Ammonium dihydrogen phosphate (ADP) crystals are widely used as the second, third and fourth harmonic generator for Nd:YAG and Nd:YLF lasers. It belongs to the tetragonal system with the space group I-42d 1. These crystals are widely used for electro-optical applications such as Q-switching for Ti–sapphire and alexandrite lasers as well as for acousto optical applications2–4. Tris(thiourea)zinc(II) sulphate (ZTS) is a semi-organic nonlinear optical (NLO) material which finds applications in the area of laser technology, optical communication, data storage technology and optical computing because it has high resistance to laser induced damage, high nonlinearity, wide transparency, low angular sensitivity and good mechanical hardness compared to many organic NLO crystals5-8. It belongs to the orthorhombic system with noncentrosymmetric space group Pca21 and point group mm2.
Growth, spectral, optical and thermal studies of rare earth neodymium(III) doped ZTS9, cerium(III) doped ZTS10,11 have been reported. We have also investigated the influence of Ce(III)- doping12 effects of ADP crystals. In the present investigation, the effect of La(III)-doping on ADP and ZTS crystals has been studied using FT-IR, XRD, SEM, EDS, UV–vis, thermal and Kurtz powder SHG measurements.
ADP (E. Merck) was purified by repeated recrystallization. ZTS was synthesized as reported earlier13. To avoid decomposition, low temperature (ZnSO4·7H2O + 3(CS(NH2)2) ï‚® Zn(CS(NH2)2)3SO4
After successive recrystallization processes, crystals were grown by slow evaporation solution growth technique (pH = 6.2).
Doping of lanthanum (5 mol %) in the form of lanthanum(III)- chloride (Aldrich) was used as such in the aqueous growth medium. The crystallization took place within 10–15 days and the high quality transparent crystals were harvested from the aqueous growth medium. Best quality and highly transparent seed crystals are used in the preparation of bulk crystals. Photographs of the as-grown crystals are shown in Fig. 2.2.1.
The FT-IR spectra of pure and doped crystals reveal small shifts in some of the characteristic vibrational frequencies (Table.2.3.1.1.) and it could be due to lattice strain as a result of La(III)- doping.
The vibrational patterns of lanthanum doped ADP exhibit slight variations as compared with pure ADP. PO4 stretching and bending vibrations are observed at ~1100, ~910 cm-1 and ~453, ~546 cm-1 respectively. Symmetric stretching vibrations of NH4+ ion are observed in the range of ~1407 cm-1. Vibrational peak at ~1280 cm-1 corresponds to OH bending. Broad peaks observed at ~3234 cm-1, ~3120 cm-1 correspond to N–H···O stretching vibrations.

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A close observation of FT-IR spectra of pure ZTS and doped specimens also reveals that the doping results in slight shifts in some of the characteristic vibrational frequencies. It could be due to lattice strain developed as a result of doping. An absorption band in the region 2750–3400 cm-1 corresponds to the symmetric and asymmetric stretching frequencies of NH2 group of zinc(II) coordinated thiourea. The absorption band observed at ~1620 cm-1 in the spectra of pure and doped specimens corresponds to that of thiourea (~1625 cm-1)14 of about the same frequency and it can be assigned to NH2 bending vibration. The CN stretching frequencies of thiourea (1122 and 1502 cm-1) shifted to higher frequencies for pure and La(III)- doped ZTS crystals (~1128 and ~1500 cm-1). The C-S is stretching frequencies (1398 and 712 cm-1)15 are shifted to lower frequencies (~1394 and ~706 cm-1) for pure and doped samples. These observations suggest that metal coordinate with thiourea through
The powder XRD patterns of La(III)-doped samples are compared with that of undoped one (Fig. 2.3.2.1). No new peaks or phases were observed by doping with inner transition metal lanthanum. However, a drastic reduction in intensity is observed as a result of doping. The most prominent peaks with maximum intensity of the XRD patterns of pure and doped specimens are quite different. The observations could be attributed to strains in the lattice. The cell parameters are determined from the single crystal X-ray diffraction analysis and the values of pure and doped crystals are given in the Table 2.3.2.1. The ionic radius of the dopant La(III) (117 pm) is very small compared with that of NH4+ (151 pm)16. Hence, it is reasonable to believe that the dopant can enter into the ADP crystalline matrix occupying predominantly substitutional positions without causing much distortion. However, the valance of the dopant is different from the host and hence one cannot expect only the simple substitutional occupancy leading to inhomogeneous strains in the crystal17. It is also appropriate to mention here that in the case of dopants having a dissimilar valance and size from the substituting element of the host lattice, due to expected strain, even small thermal/ mechanical fluctuations during the growth process lead to easy formation of structural defects18,19. It clearly shows that the crystal undergoes non-uniform strain in the lattice. The cell volume of the La(III)- doped ZTS crystals increased (Table. 2). It could be due to the small ionic radius of Zn2+ (88 pm) in comparison with that of La(III) (117 pm)17. This type of behavior (the unit cell volume of the doped materials not varying regularly with the ionic radius of the dopant) has been explained by the electron-doping effect counteracting the steric effect20.
The concentration of absorbing species can be determined using the Kubelka-Munk equation21,
The direct and indirect band gap energies obtained from the intercept of the resulting straight line with the energy axis at [F(R)hν]2 = 0 and [F(R)hν]1/2 = 0 are deduced as 5.35 eV and 5.50 eV respectively for ADP:La and 5.37 eV & 5.70 eV for ZTS:La (Fig. 2.3.3.1).
The effect of the influence of dopant on the surface morphology of ADP crystal faces reveals structure defect centers as seen in SEM images (Fig.2.3.4.1). A plate like morphology with a layered structure is exhibited. The incorporation of lanthanum in the ADP crystal matrix results in cluster of scatter centers and voids than those of the undoped specimen. The flower like morphology is observed in ZTS doped specimens. Pure ZTS contains small defect centers in the plate surface and incorporation of La(III) increases the surface roughness (Fig.2.3.4.1(b)).
The incorporation of La(III) into the crystalline matrix was confirmed by EDS performed on ADP and ZTS (Fig.2.3.5.1). It appears that the accommodating capability of the host crystal is limited and only a small quantity is incorporated into the ADP and ZTS crystalline matrix. EDS reveals that the accommodating capability of ZTS is much better than ADP as shown in Fig.2.3.5.1(b).
The amount of doping in ADP:La and ZTS:La specimens are estimated using AAS and the foreign metal ion entering into the ADP/ZTS crystal matrix is much smaller but significant. Further, the final dopant concentration within the host lattice is not proportional to the prevailing concentration of dopant in the solution at the time of the crystallization process, since the host crystal can accommodate the dopant only to a limited extent. The AAS data reveal that the La(III) ion concentration in ADP and ZTS crystalline matrix are 7.5 ppm and 11.3 ppm respectively. High incorporation of the dopant takes place in the case of lanthanide doping in ZTS compared to ADP.
TG/DTA thermogram reveals the purity of the material. The thermogram curve shows a gradual mass loss and residual mass obtained at 1000 ËšC is only 10% Fig.2.3.6.1 (a) An endothermic peak is obtained in the DTA analysis for ADP:La at a higher temperature (200ËšC) than the pure ADP crystals (191 ËšC). The melting point of the material was confirmed by using Sigma instruments melting point apparatus (200ËšC). The investigation shows that there is no physically absorbed water in molecular structure of crystals grown from the solution.
The simultaneous TG-DTA curves in nitrogen for ZTS and ZTS:La systems at a heating rate of 20 ËšC/min are given in the Fig.2.3.6.1(b). The absence of water of crystallization in the molecular structure is indicated by the absence of weight loss around 100 ËšC. The melting point of pure ZTS is 231Ëš C. A good thermal stability of ZTS:La is observed up to ~235 ËšC and the thermal behavior is not very much altered in the presence of the dopant. The sharp endothermic peak at 235 ËšC is may be due to melting point. TG curves show a gradual mass loss and residual mass obtained at 1000 ËšC is ~20 %. The sharpness of the peak shows the good degree of crystallinity of the material. No decomposition up to the melting point ensures the stability of the material for application in lasers, where the crystals are required to withstand high temperatures.
In order to confirm the influence of doping on the nonlinear optical properties (NLO) of the as-grown crystals, these were subjected to SHG test. The SHG efficiency of the materials was performed by Kurtz powder SHG method22. Input radiation used is 2.5 mV/pulse. The output SHG intensities of La(III) doped ADP and ZTS specimens give relative NLO efficiencies of the measured specimens. The doubling of frequency was confirmed by the green color of the output radiation whose characteristic wavelength is 532 nm and it indicates that the doped material exhibits second order NLO effect. The efficient SHG demands specific molecular alignment of the crystal to be achieved facilitating nonlinearity in the presences of a dopant. Incorporation of La(III) into ADP and ZTS crystalline matrix also enhances the SHG efficiency (Table.2.3.7.1) and hence La(III) is a useful dopant. The efficient SHG demands specific molecular alignment of the crystal facilitating nonlinearity in the presence of dopant or it may be due to the improvement in the crystalline perfection of ADP/ZTS crystals by low level La(III)- doping. The effect of various dopants on the SHG efficiencies of ADP/ZTS has been listed in Table.2.3.7.2. The comparative SHG oscilloscope traces of the powder samples ADP:La (blue) and ZTS:La (red) are displayed in Fig. 2.3.7.1.
The influence of La(III) doping on the ADP and ZTS crystal has been systematically studied. The reduction in the intensities observed in the powder XRD patterns and slight shifts in vibrational frequencies in FT-IR indicate minor structural variations in the doped materials. Morphological changes in the doped specimen are observed in the SEM micrographs. The studies indicate that the crystal undergoes lattice stress as a result of doping. Energy dispersive X-ray spectrum reveals the incorporation of La(III)- into the crystalline matrix of ADP/ZTS crystals. AAS studies also confirm the above observations. It is clear that the incorporation of La(III)- is comparatively high in the case of ZTS. The thermal analysis reveals the purity of the material. Enhancement in SHG efficiency is observed in ADP/ZTS as a result of La(III)- doping became of facile charge transfer.