Cryst. Res. Technol. 41, No. 3, 221 – 224 (2006) / DOI 10.1002/crat.200510563
Habit modification and improvement in properties of potassium
hydrogen phthalate (KAP) crystals doped with metal ions
S. K. Geetha1, R. Perumal1, S. Moorthy Babu*1, and P. M. Anbarasan2
1
2
Crystal Growth Centre, Anna University, Chennai – 600 025, India
Kanchi Mamunivar P. G. Centre, Pondicherry Central University, Pondicherry, India
Received 11 February 2005, accepted 6 April 2005
Published online 15 February 2006
Key words single crystal growth, solution growth, KAP, non-linear optical material.
PACS 81.10.Dn, 42.70.Mp, 78.20.-e
Potassium hydrogen phthalate (KAP) single crystals were grown by slow evaporation and slow cooling
techniques. The growth procedure like temperature cooling rate, evaporation rate, solution pH, concentration
of the solute, supersaturation ratio etc., has been varied to have optically transparent crystals. Efforts were
made to dope the KAP crystals with rubidium, sodium and lithium ions. The dopant concentration has been
varied from 0.01 to 10 mole percent. Good quality single crystals were grown with different concentrations of
dopants in the mother phase. Depending on the concentration of the dopants and the solution pH value, there
is modification of habit. Rubidium ions very much improve the growth on the prismatic faces. The
transparency of the crystals is improved with rubidium and sodium doping. The role of the dopants on the
non-linear optical performance of KAP indicates better efficiency for doped crystals. The grown crystals were
characterized with XRD, FT-IR, chemical etching, Vickers microhardness and SHG measurements. The
influence of the dopants on the optical, chemical, structural, mechanical and other properties of the KAP
crystals was analysed.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Potassium hydrogen phthalate(KAP) belongs to the series of alkali acid phthalates which crystallizes in the
orthorhombic structure with a=9.605Å, b=13.331Å and c=6.473Å and space group Pca21. There are four
chemical units of the formula K(C6H4COOHCOO), in a unit cell of KAP. It has platelet morphology with a
perfect cleavage along the {010} plane. The crystals have excellent physical properties and have a good record
for long term stability in devices [1-3]. The KAP crystals are promising materials for the qualitative and
quantitative X-ray analysis of light elements like Fe, Al, Mg, F, Si, etc., in a long and middle range spectral
area and also as good monochromators [4,5]. Also, the potassium hydrogen phthalates have piezoelectric,
pyroelectric, elastic and non linear properties useful in variety of applications [6-8]. Recently, KAP crystals
have assumed an important role in the epitaxial growth of oriented poly (1,6 –(bis(N-Carbozolyl)-2,4hexadiyne) (Poly DCH), a conjugated polymer which shows a very large (χ (3) = 10-1 esu) and fast (0.8ps)
nonlinear optical susceptibility [4,5]. The importance of the KAP crystal is related to its uniqueness as a
substrate for the growth of oriented poly DCH.
In the present work, the influence of impurities on the habit morphology [8-12], physical, chemical and
mechanical properties of KAP single crystals were studied. The metal ion impurities like sodium, lithium and
rubidium were added to the solution in the form of sodium chloride (NaCl), lithium chloride (LiCl) and
rubidium carbonate (Rb2CO3).
2
Experimental details
KAP was prepared by successive recrystallisation process. The dopant materials were added to the charge as
NaCl, LiCl and Rb2CO3 . Pure and metal ion doped crystals were grown simultaneously by slow evaporation
technique. The mother solution was prepared using the solubility relation, C(T) = 9.283 – 0.059T + 0.0058T2
____________________
* Corresponding author: e-mail: [email protected], [email protected]
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. K. Geetha et al.: Potassium hydrogen phthalate (KAP) crystals doped with metal ions
of KAP in water[5], where C(T) is the solubility of KAP in water (g/100ml of water) and T is the temperature
(ºC). A saturated solution of KAP (1250 ml) at 40˚C was prepared and the solution split up into 250ml portions
for, the simultaneous growth of pure and doped crystals. Sodium chloride concentration in the solution was
varied between 0.01, 0.1, 1 and 2 mol% for sodium doped KAP growth. Lithium chloride concentration was
varied between 1 and 2 mol % for lithium doped KAP growth. Rubidium carbonate concentration was varied
between 1 and 2 mol % for the rubidium doped KAP growth. The seeds obtained from slow evaporation
technique were used. Similar and good quality seeds were selected for the growth. The growth was carried out
in a constant temperature bath, in which the temperature was controlled with a Indtherm temperature
programmer/controller. A cooling rate of 0.1 to 0.3 K/day was employed throughout the growth run. After the
growth run, the harvested crystals (pure and doped KAP) were quickly taken out from the growth chamber and
dipped for a while in n-hexane. The grown crystals were characterized with powder XRD for structural
properties. A Rich Seifert X-ray diffractometer with CuKα radiation (λ = 1.5405 Å) was used. The FTIR
spectrum of pure and doped KAP were recorded with Shimadzu–800, FTIR spectrometer.The second harmonic
generation of the pure and doped crystals was observed using Nd: YAG laser.
3
Results and discussion
Good quality single crystals of pure and doped KAP were grown by the slow cooling method. The as grown
crystals are as shown in fig. 1. The growth rate of doped KAP crystals is low compared with that of pure KAP.
Though, the dopants were of the same valency state, ionic radius and solubility play a major role in the
segregation of the impurities and hence, the doped crystals have low growth rate. The pH of the solution
remains the same throughout the growth of doped KAP. It is anticipated that the dopants may substitute only
for potassium or go to the interstitial sites and do not disturb the H+ ions. The pH of the solution remained
same throughout the growth process during the doped crystal growth. Hence, the doped metal ions were
expected to replace the potassium ion of the KAP without affecting the carboxylic acid group, this is because,
if carboxylate hydrogen is exchanged or substituted there must be change in pH value of the corresponding
solution. The morphology of sodium, lithium and rubidium doped KAP crystals are as shown in figs. 1a, b and
c respectively. The morphology of the as grown crystals follows a similar pattern for the dopant concentrations
added to the mother solution in the present work.
Fig. 1 a) As grown sodium doped KAP crystals, b) As grown lithium doped KAP crystals g, c) as grown rubidium doped
KAP crystals.
Fig. 2 a) XRD spectra for Pure KAP, b) XRD spectra for 2 % sodium doped KAP, c) XRD spectra for 1% Lithium doped
KAP, d) XRD spectra for 2% Lithium doped KAP.
The grown crystals were crushed and subjected to powder X-ray diffraction analysis. Powder X-ray diffraction
pattern corresponding to pure, 2mol% Na doped, 1mol% Li doped and 2mol% Li doped KAP crystals are as
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Cryst. Res. Technol. 41, No. 3 (2006)
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shown in fig. 2a, b, c and d respectively. The diffraction peaks were compared with the values available in the
literature and the peaks match very well the reported values for pure KAP [13,14].
The FT-IR spectra for pure as well as doped KAP crystals were recorded from solid phase samples on a
Bruker IFS 66V model spectrophotometer. The IR spectra were recorded on Shimadzu–800, FTIR
spectrometer. The frequencies for all sharp bands are accurate to ± 1 cm–1. The calculated frequencies agree
favourably with the observed frequencies. The frequencies with their relative intensities obtained in FTIR of
pure and doped KAP and their most probable assignments are presented in table 1. The mid-IR spectrum
obtained for sodium doped KAP crystals with different dopant concentrations was shown in fig. 3.
Table 1 Vibrational frequencies obtained for pure and doped KAP crystals through FTIR studies (Species A/B).
Calculated
frequencies
cm-1
3752
3655
3604
3417
3055
1854
1826
1745
1700
1664
1545
1515
1485
1430
1385
1290
1152
1085
1005
953
905
877
854
820
800
765
715
685
620
573
551
495
453
430
405
Observed IR Frequencies and intensities
Pure
KAP doped with KAP doped with KAP Doped with 1%
KAP
0.01 M NaCl
0.1 M NaCl
by weight NaCl
3750 M
3750
3745
3743 M
3650 M
3640 W
3640 VW
3620 VW
3590 VW
3610
3600
3600 VW
3415 B
3415 B
3415 B
3415 V
3050 VW
3050 VW
3050 VW
3050 VW
1850 W
1850 VW
1825 VW
1825 VW
1743 W
1743 W
1695 M
1695 M
1695 M
1660 M
1650 M
1650 VW
1660 M
1544 S
1562 S
1562 S
1560 M
1515 W
1515 W
1485 W
1485 S
1485 S
1485 W
1434 VW
1420 VW
1420 VW
1420 VW
1382 S
1382 S
1382 S
1386 S
1288 S
1288 S
1288 S
1290 S
1153 M
1151 M
1153 M
1153 W
1087 M
1095 M
1095 M
1089 W
1010 VW
1010 VW
1000 VW
1010 W
950 VW
960 VW
960 VW
960 W
900 VW
910 VW
910 VW
910 W
875 W
875 VW
875VW
875 W
852 M
852 S
852 S
852 W
822 M
810 S
812 S
812 M
798 M
795 VW
795 VW
795 M
767 S
763 S
763 S
763 M
710 S
710 S
721 M
721 M
684 M
680 M
684 M
684 M
625 M
625 M
625 W
625 W
575 W
575 W
575 W
575 W
550 W
550 M
550 M
550 W
500 W
480 VW
500 VW
500 VW
450 W
450 M
450 M
450 VW
425 W
439 W
425 M
425 VW
410 W
408 W
410 W
410 M
Assignments
O-H asymmetrical stretching
O-H symmetrical stretching
Free O-H stretching
O-H stretching Hydrogen Bond
-C-H aromatic stretching
=C-H out of plane bending
C-H out of plane bending
Asymmetrical C=O stretching
Symmetrical C=O stretching Substituted with Na+
C=C stretching
-C= O Carboxylate ion =O Asymmetric stretching
C=C ring stretching
C=C ring stretching
O-H in plane bending
-C═O Carboxylate ion =O Symmetric stretching
C-O stretching
C-C stretching
C-C-O stretching
C-H in plane bending
C-C= stretching
O-H out of plane bending
=C-H out of plane bending
C-H out of plane bending
-C-H bending disubstituted benzene
C-H out of plane bending
C-C stretching
=C-H out of plane deformation
C-O wagging
C=C out of plane beginning
C=C-C out of plane ring deformation
C=C-C out of plane ring deformation
C=C-C deformation
C= plane bending
C=C out of plane ring bending
C=C out of plane bending
In all the spectra, the characteristic OH stretching peaks at 3750, 3650 and 3950 cm-1 are present, indicating the
substitution of sodium on the potassium site rather than on the hydrogen site. There is a large shift towards
higher energy for the peak due to carboxylate groups in the spectra of all the sodium doped crystals. This shift
towards higher energy is due to the replacement of high atomic mass potassium with lower atomic mass
sodium. In addition, the asymmetric stretching vibration of the carboxylate ion remains the same at about 1562
cm–1. The FTIR spectrum corresponding to Li doped KAP is as shown in fig. 4.
As discussed above, the carboxylate ion stretching frequency is shifted to higher energy (1565 cm-1) in this
spectrum also. The FTIR spectrum corresponding to rubidium doped KAP is as shown in fig. 5. The functional
groups present, their environment and the assigned frequencies are similar to the one obtained for Na- as well
as Li-doped crystals. The grown crystals were characterized for their nonlinear optical property using Nd:YAG
laser wavelength( λ at 1064nm). The source was focused on to the sample and the output was seen as a bright
green flash emission. The quantitative measurements of efficiency for the samples are in progress.
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S. K. Geetha et al.: Potassium hydrogen phthalate (KAP) crystals doped with metal ions
Fig. 3 FTIR spectra for sodium doped KAP crystals.
Fig. 4 FTIR spectra for lithium doped KAP crystals.
Fig. 5 FTIR spectra for rubidium doped KAP crystals.
4
Conclusions
Good quality single crystals of pure and doped KAP single crystals were grown by the slow cooling method.
The grown crystals with various dopants have similar morphology to the pure one. The powder X-ray
diffraction analysis reveals the stable lattice on doping in KAP. The FTIR analysis confirm the substitution of
potassium by the dopants rather than the hydrogen in the KAP crystals. The SHG emission was confirmed for
all the doped KAP crystals.
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