G. Bhagavannarayana

A study of the effect of annealing on Fe-doped LiNbO3 by HRXRD, XRT and FT–IR

The annealing effect on the structural perfection of Fe-doped LiNbO3 single crystals has been studied by high-resolution X-ray diffractometry (HRXRD), X-ray topography (XRT) and Fourier transform infrared (FT–IR) spectroscopy. The single crystals, prepared by mixing Li2CO3 and Nb2O5 powders in the molar ratio 48.6:51.4 with 0.05 mol% of iron at 1415 (1) K, were grown by the Czochralski (CZ) method along the [001] direction in air and poled during crystal growth by the application of a DC field. Two low-angle (tilt angle ∼1 arc minute) structural grain boundaries were observed in as-grown specimens. FT–IR spectra revealed that these crystals contain OH− and CO32− ionic defects. Grain boundaries and CO32− ionic defects were successfully removed, while the concentration of OH− ions was considerably reduced by post-growth thermal annealing at elevated temperatures.

Enhancement of crystalline perfection by organic dopants in ZTS, ADP and KHP crystals as investigated by high-resolution XRD and SEM

To reveal the influence of complexing agents on crystalline perfection, tristhiourea zinc(II) sulfate (ZTS), ammonium dihydrogen phosphate (ADP) and potassium hydrogen phthalate (KHP) crystals grown by slow-evaporation solution growth technique using low concentrations (5 x 10 -3 M) of dopants like ethylenediamminetetraacetic acid (EDTA) and 1,10-phenanthroline (phen) were characterized by high-resolution X-ray diffractometry (XRD) and scanning electron microscopy (SEM). High-resolution diffraction curves (DCs) recorded for ZTS and ADP crystals doped with EDTA show that the specimen contains an epilayer, as observed by the additional peak in the DC, whereas undoped specimens do not have such additional peaks. On etching the surface layer, the additional peak due to the epilayer disappears and a very sharp DC is obtained, with full width at half-maximum (FWHM) of less than 10 arcsec, as expected from the plane wave dynamical theory of X-ray diffraction for an ideally perfect crystal. SEM micrographs also confirm the existence of an epilayer in doped specimens. The ZTS specimen has a layer with a rough surface morphology, having randomly oriented needles, whereas the ADP specimen contains a layer with dendric structure. In contrast to ADP and ZTS crystals, the DC of phen-doped KHP shows no additional peak, but it is quite broad (FWHM = 28 arcsec) with a high value of integrated intensity, p (area under the DC). The broadness of the DC and the high value of p indicate the formation of a mosaic layer on the surface of the crystal. However, similar to ADP and ZTS, the DC recorded after etching the surface layer of the KHP specimen shows a very sharp peak with an FWHM of 8 arcsec. An SEM photograph of phen-doped KHP shows deep cracks on the surface, confirming the mosaicity. After removing the surface layer, the SEM pictures reveal a smooth surface. A similar trend is observed with other complexing agents, like oxalic acid, bipy and picolinic acid. However, only typical examples are described in the present article where the effects were observed prominently. The investigations on ZTS, ADP and KHP crystals, employing high-resolution XRD and SEM studies, revealed that some organic dopants added to the solution during the growth lead to the formation of a surface layer, due to complexation of these dopants with the trace metal ion impurities present in the solution, which prevents the entry of impurities, including the solvent, into the crystal, thereby assisting crystal growth with high crystalline perfection. The influence of organic dopants on the second harmonic generation efficiency is also investigated.

Enhancement of SHG efficiency by urea doping in ZTS single crystals and its correlation with crystalline perfection as revealed by Kurtz powder and high-resolution X-ray diffraction methods

The enhancement of second-harmonic generation (SHG) efficiency by urea doping in tristhioureazinc(II) sulfate (ZTS) single crystals and its correlation with crystalline perfection have been investigated. ZTS is a potential semiorganic nonlinear optical material. Pure and urea-doped single crystals of ZTS have been successfully grown by the slow evaporation solution technique. The presence of dopant has been confirmed and analysed by Fourier transform infrared spectrometry. The influence of urea doping at different concentrations on crystalline perfection has been thoroughly assessed by high-resolution X-ray diffractometry (HRXRD). HRXRD studies revealed that the ZTS crystals could accommodate urea up to a critical concentration without any deterioration in crystalline perfection. Above this concentration, very low angle structural grain boundaries developed and it seems the excess urea above the critical concentration was segregated along the grain boundaries. At very high doping concentrations, the crystals were found to contain mosaic blocks. The SHG efficiency has been studied using the Kurtz powder technique. The relative SHG efficiency of the crystals was found to increase substantially with the increase in urea concentration. The correlation found between crystalline perfection and SHG efficiency is discussed.

Study of the influence of dopants on the crystalline perfection of ferroelectric glycine phosphite single crystals using high-resolution X-ray diffraction analysis

Good quality and optically transparent single crystals of pure and doped glycine phosphite (GPI) were grown by both solvent-evaporation and temperature-cooling techniques. Dopants were chosen in different categories, namely transition metals (Cr, Mn, Co, Ni, Zn, Mg, Cd), rare-earth metals (Ce, Nd, La), dyes (rhodamine B, malachite green, fluorescein) and an amino acid (l-proline). The concentration of dopants was chosen depending on the category of dopants and the quality of crystallization during the growth process. The crystalline perfection of the as-grown pure and doped GPI crystals was investigated by high-resolution X-ray diffraction at room temperature. A multicrystal X-ray diffractometer employing a well collimated and highly monochromated Mo Kα1 beam and set in the (+, −, −, +) configuration was employed. Most of the crystal specimens show excellent crystalline perfection. However, grain boundaries, low-angle tilt boundaries, and vacancy and interstitial point defects were observed in some crystal specimens.

Study of point defects in as-grown and annealed bismuth germanate single crystals

Point defects and their clusters in bismuth germanate single crystals free from grain boundaries and having low density of dislocations were studied by high-resolution diffuse X-ray scattering measurements. Differences in defects in the colourless crystals (type A) and the crystals having yellow tinge (type B), which were grown with different raw materials, were investigated. In addition, interesting differences in defect structures in specimens from different regions of the same boule were investigated. Specimens with diffracting surfaces along (111), (112) and (100) planes were studied. A multicrystal X-ray diffractometer employing a well collimated and highly monochromated Mo ACT] beam and set in (+,-,-,+) configuration was employed. The diffraction curves of all the samples were quite narrow with half-widths in the range 7-11 arcsec, which are close to the theoretically expected values, if instrumental broadenings are taken into account. The observed distribution of diffuse X-ray scattering (DXS) intensity showed that not all the point defects are isolated but a significant fraction are agglomerated into clusters. Experimental data of DXS intensity were analysed by using a phenomenological model for a small concentration of dislocation loops wherein the point defects are loosely clustered with weak interactions among them. From this analysis, the cluster radius R cl , cluster volume A cl , the number of point defects within a cluster N cl and the relative concentration of the point-defect clusters among the samples were estimated. It was observed that cluster sizes do not vary from sample to sample. However, it was found that the concentration of clusters is approximately twice in the coloured sample compared with that of the colourless sample from the same boule. Annealing of the crystals at 1273 K produced an increase in point-defect clusters by a factor of ∼200. It was accompanied by a reduction in volume of clusters by a factor of ∼0.14.

Rare earth cerium doping effects in nonlinear optical materials: potassium hydrogen phthalate (KHP) and tris(thiourea)zinc(II) sulfate (ZTS)

The influence of Ce(IV) doping on ZTS and KHP crystals over a concentration range from 1 to 10 mol% in the solution during crystallization, which leads to a true concentration range from few ppm to few tens of ppm in the crystals has been investigated. The XRD and FT-IR analyses indicate that the crystal undergoes considerable stress as a result of doping. Incorporation of the Ce(IV) dopant into the crystal lattice was well confirmed by energy dispersive X-ray spectroscopy (EDS) and quantified by inductively coupled plasma (ICP) technique. The high-resolution X-ray diffraction (HRXRD) studies reveal that Ce doping in KHP leads to degradation of crystal quality whereas ZTS can accommodate Ce predominantly at the substitutional sites without any degradation of crystalline perfection. The second harmonic generation (SHG) efficiency is not influenced by Ce doping in the KHP crystals while in ZTS crystals, it is enhanced to a considerable extent correlated with moderately improved crystalline perfection.

Growth, HRXRD, Microhardness and Dielectric Studies on the NLO Material L-Alaninium Maleate

The single crystals of the NLO material, L-Alaninium maleate were grown by using the submerged seed solution method. The identity of the crystal was confirmed by single crystal X-ray diffraction. The crystalline perfection was analyzed using high resolution X-ray diffraction and it was found that the crystalline perfection is quite good. The values of the laser damage threshold and the Vicker’s microhardness are in the higher range. The dielectric studies at different temperatures showed that the dielectric constant and dielectric loss have low values at higher frequencies and these values are independent of the temperature. The details are presented and discussed. Keyword: X-ray diffraction, Growth from solution, Organic compound, Nonlinear optical materials.

Growth, spectral, thermal, dielectric, mechanical, linear and nonlinear optical, birefringence, laser damage threshold studies of semi-organic crystal: Dibrucinium sulfate heptahydrate

Dibrucinium sulfate heptahydrate (DBSH), a semi-organic nonlinear optical material, has been synthesized and single crystals were grown from water-ethanol solution at room temperature up to dimensions of 10×7×2 mm(3). The unit cell parameters were determined from single crystal and powder X-ray diffraction studies. The structural perfection of the grown crystal has been analyzed by high-resolution X-ray diffraction (HRXRD) study. FTIR and Raman studies were performed to identify the functional groups present in the title compound. The activation energy (E), entropy (ΔS), enthalpy (ΔH) and Gibbs free energy (ΔG), of the thermal decomposition reaction have been derived from thermo gravimetric (TGA) and differential thermal (DTA) analysis curves, using Coats-Redfern method. The variation of dielectric properties of the grown crystal with respect to frequency has been investigated at different temperatures. Microhardness measurements revealed the mechanical strength of grown crystal. The optical parameters, the optical band gap E(g) and width of localized states Eu were determined using the transmittance data in the spectral range 200-800 nm. The relative second harmonic efficiency of the compound is found to be 1.4 times greater than that of KDP. Birefringence and Laser damage threshold studies were carried out for the grown crystal.

Analysis on structural, SHG efficiency, optical and mechanical properties of KDP single crystals influenced by Glycine doping

Good quality single crystals of potassium dihydrogen orthophosphate (KDP) were grown with different doping concentration of Glycine by conventional solution technique in aqueous solution. X-ray diffraction study has been carried out in order to see the effect of dopant on the structural parameters of KDP. There is no additional phase was observed which was further confirmed by Raman spectroscopic analysis. The second harmonic generation efficiency was measured by using Kurtz powder technique. The relative second harmonic generation (SHG) efficiency of the grown crystals was found to be increased with doping concentration up to 2.5 mol%. Optical transmission study also revealed the same behaviour with enhancement up to 2.5 mol% concentration and later decreased but still higher than pure KDP. The mechanical strength was found to increase with increasing the doping concentration.

Crystal growth and characterization of γ-glycine grown from potassium fluoride for photonic applications.

Single crystals of γ-glycine, an organic nonlinear optical material have been synthesized in the presence of potassium fluoride (KF) by slow evaporation technique at ambient temperature. The size of the grown crystal is up to the dimension of 12 mm×10 mm×8 mm. The γ-phase was confirmed by single crystal X-ray diffraction, powder XRD and the FTIR analysis. Optical absorption spectrum reveals that the grown crystal has good optical transparency in the entire visible region with an energy band gap of 5.09 eV, which is an essential requirement for a nonlinear optical crystal. Thermal stability of the grown γ-glycine crystal was determined using the thermo gravimetric and differential thermal analyses. The NLO activity of γ-glycine was confirmed by the Kurtz powder technique using Nd:YAG laser and the grown crystal exhibits high relative conversion efficiency when compared to potassium dihydrogen phosphate (KDP).