub> KLiO11<\/sub><\/p>\n<\/td>\n<\/tr>\n\n\n Unit Cell Parameters<\/p>\n<\/td>\n | \n a=16.837 \u00c5<\/p>\n<\/td>\n | \n a=9.61 \u00c5<\/p>\n<\/td>\n | \n a=7.405 \u00c5<\/p>\n<\/td>\n | \n a=9.48 \u00c5<\/p>\n<\/td>\n<\/tr>\n | \n\n b= 6.822 \u00c5<\/p>\n<\/td>\n | \n b=13.32 \u00c5<\/p>\n<\/td>\n | \n b=9.878 \u00c5<\/p>\n<\/td>\n | \n b=6.76 \u00c5<\/p>\n<\/td>\n<\/tr>\n | \n\n c=8.198 \u00c5<\/p>\n<\/td>\n | \n c=6.48 \u00c5<\/p>\n<\/td>\n | \n c=13.396 \u00c5<\/p>\n<\/td>\n | \n c=15.39 \u00c5<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03b1=90o<\/sup><\/p>\n<\/td>\n\n \u03b1=90o<\/sup><\/p>\n<\/td>\n\n \u03b1=71.778 o<\/sup><\/p>\n<\/td>\n\n \u03b1=90o<\/sup><\/p>\n<\/td>\n<\/tr>\n\n\n \u03b2=98.85o<\/sup><\/p>\n<\/td>\n\n \u03b2=98.85o<\/sup><\/p>\n<\/td>\n\n \u03b2=87.300 o<\/sup><\/p>\n<\/td>\n\n \u03b2=105.73\u00b0<\/p>\n<\/td>\n<\/tr>\n | \n\n \u03b3 =90o<\/sup><\/p>\n<\/td>\n\n \u03b3 =90 | | | | | | | | | | | | | | | | | | | | | | | | | | | |