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Indian Journal of Pure & Applied PhysicsVol. 43, June 2005, pp. 463-468

Effect of metallic substitution on the optical, mechanical and photoconducting. properties of L-arginium diphosphate single crystals

A Joseph Ami Pragasam", S Selvakumar, J Madhavan, D Prem Anand & P Sagayaraj*Department of Physics, Sathyabama Institute of Science and Technology, Deemed University, Chennai 600 119

Department of Physics, Loyola College, Chennai 600 034

Received 9 December 2004; revised 7 February 2005; accepted 16 March 2005

L-arginium diphosphate (LADP) is one of the important analogs of LAP crystal. LADP has excellent optical, thermaland mechanical properties, which make it a potential candidate for photonics device fabrication. The present work dealswith the growth and characterization of metal (Cu2+ itnd Mg2+) substituted single crystals of LADP. The grown crystals aresubjected to single crystal XRD, AAS, FTIR, DRS spectra and microhardness studies. Microhardness studies indicate thatthe presence of dopant has enhanced the hardness of the material. The photoconductivity study reveals the positivephotoconducting nature of both pure and doped crystals.

Keywords: Non-linear optical properties, Vicker's hardness number, Metal substitution, PhotoconductivityIPC Code: HOIL21100

1 IntroductionAmino acid family crystals have been the subject of

extensive investigation for their non-linear optical(NLO) properties. L-arginine phosphate monohydrate(LAP), an NLO crystal, was first reported by Xuet all. LAP crystal has drawn the attention ofresearchers, due to its high non-linearity, widetransmission range, high conversion efficiency andhigh damage threshold. The damage threshold of LAP(>30 J/cm2) was found to be twice as high as the bestquality commercially available KDP2. As a follow-up,many new crystals belonging to LAP family havebeen investigated and reported. The crystal structuresof L-arginine dihydrate.', L-arginine chloride", L-arginine hydrochloride monohydrate.', L-arginineperchloride't', L-arginine diarsenate", L-argininumdinitrate" and a triclinic polymorph of L-argininumchloride'? have been reported. Petrosyan et al. IIdiscovered a new class of compounds II havingArg.2Ax (where Ax is one of the several inorganic ororganic acids). One of such compounds, L-argininumbis (dihydrogen phosphate) (LADP) has beensynthesized and bulk single crystal is grown by Reenaand Sagayaraj 12.Preliminary investigations reveal thatLADP possesses excellent optical, thermal,piezoelectric, piezoelastic and mechanical properties,which make it a strong candidate for photonic devicefabrication'v'". From structural point of view, LADPcrystallizes in the orthorhombic system with latticeparameters, a = 7.77 A, b = 9.68 A, c = 21.756 A, a =

~ = y = 9° and space group of P212121. The presentwork deals with the influence of metallic dopants (Cuand Mg) on the growth, structural, mechanical andoptical properties of L-arginine diphosphate (LADP)single crystals.

-2 Experimental DetailsThe stoichiometric amount of L-arginine and

orthophosphoric acid was taken with excess of doubledistilled water to synthesize LADP. The following isthe reaction.

(NH2)NHCNH(CH2)3CH(NH2)COOH+2(H3P04)+H20 -7

(NH2)3C (CH2)3CH(NH3tCOO- (H2P04)2- H20

The concentration of the Cu and Mg dopants in thedoped crystals was maintained as 0.2 M. Thestructural representation of Cu/Mg doped crystal ispresented below:

H HI I

H2N-c - COO--X --OOC-C -NH2I I

H-C-H H-C-HI I

H-C-H H-C-HI I

H-C-H H-C-HI I

H2N- C - NH2 H2N--C- NH2I I

NH2 NH2

where X represents the dopant (Cu or Mg).

464 INDIAN J PURE & APPL PHYS, VOL 43, JUNE 2005

The synthesized salts were dissolved in doubledistilled water and saturated LADP solutions wereprepared at room temperature. These saturatedsolutions were placed in a constant temperature bathkept at the desired growth temperature. Yokotaniet al. 14 have observed the problem of microbes in theL-arginine based solutions and these microbes areinhibited by the addition of H202 in the mothersolution". The added H202 forms a thin layer on thesurface of the mother solution and it protects thecontamination by organic species, evaporation of thesolvent, surface nucleation and the faces of the growncrystals. Hence, in the present work, 3 drops of H202were added in the mother solution of LADP.

2.1 SolubilityThe solubility of pure LADP and Cu2+ and Mg2+

substituted LADP in water has been determined forfour different temperatures, 30, 3S, 40 and 4SoCrespectively (Fig. 1). Recrystallized salt was used forthe studies. The solubility was determined bydissolving the solute in water in an airtight containermaintained at a constant temperature with continuousstirring. After attaining the saturation, the equilibriumconcentration of the solution was analyzedgravimetrically.

2.2 Crystal growthSolution growth procedure is based on the use of

water as a solvent, due to relatively high solubility ofLADP in it (4Sg/100ml of water at 30°C) and thefavourable thermal coefficient of solubility, the waterevaporates to produce seed crystals. After two weeks,seed crystals of LADP were used for the growth usingthe Manson jar 'crystallizer. Using the sameprocedure, the growth of Cu doped (0.2M) and Mg

:D-.-------------.Eoo~~25co~CQ)

g2)oo

-+- Pure LADP

-+- Cu-LADP

--(>- Mg-LADP

15r---~--r__,,__,--~25 :Il 35 40 45 ro

Temperature, ·C

Fig. I-Solubility curve for pure LADP, eu-LADP and Mg-LADP crystals

doped (0.2M) LADP crystals was achieved by slowsolvent evaporation at a constant temperature of 30°Cin a period of 30 to 40 days. In the present work, goodsize and optical quality crystals of LADP areconveniently grown employing slow and controlledevaporation method. Figure 2(a) shows thephotograph of as-grown LADP crystal harvested byslow evaporation method. Single crystals with adimension of 21xSx6 mrrr' are grown in a period ofSO to 60 days. The crystals are non-hygroscopic andoptically transparent.

LADP crystal grows as a polyhedron with tenvisible faces of which three are on the top and threeon the bottom. The crystals are very long in the adirection (shortest crystallographic axis). The maintruEk of the crystal has four faces, with planes (01 2)" (012) and their friedels. The planes (l00), (010)and (001) are not morphologically developed. Ihethree faces on one end of the crystal are (1lO), (1 10)and (101). In Figure 3, PQ, which is the edge of th~intersection between the adjacent faces (110) and (1 10) should be the c direction, whereas, b which shouldbe normal to a and c is not a developed edge ordirection.

Figure 2 (b and c) shows the photographs of as-grown crystals of Cu-LADP and Mg-LADP. Singlecrystals of maximum size up to 14x7xS mrrr' (Cu-LADP) and 21x8x6 mrrr' (Mg-LADP) respectivelywere harvested in a period of 30 to 40 days. TheAtomic Absorption Spectroscopic analysis (AAS) ofmetal (Cu and Mg) doped samples of LADP indicatesthat the amount of dopant ions incorporated into thecrystal lattice is far below its original concentration inthe solution (23lO ppm for Cu-LADP and for 1936ppm for Mg-LADP). .

3 Characterization

3.1 Single crystal XRD analysisThe pure and doped LADP single. crystals were

analyzed by single crystal X-ray diffraction method.A single crystal' specimen of suitable size was cutfrom a large crystal of the title compound. The singlecrystallinity was tested using a Leica polarizingmicroscope. The X-ray data were collected on anautomatic X-ray diffractometer (MESSRS ENRAFNON-IUS, The Netherlands). The structure wassolved by the direct method and refined by the full-matrix best-squares technique using the SHELXLprogram. The calculated lattice parameter values arepresented in Table 1. It is observed that the pure

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PRAGASAM et al.: PHOTOCONDUCTING PROPERTIES OF LADP SINGLE CRYSTALS 465

(a) Pure LADP crystal

(c) Mg-LADP crystal

Fig. 2-Photograph of (a) pure LADP, (b) Cu-LADP and(c) Mg-LADP crystals

p t<::::: I -~

I _

,0 t 2 10 12

•II-- - - --

--a--Fig. 3-Morphology of pure LADP crystal

Table I-Lattice parameter of pure LADJ.>tCu~LADP and Mg-LADP crystals;"";::"~'

':*.'.'~'. _.

Measurementsa (A)b (A)c (A)

LADP6.999.6721.73590°90°90°1469.143OrthorhombicP212121

Cu-LADP7.0019.68121.76090°90°90°1474.824OrthorhombicP212121

Mg-LADP6.9899.67321.96090°90°90°1484.537OrthorhombicP212121

ex.~y 0

Volume (A)3Crystal systemSpace group

>

LADP, Cu-LADP and Mg-LADP crystals haveorthorhombic structure with space group of P21212I.In the case of doped LADP crystals variations inlattice parameter as well as cell volume values areobserved. The lattice parameter values for pure LADPare in agreement with the reported value (Petrosyaneta!.ll) .

3.2 NLO property studiesKurtz and Perry proposed a powder SHG method

for comprehensive analysis of the second order non-linearity ". The Kurtz powder SHG experiments havebeen carried out for pure LADP, Cu-LADP and Mg-LADP crystals using the experimental set-up reportedelsewhere". The emissionof green radiation from the

466 INDIAN J PURE & APPL PHYS, VOL 43, JUNE 2005

sample ascertains the SHG property of all thesecrystals. The amplitude of the green radiation wasfound to increase for Cu-LADP (1.2 times) andMg-LADP (1.3 times) when compared to pure LADP.

3.3 FTIR studiesThe FfIR spectra of pure LADP, Cu-LADP and

Mg-LADP crystals were recorded on BRUKKER IFSFf-IR SPECTROMETER using KBr pellet in therange 4000 - 400 cm '. FfIR spectra of pure LADP,Cu-LADP and Mg-LADP are shown in Fig. 4. In theFfIR spectra of pure and metal doped LADP crystals,a broad envelope is seen between 2500 and 3600ern-I. This is because of hydrogen interaction with theother atoms such as N-H stretching of NH2 and C-Hstretching of CH2. The spectra show that twophosphate groups [2(H3P04)] are deprotonated and

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~~----~~--~~~~---=.~~----~~---=~~.Wavenumber. clTft

,~~=r----------~~-----.---------.r-~

20

4000 3000 2O(X) 1,;0 tOOO

Wa~ber. on"

Fig. 4-FrIR spectra of (a) pure LADP, (b) Cu-LADP and (c)Mg-LADP crystals

one is shifted to amino group [(NH3t] and another to(NH2). Hence, it can be concluded that phosphate(H3P04) is bonded to L-arginine through ionization.The C=O stretching of COO- and C-N stretching arewell resolved and positioned at 1722.0 and around2000 cm-I respectively. The intense band around 510cm ' in the' low-energy region is due to P-OHdeformation. The peak set at 1234 cm-I could beattributed to P=O stretching. The FfIR frequencyassignment of pure and metal doped LADP crystals ispresented in Table 2.

3.4 DRS spectral studiesThe DRS analysis of metal (Cu and Mg) doped

LADP crystals was carried out between 200 and 2000om covering the entire near ultraviolet, visible and

Table'2-FflR frequency assignment of pure and metal dopedLADP crystals

Wave number (cm'") AssignmentLADP Cu-LADP Mg-L~P

3424.2 3425.49 3425.66 N-H asymmetric stretching3371.27 3371.15 N-H symmetric stretching

3189.3 3191.41 3191.42 N-H stretching2979.92 2980.05 C-H stretching2645.7 2645.69 C-H asymmetric stretching2360.09 2358.97 C-N stretching1949.56 1949.36 C-N stretching

1722.1 1719.63 1719.88 C=O stretching1678.7 1679.98 1679.26 C=O stretching1656.9 1655.56· 1-655.48 N-H bending1633.2 1633.54 1633.59 NH2 deformation

1570.1 1570.07 N-H bending1534 1534.21 1534.26 NH3+ symmetric deformation,

NH2 in-plane deformation1473,3 1473.57 1473.57 CH2 in plane deformation

1456.49 1456.47 O-H symmetric stretching1395.56 1395.57 C-O symmetric stretching

1333.3 1334.13 1334.06 CH~ wagging1313.1 1313.32 1313.25 C-N stretching1280.5 1281.7 1281.45 P=O stretching1233.3 1234.3 1234 P=O stretching

1169.42 1169.35 NH2 wagging1110.6 1109.47 1109.55 y (P04)1025.3 1023.66 1023.26 C-O stretching

987.33 989.59 C-H out of plane deformation943.8 942.97 943.37 CH2 Rocking

911.55 911.8 CH2Rocking879.4 878.74 878.83 C-C stretching

768.33 768.4 COO- scissoring742.9 742.1 742.21 NH2 out of plane bending

626.47 627.01 O-H out of plane deformation530.3 527.18 527.72 y (P04)510.4 510.32 510.52 P-OH deformation

493.45 493.75 COO- rocking

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PRAGASAM et al.: PHOTOCONDUCTING PROPERTIES OF LADP SINGLE CRYSTALS 467

I1200 -I

-+-PL.re lAOP.11100.

(012) I1000

--- Pt..m LADP-IIiQ. 900 (012)

~ i> 800 1-lIf- Cu-lADP- I:I:

I (012) I700

600 I--Mg-LADP- I2000 i (012)

50060 ~I ------1

20 40

iLoad P (9.8x10.J N) I

1.6·

!=;re~1.5 I (012)(n=1.6893)

1.4 ___ Pure LADP- I

Q. 1.3 (012)(n=1.8229) I8'

...,... Cu-LADP- 1\••.• 1.2

1.1(012)(n=1.781) \i

oI- I III

1 1 1 1 -4-Mg-LADP- 1\500 1000 1500 2000

I (012)(n=1.7472) I

Wavefength, nm 0.9 I IiLOS 1 1.5 2 I

(b) Mg-LADP crystal Logd I

near infrared regions using the Varian Carry 5Emodel spectrophotometer. The spectra are presentedin Fig. 5. In both the spectra, the level of absorption isvery low in the region between 200 and 1350 nm.Since the transparency of the crystals in the entirevisible region reveals their suitability for NLOproperties.

<

3.5 Microhardness studiesMicrohardness studies have been carried out on

pure and metal doped LADP single crystal using aLeitz Wetzlar Vicker's microhardness tester. Thestatic indentations were made at room temperaturewith a constant indentation time of 10 second. Theindentation marks were made on the (012) and (0 1 2)planes by varying the load from 5 to 50g -.Above 50gmicro cracks were observed above the impression andhence readings were not taken for higher loads.Vicker's microhardness values have been calculatedusing the relation:

Hv = 1.8544 pld' M Pa (or) kg mm"

01-"-- --

500 1000Wavelength, nm

1500

(a) Cu-LADP crystal

rr-:

Fig. 5-0ptical absorption spectra of (a) Cu-LADP and (b) Mg-LADP crystals

where p is the applied load and d is the mean diagonallength of the indenter impression.

Graphs have been plotted between hardnessnumber (Hv) and applied load p for both pure andmetal doped LADP crystals. Fig. 6 indicates that theHv values are different for different orientations,suggesting the anisotropic nature of the hardness forpure LADP crystal. Th~ hardness number on (012)plane is more than (0 1 2) plane. The work hardeningcoefficient n of the samples has been determined from .the slope of the log p versus log d plot (Fig. 7).

In the case of metal (Cu and Mg) substitutedLADP, there is a marginal increase in the hardnessvalues, which confirm the presence of metals in thegrown crystals. The microhardness results on LADPcrystals strongly suggest that this class of materialshave moderate mechanical property in comparison toother popular NLO materials 17, 18 like LAP, KB5, ZTS,Urea, NMU, etc.

3.6 Photoconductivity studiesThe photoconductivity of pure LADP, Mg-LADP,

and Cu-LADP crystals were measured using aKeithley 485 picoammeter. The detail of the

Fig. 6-Plot of Hv versus p of pure and doped LADP crystals

Fig. 7-Plot of logp versus log d of pure and doped LADPcrystals

468 INDIAN J PURE & APPL PHYS, VOL 43, JUNE 2005

[--+-- Pure LADP- i

(D.C)_____Pure

i (P.C)

!--ir-Cu-LADP·(D.C)

'""",*-Cu-LADP.. (P.C)

I-.-Mg-LADP_ ,(D.C)

-+-Mg-LADP-(P.C)

500 1000 1500 2000 2500

Electric Field (E) (V/cm)

3000 3500

Fig.8-Plot of applied electric field (E) versus photo current (PC)/ dark current (DC) for pure and metal doped LADP crystals

experimental set-up used in the present work isreported elsewhere". The conductivity of the sampleswere measured by fixing electrodes with silver paint.The dark conductivity of the sample was measured byconnecting the sample in series to a de power supplyand a picoammeter. The electrical contacts were madeat spacing (d) of about 2 mm on the sample usingsilver paint. The de input was increased from 0 to 300V in steps and the corresponding dark current wasrecorded for 'pure and doped LADP crystals. Tomeasure the photoconduction, the sample isilluminated with a halogen lamp (lOOW) by focusinga spot of light on the sample with the help of a convexlens. The de input was increased from 0 to 300 V insteps and the photocurrent was measured by the.electrometer. The variations of photocurrent (lp)and the dark current (ld) with applied field areshown in Figure 8 for pure LADP, Cu-LADP andMg-LADP crystals respectively. It is observedfrom the conductivity plots that both pure and themetal substituted crystals exhibit positivephotoconducti vity.

4 ConclusionIn the present work, metal (Cu and Mg) doped

single crystals of LADP are grown by slowevaporation method in a period of 30 to 45 days. TheAAS result suggests that the amount of metallicdopant incorporated into the crystal lattice is farbelow its original concentration in the solution. TheXRD studies of metal doped crystals reveal that thecrystal structures are the same (orthorhombic) as thatof pure LADP, but the cell volumes are changed dueto doping. In FTIR analysis, the stretching vibrationsof the various functional groups such as NH2, (NH3t,COO- and (H2P04f indicate that the L-argininemolecule might be protonated to give an improvementof the dipolar strength of the crystals. The NLOstudies confirm that the grown crystals are non-linear

in nature and the metal substitution has enhanced thenon-linearity of the crystals. The photoconductivitystudies indicate that both pure and metal substitutedLADP crystals exhibit positive photoconductivity.Owing to its good transparency, chemical stability,mechanical strength and dipolar strength, LADPcrystal is a promising material for NLO applications.

AcknowledgementThe authors thank Prof P Ramasamy and Dr

Dhanasekaran, Crystal Growth Centre, AnnaUniversity, Chennai (Microharness study), Dr BabuVarghese, SAIF, lIT, Chennai (XRD, FTIR and UV),Dr S Ganesan, Department of Physics, AnnaUniversity, Chennai (NLO study) and Dr ARamanand, Head, Department of Physics, LoyolaCollege, Chennai (Growth and photoconductivitystudy) for their help, support and encouragement. Theauthors would like to thank University GrantsCommission, New Delhi, for funding this researchwork.

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