51 2011 PhysicaB Non-linear Opt Mater L-LHN Riscob Sept2011

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Crystalline perfection, optical and dielectric studies on L-histidine nitrate:A nonlinear optical material

B. Riscob a, S.K. Kushwaha a, Mohd. Shakir a, K. Nagarajan b, K.K. Maurya a, D. Haranath c, S.D.D. Roy c,G. Bhagavannarayana a,n

a National Physical Laboratory (CSIR),Dr. K.S. Krishnan Road, New Delhi 110012, Indiab Deptartment of Physics, PSG College of Arts and Science, Coimbatore 641015, Tamil Nadu, Indiac Department of Physics, Nesamony Memorial Christian College, Marthandam 629165, India

a r t i c l e i n f o

Article history:

Received 14 December 2010

Received in revised form

29 August 2011

Accepted 1 September 2011Available online 7 September 2011

Keywords:

Crystal growth

Optical materials

Photoluminescence spectroscopy

Optical properties

Raman spectroscopy and scattering

Dielectric properties.

a b s t r a c t

Single crystals of L-histidine nitrate (LHN), a recently investigated nonlinear optical material, were

grown by conventional solution technique. Crystal structure and vibrational modes of the grown

crystals were confirmed by powder X-ray diffractometry and FT-Raman spectrometry, respectively.

Crystalline perfection of the grown crystals was evaluated by employing an in-house developed high-

resolution X-ray diffractometer (HRXRD) and it was found that the grown crystals were free from

structural grain boundaries and the perfection was reasonably good. However, HRXRD could reveal the

fact that the crystals contain predominantly the interstitial point defects. The birefringence was

measured over a range of wavelength between 5480 and 5630 A and it was found that its value is

nearly constant and 10 times higher than that of KDP. The optical band gap was found to be �3.73 eV.

The photoluminescence excitation and emission spectra for single crystals were recorded. The SHG

efficiencies of LHN samples of different particle sizes were measured by the Kurtz and Perry technique

and they removed the ambiguity in the values reported differently in the literature. Dielectric

properties were studied as a function of temperature over a wide range of frequency. The optical

and dielectric studies along with the crystalline perfection reveal that the LHN crystal could be a good

candidate for nonlinear optical devices.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Due to unique properties the nonlinear optical (NLO) singlecrystals have promising applications in the area of photonics suchas high-speed information processing, frequency conversion, opti-cal communication, high optical disk data storage, etc. [1–4].Birefringent crystals are key materials for optical isolators, circu-lators and beam displacers needed for devices for optical commu-nication [5]. Up to now several hundreds of donors and acceptorssubstituted delocalized P electron systems have been reported,which show NLO properties. In this respect, amino acids are alsofound to be interesting materials for NLO applications. Theimportance of amino acids for NLO application lies in the fact thatalmost all amino acids contain an asymmetric carbon atom andcrystallize in non-centrosymmetric space groups. Therefore in therecent years many amino acid materials such as L-arginine maleatedihydrate (LAMD), L-histidine hydrochloride monohydrate, L-argi-nine formomaleate (LAFM), L-valinium fumarate (LVF), etc. [6–9]

with good NLO efficiency have been synthesized and their singlecrystals have been grown and characterized. To date, the birefrin-gent crystals widely used are the natural calcite and YVO4.However, calcite crystal has serious problems of optical inhomo-geneity, small size and difficulty to process precisely due to itscleavability, while YVO4 crystals with large size and high qualityare difficult to obtain [10,11]. It is therefore necessary to search fora new birefringent crystal having high quality, large size, goodchemical stability and mechanical workability.

The present material LHN is a known NLO material, crystallizesin orthorhombic crystal structure with space group P212121 andon this material few other studies were also reported like FT-IR,FT-Raman, mechanical, thermal and room temperature dielectricbehavior [12–14]. However, to the best of our knowledge, thereare no report(s) on crystalline perfection, birefringence andoptical studies like band gap, photoluminescence, nonlinearitywith different particle sizes and dielectric behavior with tem-perature on this crystal, which are also very important para-meters to know their use for device applications.

In the present investigation, single crystals of LHN were grownby the slow evaporation solution technique (SEST). The structuraland vibrational studies were carried out for the grown crystals

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Physica B

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.physb.2011.09.004

n Corresponding author. Tel.: þ91 11 45608261; fax: þ91 11 45609310.

E-mail address: [email protected] (G. Bhagavannarayana).

Physica B 406 (2011) 4440–4446


powder XRD and FT-Raman analysis. The crystalline perfection ofthe grown crystals was assessed using high resolution X-raydiffractometry (HRXRD). The birefringence measurement wascarried out. The optical band gap was also calculated usingUV–vis. study. The photoluminescence emission and absorptionstudies were carried out by using a photoluminescence (PL)spectrophotometer. SHG efficiency of the material with differentparticle sizes was also measured by the Kurtz and Perry techni-que. The dielectric studies over a wide range of frequency for thegrown crystal along [0 0 1] were performed at and above theroom temperature.

2. Experimental details

2.1. Synthesis and crystal growth

The title material was synthesized from L-histidine and nitricacid in 1:1 molar ratio [12]. The product was purified by arepeated recrystallization process and used for the growth ofsingle crystals using double distilled water as the solvent. Thesaturated solution of recrystallized LHN was prepared at 308 Kwith continuous stirring for 24 h. The filtered solution in a beakercovered with perforated lid was housed in a Eurotherm controlledconstant temperature bath with good stability (70.01 1C) at308 K. In a time span of 20 days well faceted transparent singlecrystals were harvested and the photograph of such a typicalgrown crystal is shown in Fig. 1. The observed morphology ofgrown crystals was found to be the same as that reported [12].

2.2. Analysis techniques

For confirmation of the crystal system and space group as wellas determining lattice parameters, a homogeneous powderedspecimen of LHN crystals was subjected to PW1830 PhilipsAnalytical Powder X-ray diffractometry with nickel filtered CuKa

radiation (35 kV, 30 mA). The data was recorded by operating thediffractometer in the angular range of 10–601 of 2 y at the scan

rate of 0.011/s. The single crystal specimens with natural facetedsurfaces were subjected to Perkin Elmer GX 2000 FT-Ramanspectrometry to carry out the vibrational studies in the wavenumber range 100–3500 cm�1 at 293 K.

Crystalline perfection of LHN crystals was evaluated using amulticrystal X-ray diffractometer designed and developed atNational Physical Laboratory [NPL] [15]. The well-collimatedand monochromated MoKa1 beam (0.709261 A) obtained fromthe three monochromator Si crystals set in dispersive (þ ,� ,�)configuration has been used as the exploring X-ray beam. Thespecimen crystal is aligned in the (þ ,� ,� ,þ) configuration.Because of the dispersive configuration, though the lattice con-stants of the monochromator crystal(s) and the specimen aredifferent, the unwanted dispersion broadening in the diffractioncurve (DC) of the specimen crystal is insignificant. The rocking ordiffraction curves were recorded by changing the glancing angle(the angle between the incident X-ray beam and the surface of thespecimen) around the Bragg diffraction peak position yB (taken aszero for the sake of convenience) starting from a suitable arbitraryglancing angle and ending at a glancing angle after the peak sothat all the meaningful scattered intensities on both sides of thepeak were included in the diffraction curve. The specimen wasrotated about the vertical axis, which is perpendicular to theplane of diffraction, with a minimum angular interval of 0.400. TheDC was recorded by the so-called o scan wherein the detectorwas kept at the same angular position 2yB with wide opening.This arrangement is very appropriate for recording the short-range order scattering caused by defects or by the scattering fromlocal Bragg diffractions from agglomerated point defects or fromlow angle and very low angle structural grain boundaries [16].Before recording the diffraction curve, to remove the non-crystal-lized solute atoms that remained on the surface of the crystal andalso to ensure the surface planarity, the specimen was first lappedand chemically etched in a non-preferential etchant of water andacetone mixture in the 1:2 volume ratio. This process also ensuresthe removal of possible complexating surface layers [7].

The optically polished single crystal of LHN was subjected tobirefringence measurements using the channeled spectrummethod [17] with a halogen lamp of 1000 W as a source. Theexperimental set-up used in the present study is shown in Fig. 2.The beam obtained from the source (S) was collimated by thecollimator (C) and optically polarized by a polarizer (P). Thepolarizer and analyzer (A) were placed in crossed positions, andthe crystal (Cr) was placed in between them in such way that itsoptic axis was perpendicular to the incident ray. The transmittedlight components from the analyzer interfere and interferencepatterns were observed through the constant deviation spectro-meter (CDS). For each dark band, the corresponding wavelengthwas read out directly from the drum of the CDS.

The UV–vis absorption study was carried out using aJASCO V-570 spectrophotometer in the wavelength range of190–900 nm. The photoluminescence excitation and emission

Fig. 1. Photograph of the SEST grown LHN single crystal. The arrow indicates the

(0 0 1) face of the crystal.

S CDSC P ACr

Fig. 2. Block diagram of the birefringence set-up.

B. Riscob et al. / Physica B 406 (2011) 4440–4446 4441


spectra were recorded for the single crystals using a Perkin ElmerLS-55 Fluorescence spectrofluorometer. To measure the SHGefficiency of the LHN single crystals, well crushed powders ofas-grown crystals were sieved with standard test sieves ofdifferent sizes 25, 50, 100 and 125 mm, respectively, to achievehomogeneous particles. These powders were filled properly in aglass microcapillary of 1 mm inner diameter and subjected to theKurtz and Perry powder technique [18]. A standard potassiumdihydrogen phosphate (KDP) crystal was used as a reference tocompare the SHG values. A Q-switched Nd:YAG [Spectra Physics(DCR-II)] laser with a fundamental wavelength 1064 nm, inputenergy 5.15 mJ pulse�1, 8 ns pulse duration, 10 Hz repetition rateand 1 mm spot diameter was used in 901 scattering geometry as asource. The laser radiation was made incident on the specimensample, and the produced green output radiation from the speci-men was detected by a photomultiplier tube (PMT) coupled witha filter. The signal from the PMT was used to assess the relativeSHG efficiency of the grown crystals of different particle sizes incomparison to that of the KDP reference crystals. The experi-mental conditions were kept the same for all the samples.

The dielectric properties of the grown crystals as a function oftemperature were studied using a PSM1735 NumetriQ highfrequency impedance analyzer. Before subjecting to the analyzer,the electrodes were made using silver paste on the parallelopposite surfaces of a rectangular specimen. The specimen holderwith crystal was housed in a furnace with the temperaturestability up to 71.0 1C controlled by a Eurotherm temperaturecontroller.

3. Results and discussion

3.1. Powder X-ray diffraction

The recorded Powder X-ray diffraction pattern for LHN isshown in Fig. 3. The sharp nature of the diffraction peaks indicatesgood crystallinity of the grown crystals. The recorded data wasused to calculate lattice parameters using Proszki software fromwhich it was confirmed that the grown crystals belong toorthorhombic system with non-centrosymmetric space groupP212121. The calculated lattice parameters are a¼5.2601,b¼7.1379 and c¼25.0209 A and found to be in good agreementwith the earlier reported values [12].

3.2. Vibrational studies

The recorded FT-Raman spectrum for LHN single crystal isshown in Fig. 4. The unused portion of the spectrum was removedto visualize the analyzed peaks with clarity. The observed peaksin the spectrum are assigned to their corresponding bonds andfunctional groups and tabulated in Table 1. The peaks at 1316,1174, 1040, 843 and 722 cm�1 correspond to asymmetric stretch-ing, symmetric as well as asymmetric deformation, symmetricstretching, symmetric and asymmetric bending, respectively. Thepeaks corresponding to ring deformation confirm the propersynthesis of LHN. The peaks at 224 and 202 cm�1 are due tolattice vibrations. The sharpness with high intensity of the peaksindicates that the grown crystals are almost free from thestructural defects [19], which is further elucidated by the follow-ing HRXRD analysis.

3.3. High resolution X-ray diffractometry

Fig. 5 shows the high-resolution diffraction curve (DC)recorded for a typical SEST grown LHN single crystal using(0 0 4) diffracting planes in the symmetrical Bragg geometry byemploying the multicrystal X-ray diffractometer with MoKa1

radiation. As seen in the figure, the DC contains a single peakand indicates that the specimen is free from structural grainboundaries. The FWHM (full width at half maximum) of the curveis 7500, which is somewhat more than that expected for an ideallyperfect crystal from the plane wave theory of dynamical X-raydiffraction [20] but close to that expected for a nearly perfectcrystal. It is interesting to see the asymmetry of the DC. For aparticular angular deviation (Dy) of glancing angle with respect tothe peak position, the scattered intensity is much more in thepositive direction in comparison to that in the negative direction.This feature clearly indicates that the crystal contains predomi-nantly interstitial type of defects than that of vacancy defects.This can be well understood by the fact that due to interstitialdefects (self-interstitials or impurities at interstitial sites), whichmay be due to fast growth and/or impurities present in the rawmaterial, the lattice around these defects undergoes compressivestress [21], the lattice parameter d (interplanar spacing) decreasesand leads to gives rise to more scattered (also known as diffuseX-ray scattering) intensity at slightly higher Bragg angles (yB) as d

and sin yB are inversely proportional to each other in the Braggequation (2d sin yB¼nl; n and l being the order of reflection and

Fig. 3. Powder XRD pattern for the LHN crystals. Fig. 4. FT-Raman spectra for LHN single crystal.

B. Riscob et al. / Physica B 406 (2011) 4440–44464442


wavelength, respectively, which are fixed). However, the singlediffraction peak with reasonably low FWHM indicates that thecrystalline perfection is fairly good. The density of such interstitialdefects is however very meager and in almost all real crystals,including nature gifted crystals, such defects are commonlyobserved [22] and are many times unavoidable due to thermo-dynamical conditions and hardly affect the device performance.More details may be obtained from the study of high-resolutiondiffuse X-ray scattering measurements [15], which is however notthe main focus of the present investigation. It is worth mention-ing here that the observed scattering due to interstitial defects is

of short order nature as the strain due to such minute defects islimited to the very defect core, the long range order could not beexpected and hence the change in the lattice parameter of thecrystal is not possible [23]. It may be mentioned here that theminute information like the asymmetry in the DC could bepossible as in the present sample only because of the high-resolution of the multicrystal X-ray diffractometer used in thepresent investigation.

3.4. Birefringence analysis

Birefringence is the difference between the magnitudes ofrefractive indices of ordinary ray and extraordinary ray. Thischaracteristic behavior of birefringence crystal can be used inpolarization devices and beam reflector [24]. The birefringence ofthe grown crystal was calculated using the formula Dn¼kl/t [25],l being the wavelength, t is the thickness of the crystal and k isthe fringe order. In the present experimental study, the thicknessof the crystal was around 0.19 mm. The graph drawn betweenbirefringence and wavelength is shown in Fig. 6. As seen in thegraph, the value of birefringence lies between 0.3432 and 0.3406over the wavelength region of 5480–5630 A. Such a nominalvariation of birefringence shows the suitability of LHN crystalsfor efficient second harmonic generation (SHG) and opticaldevices like polarizers and beam reflectors. The birefringencevalue of the titled material is 10 times higher than that of thestandard KDP crystal. The average value of retardation ! (thick-ness of the sample�birefringence) was also calculated and it wasfound to be 6.496�10�5. LHN crystal shows high birefringencevalue compared to other materials like calcite [10], potassiumacid phthalate (KAP) [26], L-histidine bromide (LHB) [27], etc.

3.5. UV–vis spectroscopy analysis

To calculate the optical band gap, the absorption spectra of thegrown crystal were recorded. First the optical absorption coefficient(a) was calculated from the absorbance [Fig. 7(a)] using the relation

a¼ Absorbance=t ð1Þ

t being the thickness of the crystal. The relation between the opticalband gap (Eg) and the optical absorption coefficient (a) near theabsorption edge can be written as [28]

ahn¼ Aðhn�EgÞ1=2

ð2Þ

Table 1FT-Raman peak positions with their assignments for LHN crystal.

Peak Position (cm�1) Assignments

3157 CH stretching

2974 CH2 asymmetric stretching

2948 CH2 symmetric stretching

1629 NH3 symmetric deformation

1495 CN stretching

1431 CH2 deformation

1353 CH bending

1316 NO3 asymmetric stretching

1272 CH wagging

1245 CH2 twisting

1201 RingþNH stretching

1174 NO3 symmetricþNO3 asymmetric deformation

1086 C–N symmetric stretching

1040 NO3� symmetric stretching

992 Ring deformation

944 C–C stretching

926 C–C stretching

900 Ring deformation

843 NO3� symmetric bending

814 O–C–O deformation

742 O–C–O deformation

722 NO3� asymmetric bending

704 O¼C–O deformation

626 Ring puckering

544 C–C–O deformation

397 C–C–C in plane vibration



247 lattice vibrations

202 lattice vibrations

Fig. 5. High-resolution diffraction curve recorded for a typical LHN single crystal

using (0 0 4) diffracting planes in symmetrical Bragg geometry with MoKa1

radiation. The inset shows the schematic of the compressed lattice around an

interstitial defect core.

Fig. 6. Birefringence vs. wavelength for LHN crystal.



A is a constant, Eg is the optical band gap, h is the Plank’s constantand n is the frequency of incident photon. The plot of (ahn)2 vs. hn isshown in Fig. 7(b). The band gap of the grown crystal was obtainedas �3.73 eV from the intercept on the X-axis with the extrapolatedstraight line. The transparency of the grown crystal was found to bein good agreement with the reported one [12]. The high transmis-sion in the entire visible region also confirmed the colorless natureof the grown crystal. Hence the titled compound could be a goodcandidate for fabrication of NLO devices.

3.6. Photoluminescence studies

The PL excitation spectrum [Fig. 8(a)] of LHN single crystal wasrecorded in the ultra violet region, corresponding to the emissionwavelength �385 nm. A strong absorption band was observed at�241 nm. The PL emission spectrum [Fig. 8(b)] for the excitationof the specimen crystal at 241 nm was recorded in the wave-length range from 300 to 700 nm. A strong emission band wasobserved at �385 nm. The emission spectrum indicates that theLHN crystal is efficient for the absorption of ultraviolet light andemission of light in violet region. The small band at �595 nmmay be due to minute lattice defects, whereas high intensity anddiscrete nature of the major emission band at �385 nm indicatethat the grown crystals are free from the major defects like grainboundaries as revealed by HRXRD results.

3.7. Second harmonic generation (SHG) studies

Depending upon the coherence length of the laser beam, theSHG efficiency increases with increasing particle size and attainsconstant value at higher particle sizes [29]. Different SHG valuesare reported previously for the title material [12,13]. Theobserved values of SHG efficiency with different particle sizesare given in Table 2. The variation of SHG with particle size isshown in Fig. 9. We have repeated our experiments several timeson the title material with different particle sizes to get the actualvalue of SHG. The observed saturated value of SHG is found to be0.8 times to that of standard KDP and seems to be the correctvalue, which is well matched with that of Ref. [13]. This kind ofparticle size dependency on SHG intensity exists only in phase-matchable materials [30]. Hence this crystal can be used as anefficient frequency doubler and optical parametric oscillator.

Fig. 7. (a) Absorption spectrum and (b) Optical band gap for the LHN crystal.

Fig. 8. The recorded photoluminescence: (a) excitation spectrum with peak at

241 nm and (b) emission spectrum with peak at 385 nm, for LHN single crystals.

Table 2Particle size dependent SHG intensities for LHN crystals.

Sample/Particle size SHG Signal 2o (mv) Efficiency with respect to KDP

Pure KDP (standard size) 26.0 1.0000

LHN–25 mm 9.5 0.3653

LHN–50 mm 19.0 0.7308

LHN–100 mm 20.9 0.8038

LHN–125 mm 20.8 0.8000



3.8. Dielectric studies

The dielectric properties of the nonconducting optical singlecrystals are well correlated with electro-optic properties [31].Dielectric constant (er) and dielectric loss (tan d) for LHN crystalrecorded on the prominent face of crystal (0 0 1) with differenttemperatures over a wide range of frequency (10 kHz–15 MHz)are shown in Fig. 10(a) and (b), respectively. The dielectricconstant is higher at lower frequencies, then decreases sharplywith frequency and after that it remains almost constant over theentire higher frequency range. The decrease of dielectric constantin higher frequency region may be due to the fact that the dipolescannot follow up the fast variation of the applied field. The highervalues of er at lower frequencies may be due to contribution fromall the four types (space charge, dipolar, ionic and electronicpolarization) of polarizations, but at higher frequencies, only ionicand electronic polarizations contribute. At room temperature,er has a smaller value over the entire frequency range. As thetemperature increases, er also increases at lower frequencieswhereas at higher frequencies it remains the same as that of roomtemperature. From Fig. 10(b) it is clearly visible that the dielectricloss (tan d) for LHN at room temperature is almost zero. Atelevated temperatures, tan d increases at lower frequencies butat higher frequencies it remains almost negligible. Fig. 10(c) showsthe ac conductivity sac vs. applied frequency, which was calcu-lated from the following formula:

sac ¼ 2pneoertand

eo is the vacuum dielectric constant, er the relative dielectricconstant for the LHN crystal and n is the frequency of applied acfield. The Nyquist/cole–cole plots at 300 and 323 K were calcu-lated and are shown in Fig. 11(a), which show the near semicircleshape. However, at the higher temperatures we could not achievethe proper shape of the curves and they could not be fitted withany available model. The slight change in the radius may beattributed to the increase in dc part of conductivity [32], as shownin ac conductivity plots. The relaxation time (t) corresponding tothe peak of Cole–Cole plots have been calculated using the relationoMt¼1; oM(¼2pfM) is the frequency at which Z00 is maximum.For both the temperatures 300 and 323 K there is no shift in thepeak positions and the value of t is found to be 1.89�10�5 s. Thedata points are well fitted with the simulated plots by electro-chemical impedance spectroscopy (EIR) analyzer software (http://www.abc.chemistry.bsu.by/vi/analyser/). The evaluated constantphase element (CPE) coefficient n at 300 and 323 K are 0.65 and

0.6, respectively. To get the correct value of fM, which is 8422 Hz,frequency dependent plots of Z00 were used [Fig. 11(c)]. Theactivation energy (DE) calculated using the relation sac¼so

exp(�DE/kT) (so being a constant, T the absolute temperature

Fig. 9. Variation of SHG with particle size of LHN sample.

Fig. 10. (a) Dielectric constant and (b) dielectric loss curves with frequency for

LHN single crystals at different temperatures.



and k the Boltzmann constant) at various temperatures is shownin Fig. 11(b).

4. Conclusions

Single crystals of LHN have been grown by the conventional SESTmethod. The crystal structure and synthesis of the compound havebeen confirmed by powder XRD and FT-Raman, respectively. Crystal-line perfection of the grown crystals has been assessed using a

HRXRD and the results revealed that the grown single crystals arefree from the major defects like grain boundaries. The birefringencevalue was found to be 10 times higher than that of KDP and is stableover the characterized wavelength range. The optical band gap wasdetermined from UV–vis studies and found to be �3.73 eV. Thephotoluminescence excitation and emission spectra revealed that thecrystals show the prominent emission for the violet radiation at 385nm. The SHG efficiency of LHN samples was confirmed as 0.8 timesthat of standard KDP. The ambiguity in the reported values has beenremoved by studying the SHG at different particle sizes. Dielectricconstant and dielectric loss studies over a wide range of frequencyindicate that the grown LHN single crystals are free from the majordefects in tune with HRXRD. The present studies on LHN singlecrystals reveal excellent optical and dielectric properties along withgood crystalline perfection and hence LHN may be a good candidatefor the fabrication of optical devices.

Acknowledgement

Authors are thankful to the Director, NPL for his continuousencouragement to carry out the present investigation. They arealso grateful to Dr. Manju Arora, NPL, for recording FT-Ramanspectra. Riscob acknowledges Director, NPL, for giving him anopportunity to work at NPL during his M.Phil. project work inCrystal Growth and Crystallography Section.

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0.0 5.0M 10.0M 15.0M 20.0M0.0

2.0M

4.0M

6.0M

8.0M

10.0M 300 K 323 K

Z"

Z'

ωM

τ = 1

τ = 5.13x10-5s

Simulated 300 K Simulated 323 K

1k 10k 100k 1M 10M0.0

2.0M

4.0M

6.0M

8.0M

Z"

Frequency (Hz)

300 K 323 K

300 320 340 360 380 400 4205.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

activ

atio

n en

ergy

(eV

)

T (K)

Fig. 11. (a) Nyquist/Cole–Cole plots at temperatures 300 and 323 K, (b) Z00 vs.

frequency plots at temperatures 300 and 323 K and (c) activation energy for the

grown LHN single crystal at different temperatures.


51 2011 PhysicaB Non-linear Opt Mater L-LHN Riscob Sept2011

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