Morphology and Physical Properties of L-Arginine Trifluoroacetate Crystals X. J. Liu, Z. Y. Wang, X. Q. Wang, G. H. Zhang, S. X. Xu, A. D. Duan, S. J. Zhang, Z. H. Sun, and D. Xu*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2270–2274
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong UniVersity, Jinan 250100, China ReceiVed September 30, 2007; ReVised Manuscript ReceiVed January 16, 2008
ABSTRACT: Morphology of a novel semiorganic nonlinear optical crystal, L-arginine trifluoroacetate (+(NH2CNH(CH2)3CH(NH3+)COO- · CF3COO-) has been indexed. The crystal has nine developed facets with major ones (1j 01) (1j 00) and (001). The dielectric constant and dielectric loss were recorded both as function of frequency and temperature in detail. The measured specific heat values of LATF are larger than those of LAP and KDP. The thermal expansion is highly anisotropic along different directions. The refractive indices and optical damage threshold have also been investigated for the first time. 1. Introduction In recent years, an intense worldwide effort has been focused on the design and development of highly efficient organic nonlinear optical (NLO) materials. In the organic class, crystalline salts of amino acids are one of the directions for searching new NLO materials, which recently has attracted considerable interest.1–4 Amino acids and their complexes are promising materials for NLO applications as they contain a proton donor carboxyl acid (-COO) group and the proton acceptor amino (-NH2) group. All the amino acids except glycine contain chiral carbon atom and crystallize in noncentrosymmetric space groups. L-Arginine is an amino acid that forms a series of complexes upon reaction with different acids.5–8 The discovery of NLO properties of L-arginine phosphate monohydrate (LAP) has played an important role both in attracting the attention to crystalline salts of amino acids (in particular to L-arginine9,10) and also working out the conception of semiorganic crystals.11 In our earlier publications,12–15 the details regarding growth aspects, crystal structure, surface morphology and some properties of a new NLO material L-arginine trifluoroacetate (LATF) single crystal have been reported. As a continuation of our research project work, preliminary measurements as to crystal morphology, electrical, thermal, and NLO studies for LATF are discussed in the present contribution, as they are not found in any of the literature. 2. Experimental Procedures The single crystal growth of LATF crystal has been elaborated in our previous study.13 Very recently, larger bulk crystals have been harvested after a period of nearly 30 days. The crystal is found to be highly transparent and absence of visible inclusions and nonhygroscopic in nature. Interestingly, microbial growth was not investigated in the entire growth period even after 1 month. The morphology and different facets of the crystal planes were identified by using a Rigaku D/Max-γA diffractometer. The dielectric studies were performed on (1j 01) plane using HP4592A impedance analyzer. Each sample having dimensions of 5 × 3 × 2 mm3 with conductive silver coating on the opposite facets was placed between the two copper electrodes and thus a parallel plate capacitor was formed. The specific heat was calculated from the DSC curve obtained using a Perkin-Elmer differential scanning calorimeter (Diamond) in the range of 300.02–350.02 K. The thermal expansion coefficients were measured * Corresponding author. Tel.: 86-531-8836-4233. E-mail:
[email protected],
[email protected].
Figure 1. (a) Morphology of LATF single crystal. (b) Photograph of the growth layers observed on (1j 01) face (200×). along [100], [010], [001] and in a direction normal to the (1j 01) plane in temperature range from 298 to 348 K by using a Perkin-Elmer thermomechanical analyzer (TMA). The as-grown crystals were polished in order to make the probe in TMA equipment contact closely with their surfaces, thus guaranteeing the accuracy of the data. Several monochromatic sources in the visible and near IR, generated by Hg, H, He, and Na lamps, were used to measure the values of the refractive indices. The laser-induced damage threshold values on (1j 01) plane was determined using a linearly polarized Nd3+:YLiF4 laser for 20 ns laser pulses at a wavelength of 1053 nm operating in a transverse and longitudinal single mode.
3. Results and Discussion 3.1. Crystal Morphology. It was observed that the growth rate of LATF along the b-axis is much higher than that along the a- and c-axes, which resulted in elongation along the [010] direction. As depicted in Figure 1a, the morphology is a polyhedron with nine symmetrically independent facets. Among these facets, the crystal has three prominent flat facets. The major flat facet is indexed as (1j 01) plane and the other two developed planes have been indexed as (1j 00) and (001). The longest edge of the crystal coincides with b direction, which is the shortest crystallographic axis as in the case of many other well-known crystals.16 The grown crystal belongs to the monoclinic system with lattice parameters a ) 10.581(2) Å, b ) 5.7100(10) Å, c ) 10.861(2) Å, β ) 106.81(3)°.12 The (1j 01) habit facet consists of macro steps, which are illustrated in the optical micrograph (Figure 1b). The linear steps are along the b-direction and are a manifestation of two-dimensional layer growth.
10.1021/cg7009513 CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
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Figure 2. Variation of dielectric constant (a) and dielectric loss (b) of LATF with log frequency at room temperature, respectively; (c) variation of dielectric constant of LATF with temperature. Table 1. Comparative Dielectric Constants of Some L-Arginine Derivative NLO Crystals at the Frequency of 100 KHz at Room Temperature crystal dielectric constant
LATF
LAP21
LADP21
LADI22
5.25
4.78
5.4
4.97
3.2. Dielectric Studies. The dielectric properties are correlated with the electric field distribution within crystal. The prominent (1j 01) plane was polished on a silk cloth with fine-grade alumina powder and methanol as lubricant. A small cylindrical heater whose temperature was recorded by a thermocouple was used to house the samples. Panels a and b in Figure 2 show variation of dielectric constant (r) and dielectric loss (tanδ) dependence of frequency (500 Hz to 5 MHz) at room temperature, respectively. The dielectric constant is relatively high in the lower frequency region and it decreases with increasing frequency and becomes almost saturated beyond 100 kHz. This may be due to the interfacial polarization, in which the mobile charge carriers are interdicted by a physical barrier which restrains generating a localized polarization of the material.17 Table 1 shows that the dielectric constant of LATF is comparable to that of other L-arginine derivative crystals. The higher
Figure 3. Variation of specific heat with temperature.
values of dielectric loss at low frequencies originates from spacecharge polarization mechanism of molecular dipoles, and the
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Figure 4. Orientation schematic diagram of the two processed samples. Table 2. Data of the Thermal Expansion Coefficients of Some NLO Crystals compd
thermal expansion coefficients
LATF LAP27 DLAP27 UDT28 NPNa dihydrate29
R11 ) 51.2 × 10-6 K-1, R31 ) 2.90 × 10-6 K-1 R22 ) 7.50 × 10-6 K-1, R33 ) 16.4 × 10-6 K-1 R11 ) -17.5 × 10-6 K-1, R22 ) 9.60 × 10-6 K-1 R33 ) 92.5 × 10-6 K-1 R11 ) 57.4 × 10-6 K-1, R31 ) 5.00 × 10-6 K-1 R22 ) 8.70 × 10-6 K-1, R33 ) 18.3 × 10-6 K-1 R1 ) 52.3 × 10-6 K-1, R2 ) 38.6 × 10-6 K-1 R3 ) 35.7 × 10-6 K-1 R1 ) 74.8 × 10-6 K-1, R2 ) 4.30 × 10-6 K-1 R3 ) 19.9 × 10-6 K-1
Table 3. Sellmeier coefficients derived from the measured refractive indices Sellmeier coefficients (nm2)
nx
ny
nz
a b c d e
2.13802 0.01741 0.03407 -0.00634 0.03348
2.24612 0.05939 0.00851 -0.04295 0.00883
2.42672 0.06279 0.03308 -0.04512 0.03325
characteristic of low dielectric loss at high frequencies clarifies that the grown samples possess enhanced optical quality with lesser defects.18,19 The plot of dielectric constant as a function of various temperatures at different frequencies is given in Figure 2c. It is obvious that the variation of dielectric constant with temperature is small, which infers that the crystals are of good chemical homogeneity.20 3.3. Specific Heat. The specific heat data of crystalline solids are generally described by the Debye lattice theory in terms of harmonic frequency spectrum. But in our case owing to the complex structure of LATF it is difficult to account for the measured specific heat with the predictions of the lattice theory. The spectral dependence of the specific heat of the temperature is plotted in Figure 3. From this curve, it can be seen that the specific heat of LATF crystal is linear with temperature and increases from 1.182 to 1.452 J g-1 K-1 in the measured temperature range. The molar mass of LATF is 288.24 g/mol, and the Cp of LATF is also equal to 340.70–418.52 J K-1 mol-1 in the temperature range of 300.02–350.02 K. These results suggest that LATF crystal has a comparatively large specific heat, when compared to LAP and potassium dihydrogen phosphate (KDP).23 Intuitively, one can
Figure 5. Dispersion of the three principal refractive indices and calculated curves from the Sellmeier coefficients.
presume that LATF would have much higher laser damage threshold because a rise in temperature with laser irradiation is one of the mechanisms by which damage is caused. 3.4. Thermal Expansion Measurements. The thermal expansion coefficient data is an important parameter in the growth of crystal, not only from the thermophysical property point of view but also from the standpoint of the mechanical behavior of the material. Under intense laser beam irradiation, the optical absorption of the crystal causes thermal gradients that disturb its phase matching properties and even give rise to mechanical stresses. Moreover, if the thermal stresses are sufficiently high, the crystal will fracture. The fracture temperature is inversely proportional to the thermal expansion coefficient.24 For a single crystal under no external influence, its thermal expansion coefficient is compliant with the crystal symmetry. Therefore, for monoclinic LATF crystal, it normally has high anisotropy. The thermal expansion coefficient is a second rank tensor. The thermal expansion tensor referred to axes in the conventional orientation is25
[
a11 0 a31 [aij] ) 0 a22 0 a31 0 a33
]
(1)
Because LATF is monoclinic, one of the principal axes (Y) of the thermal expansion ellipsoid coincides with the crystallographic b axes. The other two principal axes are in the (010) plane, and they can be determined by measuring the thermal expansion coefficients along three arbitrary directions in the (010) plane. Thus, measurements along four different directions including one along the b axis are necessary to describe the thermal expansion ellipsoid. Two samples cut along four different orientations were made from single crystal, as sketched in Figure 4. In the (010) plane, the three orientations ξ ) 0, 90, and 106.81° with respect to the crystallographic c axis were obtained and their measured mean linear thermal expansion coefficients are R1 ) 16.4 × 10-6 K-1, R2 ) 51.2 × 10-6 K-1, and R3 ) 46.7 × 10-6 K-1, respectively. The three coefficients R11, R31, and R33 can be calculated by a method presented in
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Table 4. Experimental Results of the Angle between the Optical Axes 2V λ (nm) 2V(deg)
365.0 77.875
404.7 79.804
435.8 80.697
486.1 81.189
546.1 81.299
the literature.26 The operator ξ formed by the angles ξ1, ξ2, and ξ3 is determined as
[
sin 2ξ1 sin2ξ1 cos 2ξ1 [ξ] ) sin 2ξ2 sin2ξ2 cos 2ξ2 sin 2ξ3 sin2ξ3 cos 2ξ3
]
(2)
The transformation matrix [T] is
[
0 1 0 [T] ) ([ξ]T[ξ]-1[ξ]T) ) 0.1510 1.6550 -1.8060 1 0 0
]
(3)
Therefore, the three crystallographic expansion coefficients can be obtained by means of the following equation
[ ] [][ ]
a11 a1 51.2 a31 ) [T] a2 ) 2.9 × 10-6 K-1 a33 a3 16.4
(4)
In addition, the expansion coefficient R22 in the direction of b axis was also determined experimentally to be 7.5 × 10-6 K-1. The anisotropic thermal expansion measurements were helpful to investigate the cracking problem of LATF crystal. During the bulk crystal growth, temperature gradient is necessarily present and additional gradients are superposed transiently in slow cooling technique. This may be the reason why cracks sometimes appear during the growth process. In particular, the expansion coefficients of LATF were found to be smaller in magnitude than those of some solution growth NLO crystals (Table 2), which make LATF suitable for laser and NLO applications. 3.5. Refractive Indices. According to the space group P21, theLATF single crystal is optically biaxial and it has three corresponding principal refractive indices. The classical V-prism method was used to measure the refractive indices for different wavelengths at room temperature. For that, the incidence direction of the monochromatic collimated light is normal to one side face of the V-prism, and then the light beam passed through the V-prism and the crystal sample. For interpolation of the data, the least-squares routine was used to fit the refractive indices according to the following Sellmeier analytical equation of type
n2(λ) ) a + b/(λ2 - c) - d/(e - λ2)
(5)
where λ is the wavelength in nanometers and a, b, c, d, and e are Sellmeier coefficients. For each refractive indices, the five coefficients are listed in Table 3. The dispersion curves obtained from the fit in terms of the above equation and the experimental values are in good agreement, which is plotted in Figure 5. The crossing angle between the optical axes 2V can be calculated by the equation
tan V )
nz nx
√
n2y - n2x n2z - n2y
(6)
where V is the angle between the largest main axis of the optical indicatrix and the optical axis. As it is seen from Table 4, the
587.5 81.276
589.3 81.277
656.3 80.990
667.8 80.961
706.5 80.829
obtained values of 2V are smaller than 90°, which indicates that LATF crystal is optically positive biaxial crystal and the z axis is bisectrix of acute angle of 2V. 3.6. Laser-Induced Damage Threshold. One of the most important considerations in the choice of a material for NLO applications is its optical damage tolerance. Because of the high optical intensities involved in nonlinear processes, the NLO materials should endure high power intensities.30 For this measurement, 0.1 cm diameter beam is focused onto the sample with a 10 cm focal length lens. A well-polished sample with clean surface was taken for this present study. During laser radiation, the power meter records the power density of the input laser beam for which the crystal becomes damaged. The single pulse energy is 300 mJ. The damage threshold is calculated as follows
I)
4E P E E ) ) ) 2 S tA tπd2 πd t 4
(7)
where I and P are the power density and power of incident light, respectively, E is the single pulse energy, t is pulse width, and A and d are the area and diameter of the circular spot, respectively. The result is found to be 2 GW/cm2 and this value can be enhanced in case for a better crystalline quality. Therefore, LATF crystal possesses a relative high optical damage threshold. The subject of the material damage is of great importance to the design and successful operation of NLO devices. 4. Conclusions High optical quality bulk crystals of LATF have been successfully grown by slow-cooling technique. The crystal is a polyhedron with nine developed facets, and the (1j 01) facet is the most prominent one. The dielectric constant decreases with the increasing frequency but attains the saturation for frequencies larger than 100 kHz. The specific heat changes little in the measured temperature range of 300.02–350.02 K. The thermomechanical analysis shows that the crystal has lower expansion coefficients when compared to many other NLO materials. The refractive indices measurements reveal that the crystal has large values of refractive index and birefringence and is phasematchable. Apart from that, the crystal possesses a relative high optical damage threshold. Hence, the aforesaid results make LATF crystal a good candidate for the NLO applications. Acknowledgment. This work is supported by the grants (Nos. 60608010 and 50772059) of National Natural Science Foundation of China (NNSFC) and the foundation for the Author of National Excellent Doctoral Dissertation of PR China (No. 200539).
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