CRYSTAL GROWTH & DESIGN
Growth of Triglycine Sulfate Single Crystals Doped with Pt(IV) and L-Alanin
2003 VOL. 3, NO. 3 393-395
Jan Novotny´,* Zdislava Podvalova´, and Jirı´ Zelinka Department of Optoelectronics, Institute of Radio Engineering and Electronics, Academy of Sciences of the Czech Republic, Prague, Czech Republic Received February 20, 2003
ABSTRACT: Single crystals of triglycine sulfate (TGS) doped with Pt4+ and L-alanin (LATGS/Pt(IV)) were grown from aqueous solution by means of the slow cooling method. Surface morphology, domain structure, and P-E hysteresis loops have been investigated. The possibility of catalyzed growth of 〈001〉 crystal pyramids on the basis of metal-glycine complexes is suggested. We have found on the basis of experimental results that LATGS/Pt(IV) crystals are excellent materials for construction of infrared detectors. Introduction The exploitation of the ferroelectric triglycine sulfate (TGS) in some technical applications1 has increased the need for high-quality single crystals with stabilized domain structures.2 Pure TGS shows a typical secondorder ferroelectric phase transition at the Curie point TC ) 49 °C. Below TC, when the crystal possesses the polar symmetry of group 2 of the monoclinic system, spontaneous polarization Ps arises along the b-axis. The real crystals are split into antiparallel domains. Ferroelectric properties, mainly internal electric field Eb, dielectric losses tan δ and domain structure have been found to change under the influence of various doping elements.3 The first information on properties of the doped TGS single crystals with Pt2+ was reported in 1985.4 We propose on the basis of glycine complex structure that doped TGS crystals with Pt4+ and with L-alanin could contain better properties from the point of view of pyroelectric properties.5 In this paper, we report the procedure of single crystal growth of full-facetted LATGS/Pt(IV) crystals from solutions with various content of PtCl4 and L-alanin as well as the effect of doping on the crystal form and growth rate. On samples prepared from different growth pyramids of grown crystals, we preferentially studied the shape of domain structure and parameters which are determined from P-E hysteresis loops. We also discuss the use of LATGS/Pt(IV) grown crystals as materials for infrared detector preparation. Experimental Procedures TGS single crystals doped with Pt4+ and with L-alanin were grown from aqueous solutions by the slow cooling method.6 The growth process was performed in the range of 45-30 °C; the temperature was controlled by an automatic program with a cooling rate of 0.02 °C/h. A survey of selected doped LATGS/ Pt(IV) grown crystals is in Table 1. Typical habit of LATGS/Pt(IV) grown crystals (1.5 mol % PtCl4 and 47 mol % L-alanin in the growth solution) is shown in Figure 1. In this figure, the habit of pure TGS single crystals is presented for comparison. * To whom correspondence should be addressed. Institute of Radio Engineering and Electronics, Academy of Science of the Czech Republic,Chaberska´ 57,Prague18251,CzechRepublic.E-mail:
[email protected].
Figure 1. General crystal forms of pure and doped TGS single crystals. Table 1. Survey of Doping Levels of LATGS/Pt(IV) Single Crystals crystal
PtCl4 (mol %)
LATGS/Pt(IV)
0.38 1.1 1.5
L-alanin
(mol %)
53 47 47
The main reason for the change in crystal morphology is the modification of the growth rates in crystallographic axes. We found from kinetic growth rate data that growth velocity of nonpolar 〈001〉 and 〈101〉 sectors of the doped TGS crystals are higher than those for pure TGS single crystals. The growth results are presented in Figure 2. This phenomenon is very important for defined growth of nonpolar 〈001〉 TGS pyramids doped with some heavy metal ions destined for applications.7 The domain structure is a very important factor in the preparation of pyroelectric detectors8 because for this purpose we need the crystal with reduce domain mobility in the form of “internal” electric field, the so-called internal bias field Eb. Therefore, we have revealed the static domain structure by room etching of the samples that were b-cut from 〈110〉 growth pyramids. The visualized domain structure was investigated by Nomarski contrast optical microscopy. The typical domain structure of LATGS/Pt(IV) and pure TGS single crystals is in Figure 3. In this figure, we see that TGS single crystals doped with Pt(IV) ions possess a nearly single domain structure which is important for construction of pyroelectric detectors. To measure the dependence of spontaneous polarization P on the electric field E (P-E hysteresis loops), a significant number of b-plate samples were cut from 〈001〉 and 〈101〉 pyramids by a wet-thread saw and by cleavage. Hysteresis loops were observed with the help of the Sawyer-Tower circuit
10.1021/cg034027g CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003
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Figure 2. Dependence of the growth rate of the 〈001〉 face of LATGS/Pt(IV) crystals.
Novotny´ et al.
Figure 5. Distribution of coercive field Ec on the cross section of single crystals.
Figure 3. Typical domain structure doped (a) and nondoped (b) TGS single crystals.
Figure 6. Distribution of dielectric constant r on the cross section of single crystals.
Figure 4. Distribution of spontaneous polarization Ps on the cross section of single crystals. at an applied voltage of 250 V, frequency of 50 Hz, and temperature of 22 °C. The main physical properties of TGS doped grown crystals have been determined on the basis of hysteresis loop measurements. These results are demonstrated in Figures 4-7 for LATGS/Pt(IV) crystals, marked in Table 1. We have found from these measurement data that LATGS/ Pt(IV) crystals are highly suitable for infrared applications, such as military systems, astronomical telescopes, earth observation cameras, gas analyzer monitors, and Fourier transform infrared instrumentations. Therefore, we measured the pyroelectric coefficient p, permittivity r, and dielectric losses tan δ. From the obtained data, we calculated figures of merit9 of prepared detectors,10 which are defined by the following equations:
p F1 ) , cj
FU )
p , jcr
FD )
p cj(r tg δ)1/2
where cj is the volume specific heat of TGS. The measured data and calculated figures of merit for our detectors are shown in Table 2.
Figure 7. Distribution of internal field Eb on the cross section of single crystals.
Results and Discussion It has been ascertained by morphological studies that impurities Pt4+ with L-alanin have a great influence on crystal habit. The rate growth of LATGS/Pt(IV) crystals in the direction of the c-axis is higher than that of pure TGS single crystals. These two types of impurities, e.g., PtCl4 and L-alanin, have the most profound influence on physical properties of grown crystals. From physical measurements, we found that values of Ec, Eb, and tan δ are sufficiently homogeneous in volume. The growth
Triglycine Sulfate Single Crystals
Crystal Growth & Design, Vol. 3, No. 3, 2003 395
Table 2. Materials Constants of Infrared Detectors Prepared from LATGS/Pt(IV) Single Crystals samples
1
2
4
5
average
units
Ec Eb Ps p dp/dT r tg δ FU FI FD
114 511 0.0256 433.3 4.0 32.2 0.0025
120 523 0.0248 405.4 3.9 32.1 0.002
119 323 0.0246 460.9 4.0 31.6 0.003
103 297 0.0245 455.2 4.1 31.2 0.0025
114 414 0.0249 440 4.0 31.8 0.0025 0.6 1.7 × 10-10 2 × 10-4
kV/m KV/m C/m2 µC/m2K %/°C m2/C m/V Pa-1/2
velocity of doped TGS crystals in direction of 〈001〉 and 〈101〉 growth pyramids are higher than that ofpure TGS crystals and depends on concentration of Pt4+ in the growth solution. Increase of the growth rate of the 〈001〉 plane of doped crystals is caused by a decrease of the of the deposition process.11 This Gibbs energy ∆G (001) h fact is very important for the growth of high volume nonpolar 〈001〉 growth pyramid of the doped crystals.12 We suggest for this purpose a special flow-type crystallizer for the growth of large crystals on oriented planar crystal seeds.13 We suppose that in the new crystallizer under precise growth conditions (constant supersaturation and defined hydrodynamic conditions) we can obtain LATGS/Pt(IV) single crystals with high structural quality and with homogeneous physical properties for the manufacture of high sensitivity pyroelectric detectors suitable for infrared applications.
Acknowledgment. We wish to thank F. Sroba´r for advice and helpful discussions. The work was supported by the Academy of Sciences of the Czech Republic under project No. S2067204 and Key Research Area KSK 1010601. References (1) Whatmore, R. W. Rep. Prog. Phys. 1986, 49, 1335-1386. (2) Banan, M.; Lal, R. B.; Batra, A. J. Mater. Sci. 1992, 27, 2291-2297. (3) Lal, R. B.; Batra, A. K. Ferroelectrics 1993, 142, 51-60. (4) Brezina, B.; Havra´nkova´, M. Ferroelectrics Lett. 1985, 4, 8187. (5) Novotny´, J. et al. Internal report Z-2032/A, IREE, Prague 2000. (6) Prokopova´, L.; Novotny´, J.; Micka, Z.; Malina, V. Cryst. Res. Technol. 2001, 11, 1189-1195. (7) Novotny´, J.; Prokopova´, L.; Micka, Z. J. Cryst. Growth 2001, 226, 333-340. (8) Neumann, N. Ferroelectrics 1993, 142, 83-92. (9) Whatmore, R. W.; Osbond, P. C.; Shorrocks, N. M. Ferroelectrics 1987, 76, 351-359. (10) Novotny´, J. et al. Internal report Z-2043/A, IREE, Prague 2001. (11) Novotny´, J. Proceedings of SPIE, Vol 4710. Int. Conf. Thermosense XXIV, 1-4 April 2002, Orlando, FL, USA. Int. Conf. Thermosense XXIV, 2002. (12) Brezina, B; Havra´nkova´, M.; Va´×f0a M. Cryst. Res. Technol. 1992, 27, 13-20. (13) Novotny´, J. Proc. of Int. Conf. Material Weeks, Munich, Germany, 2001.
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