eduzhai > Physical Sciences > Materials Sciences >

Vibrational spectra of pure and doped DTGs crystals

  • sky
  • (0) Download
  • 20211030
  • Save
https://www.eduzhai.net American Journal of Materials Science 2015, 5(3A): 48-54 DOI: 10.5923/s.materials.201502.08 Vibrational Spectral Studies of Pure and Doped DTGS Crystals Arun K. J.1,2,*, A. K. Batra1,*, M. D. Aggarwal1, Almuatasim Alomari1 1Department of Physics, Chemistry, and Mathematics (Materials Science Group), Alabama A&M University, Normal, Alabama, USA 2Department of Physics, Sree Kerala Varma College, Thrissur, Kerala, India Abstract Single crystals of deuterated triglycine sulfate (DTGS), DTGS doped with deuterated phosphoric acid (DTGS(0.8)P(0.2)) [DTGSP], and L-alanine (DTGS(0.8)P(0.2)—LA (2 gm)) [DTGSP-LA] are grown from deuterated water solution containing glycine, D2SO4, D3PO4, and L-alanine. Vibrational spectroscopic analysis has been under taken to examine the configuration of glycinium ion formation and the changes in the nature of hydrogen bonding due to the partial substitution of SO42- by PO43-. Fourier Transform Infrared (FT-IR) and FT-Raman analysis indicates the presence of both zwitter and glyciniumions in the doped DTGS crystals, which is due to the low incorporation of dopants into the crystal lattice, which supports Hoshino’s theory of spontaneous polarization reversal. Nuclear magnetic resonance studies (1H, 13C, 31P) revealed the possible bond formation during the formation of the compound. Keywords Ferroelectric crystal, Infrared spectroscopy, Raman spectroscopy, Nuclear magnetic resonance 1. Introduction The TGS crystal family, with the chemical formula (NH2CH2COOH)3⋅H2SO4, is one of the well-known ferroelectric materials useful for room temperature IR detector applications. Pyroelectric detectors based on TGS are uniformly sensitive to radiations in the wavelength range from ultraviolet to far-IR and do not require cooling as compared to quantum detectors, where low cooling is required. The crystal structure of TGS is monoclinic below Tc (49°C). There are two chemical formulas in the elementary cell of the lattice, and the crystal is monoclinic with the space group P21 in the ferroelectric phase. Above the Tc, in the paraelectric phase, there are two mirror planes at y = 0.25 and 0.75, and space group P21/m. The ferroelectric effect is associated with the asymmetrical arrangement of the glycine group G I versus the mirror plane and the proton exchange between glycine groups G II and G III associated with the exchange role of zwitterions between two groups [1]. The b-cut/(010) crystals are used for detector fabrication, which is also a cleavage plane. Single crystals of TGS are usually grown from an aqueous solution by the method of temperature lowering or solution evaporation [2-7, 9-16]. TGS crystals weighing more than 100 g have also been grown from solution with ethyl alcohol addition [8]. * Corresponding author: drarunkj@gmail.com (Arun K. J.) ashobatra@gmail.com (A. K. Batra) Published online at https://www.eduzhai.net Copyright © 2015 Scientific & Academic Publishing. All Rights Reserved To counter the drawbacks cited above while maintaining the same basic structure, many modifications to TGS have been proposed and evaluated for pyroelectric infrared (PIR) detection applications. These modifications include deuteration of TGS (DTGS). L-alanine has the effect of introducing an internal electric field to produce poled TGS crystals and stabilizing the spontaneous polarization. This is because the alanine molecules possess an extra methyl group that prevents them from rotating within the lattice. The dipole it possesses is thus fixed with respect to the crystal structure and does not disappear at the Tc. The L-alanin LATGS crystals can be thermally cycled through the Tc. They will retain their spontaneous polarization, thereby avoiding the poling process. There is an increase in figure of merit (FOM) depending upon the L-alanine content in the TGS lattice. The uniformity of L-alanine doping in the TGS lattice is also difficult to attain. It can be inferred that deuteration of TGS provides a marked improvement to the RV and also increases the Tc to 60°C, depending on the deuteration level. This is extremely desirable as it increases the temperature range of detector operation. Bye and Keve [6] studied the pyroelectric properties of x-ray irradiated/field-treated TGS detectors and compared these with LATGS detectors. The improvements in FOMs of x-ray/field-treated TGS detectors were comparable to L-alanine-doped TGS detectors. In the recent past, doped TGS crystals with organic molecules have been reported with encouraging results such as L-lysine, urea, urea + L-alanine, thiourea, guanidine, and others [11-16]. A higher performance and increase in range of the operational temperature were offered by TGFB American Journal of Materials Science 2015, 5(3A): 48-54 49 (Triglycine fluoberyllate) and DTGFB. However, the growth and processing of these crystals is dangerous due to the presence of toxic beryllium. Deuterated Triglycine (DTGS) crystal has a Curie temperature, which can be raised to 57-62°C thus the temperature ranges of the devices made of it can also be raised. L-alanine doped deuterated triglycine sulfate (DTGS-LA) crystal creates an inherent bias field due to the doping, preventing the crystal from depolarization, and the pyroelectric property is greatly increased. The pyroelectric figure of merit (FOM = p/ε; p-pyroelectric coefficient, ε-dielectric constant) demands low permittivity and high pyroelectric coefficient. Substitution of glycine with alanine has improved the ferroelectric properties. The amino acids mixed TGS have low permittivity values compared with pure TGS. In the present investigation, deuterated phosphoric acid and the amino acid L-alanine have been substituted in order to improve the pyroelectric figure of merit of DTGSP. The amino acid alanine chosen has a similar structure to glycine. Studies on the advantages of DTGS and doped compounds over TGS as IR detectors have been reported. In the present work, pure and doped DTGS, and DTGSP with L-alanine are grown from solution. We have chosen this study to understand the incorporation of dopant and its bonding nature, with a view of obtaining an insight into the structural aspects. 2. Experimental Procedure Deuterated triglycine sulfate (DTGS) was synthesized from the extra pure glycine (99.99%) deuterated sulfuric acid (D2SO4) and deuterated water (D2O) as the solvent according to the reaction 3(NH2CH2COOH) + D2SO4 → (NH2CH2COOH)3 D2SO4 Deuterated triglycine sulfo phosphate (DTGSP) was obtained by the chemical reaction of glycine, deuterated sulfuric acid, deuterated phosphoric acid, and deuterated water as the solvent with the following chemical composition: 3(NH2CH2COOH) + (1-X) D2SO4 + X D3PO4 → (NH2CH2COOH)3 (1-X) D2SO4 X D3PO4 where X= 0.2. DTGSP doped with 2 grams of L alanine (DTGSP-LA) is also prepared in the present study. Impurity concentration was minimized by successive recrystallization. The final solution was filtered using 0.1 µm filter paper and kept in a beaker covered with perforated sheets to avoid contamination. After three days of evaporation, solution becomes supersaturated and small crystals were found to grow inside the beaker and are allowed to grow for a maximum possible dimension, and were carefully harvested and subjected to vibrational spectral characterization studies by Fourier transform infra red (FTIR) and FT-Raman analysis, and is also analyzed by nuclear magnetic resonance studies. FTIR spectral analysis in the range of 500-3500 cm-1 was carried out by Thermo Nicolet FTIR spectrometer with a resolution of 8 cm-1 using Globar (ETC Everglo) as the source of IR Radiation. FT-Raman spectra were recorded with a portable Horiba scientific Raman micro spectrometer in the range of 250-2500cm-1 using 785nm laser source. Nuclear magnetic resonance spectra of the grown samples were recorded with Anasazi Eft-60 NMR spectrometer in the 1H, 31P, and 13C modes. 3. Results and Discussion 3.1. FT-IR Spectroscopic Analysis Figure 1 depicts the FT-IR spectra of pure DTGS, DTGSP, and DTGSP-LA. In order to understand the existence of dopant and its bonding nature, with a view of obtaining an insight into the structural aspects of increased pyroelectric figure of merit, the present study has been undertaken. The observed wave numbers, relative intensities obtained from the recorded spectra, and the assignments proposed for the pure and doped DTGS crystal are given in table 1. Degenerated modes of NH3 bending and C=O, NH4, C-H and O-H stretching vibrations are observed. More bands were located at the same positions as those of pure TGS. Doped samples provide less resolution of bands. Some bands are broadened and some are narrowed. Degeneracy is more for doped samples than that for pure ones of DTGS. The internal modes of vibrations of the polyatomic group are known to occur in the 4000-500 cm-1 range, while the lattice modes of vibrations due to the motion of one polyatomic group relative to another within the unit cell occur in the 500-10 cm-1 range where most inorganic compounds absorb IR radiation. The hydrogen bonding has a tendency to shift the stretching frequencies to a lower number. The vibrational peaks occurring at 1497 cm-1 and 1603 cm-1 are associated with the symmetric and antisymmetric vibrations of coorespectively, and indicate the presence of glycine as a zwitter ion. The strong sharp band at 1703 cm-1 due to the stretching of C=O indicates the presence of the glycine molecule as NH3+CH2COOH (glycinium ion) and strongly suggests the presence of both zwitter and glycinium ions. Normally, in the undoped DTGS crystals, the rear three glycine molecules per molecule of H2SO4, two exist as a glycinium ion and the third in a zwitter ion configuration. The existence of the glycine in the zwitterion configuration does not seem to be convincing for the case of H3PO4-doped TGS samples, because the hydrogen in H3PO4 would prefer to be accepted by the COO- of the zwitterions to form a glycinium ion rather than remain isolated in the crystal lattice [3]. Hence, all glycine molecules would rather exist as glycinium ions. In contrast, the presence of both the zwitter and glycinium ions in doped samples can be 50 Arun K. J. et al.: Vibrational Spectral Studies of Pure and Doped DTGS Crystals understood only in terms of the very low incorporation of H3PO4 into the crystal lattice as evidenced from the compositional analysis. Since no major shifts or additional peaks corresponding to the internal modes in the mid-IR region have been observed, the formation of any other complex compound in the doped DTGS crystals is ruled out. In order to ascertain the presence of phosphorus as PO43or HPO42-, doped samples were investigated in the infra- red region of the spectrum. An absorption peak around 576 cm-1 was observed in all the samples attributing the SO42- ion vibration. The peaks at 563 cm-1 and 542 cm-1 in the doped samples clearly confirm the presence of phosphorous [6] It is found that the IR spectra of doped TGS crystals closely resemble those of the undoped crystals with a slight shift suggesting that the crystal structures are similar with internal symmetry arrangements. Table 1. Observed IR bands and their assignment for un-doped and doped DTGS DTGS wave number (cm-1) DTGS(0.8)P(0.2) wave number ( cm-1) 567w 576w, br 613s 643vw 668m 862m 897s 977vw 1013w 1050w 1078w 1121w 1299s 1375vw 1424m 1495m 1535vw 1569vw 1614w 1702s 1737vw 1862vw 2017-2536vw 563m - 576 611m 644vw 684w 861m 892s 927w 974s 1017vw 1076s 1108-1127m 1300s 1378m 1497s 1528w 1603m 1703s 1739vw 1863w 2008-2538vw DTGS(0.8)P(0.2)-LA wave number (cm-1) 542w 563m 577w 611m 645w 667w,sh 685w,sh 864 896s 933w 976s 1011vw 1050w 1080s 1113m 1305s 1362w 1412 1500s - 1615w 1703w 1738w 1868w 2026-2551 vw Assignment of vibration PO43PO43γCOOνSO42δC-N NH3+ COO-scissoring νS-H δC-C out of plane ρNH3 νP-O-H νC-C-N ωCH2, δC-C νC-N νC=O ρCH2,νC-C δCH2 glycine δCH2 glycine νsCOOνC-S νNH3+ δNH3+ νasCOOνasCOOνasCOOνC=O Amide group Combinations Overtones Figure 1. FTIR Vibrational spectra of pure and doped DTGS crystal ν, stretching; δ, deformation; ρ, rocking; ω, wagging; γ, bending 3.2. FT-Raman Spectral Analysis Raman spectra of the pure and doped DTGS crystals were recorded in the range from 300-1800 cm-1 is shown in figure 2. By using the spectral data of SO4, PO4, CH2, NH3, C-H, and by comparing the spectra of the different samples, we have assigned the observed lines to the vibrations of the characteristic groups. The vibrational assignments are given in table 2. The low frequency vibrations are assigned to vibration and translation of glycine molecules relative to their linking bonds. The vibrations from 319-400cm-1 are assigned to the vibrations and translations of glycine molecule. The relative intensities of the glycine internal vibration remain almost unchanged, which confirms that doping with D3PO4 and L-alanine does not alter the crystal structure with internal arrangements in the symmetries of groups in the unit cell. According to Hoshino [20] one of the glycine groups exists as a zwitter ion while the other two glycine groups have protonated carboxyl groups, which have taken protons from the sulphuric acid. American Journal of Materials Science 2015, 5(3A): 48-54 51 shifted to a considerable range compared to pure DTGS. A change in the intensities of peaks was also observed. The amount of polarizability change will determine the Raman scattering intensity. So, we can infer that the change in the intensity of the peaks may be due to incorporation of dopants. Table 2. Vibrational assignment for observed Raman spectra of pure and doped DTGS crystal DTGS wave number (cm-1) DTGS(0.8)P(0.2) wave number ( cm-1) DTGS(0.8)P(0.2)-LA wave number (cm-1) Assignment of vibration 322 319w - 341 319w - δCCN (GI) δCCN (GII) - 400 393 w δCCN (GIII) 445 443 445 m,sh ν2SO42- 492 496 496 m C-CO bend 573 572 608 609 570 m 609 w C-CO bend ωCOO 621 622 623 m ν4SO42- 659 658 658 sh O-C-O bend ( GIII) - - 767 w - - 813 w - - 847 m 862 863,886, 911 866 m C-C stretch 884 (3 lines, W) 884 C-C stretch 973 1037 1110 1130 1163 - 973 1035 1110 1131 1160 - 975 1038 1110 1145 ν1SO42νC-N NH3 rocking NH3 rocking NH3 rocking NH3 rocking Figure 2. FT - Raman spectra of pure and doped DTGS crystal The spontaneous polarization reversal in TGS is due to the proton transfer between glycine and glycinium ions. The switching mechanism can be confirmed by the existence of a dipolar ion in the unit cell of TGS. The bands at 1359 cm -1 and 1411 cm-1 indicate the presence of the zwitter ion in the DTGS crystal. Considering the fact that PO43- ions are replacing the SO42ions partially as evidenced by the vibrational spectra analysis, the extra proton provided by D3PO4 molecule is expected to combine with the zwitter ion and hence, no transformation takes place between the glycinium ion and the zwitter ions wherever PO43- ions are replacing SO42- ions. The NH3 stretching modes appear as broad bands and therefore, could not be assigned and are not used for analyzing the effect of alanine doping. The bands at 1110 cm-1 and 1604 cm-1 are sensitive to doping. In the FT- Raman spectra of DTGS doped with D3PO4 and D3PO4 with alanine, some peaks were 1304 1316 1411 1438 1479 1535 1604 1672 1703 1320 1413 1438 1480 1538 1605 1674 1738 1269 1304 1359 1411 1440 1481 1603 1676 1740 CH2wagg/twist CH2wagg/twist CH2wagg/twist νC-O Sym (GI/GII) δCH2 δCH2 δNH3 asymm δNH3 asymm νC=O (GI) Amide Amide 3.3. Nuclear Magnetic Resonance (NMR) Spectra Analysis Carbon-13 NMR (13C NMR) is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as the proton NMR identifies 52 Arun K. J. et al.: Vibrational Spectral Studies of Pure and Doped DTGS Crystals hydrogen atoms. The 13 C NMR is directly above the carbon skeleton not just the proton to which it is attached. The number of signals tells the number of different carbons, and the splitting signal tells the number of hydrogen attachment, which depends on resolution and concentration due to the low isotopic abundance. In our study, samples are dissolved in deuterated water and 13C NMR spectra are recorded for DTGS, DTGSP and DTGSP-LA, and are shown in figure 3. The peak at 167 ppm found in all the three samples is due to C=O, and the one around 37 ppm found in all the three samples is due to R3-CH bond, wherein the R groups won’t necessarily be simple alkyl groups. The peak at 46ppm confirms C-NH2 and C-CH3 forms a peak at 12 ppm in the sample, which validate our study. tetra methyl silane (δ=0 ppm) and solvent protons must not be allowed to interfere. Hydrogen nuclei are sensitive to the hybridization of the atom to which it is attached and to electronic effects. De-shielded nuclei resonate at higher δ values whereas shielded nuclei at lower values. Electron withdrawing causes a down field shift of 1-4 ppm for H atoms on aliphatic C atom. The labile protons (-OH, -NH2, -SH) have no characteristic chemical shift. Such resonance can be identified by the disappearance of a peak when treated by D2O as deuterium will replace a protium atom and is called D2O shake. Acidic protons may be suppressed when a solvent containing acidic deuterium ions is used. In our study, we have used acidic D3PO4, D2SO4, and D2O for making DTGS and DTGS doped with deuterated phosphoric acid. The 1H NMR spectra of the samples recorded are shown in figure 4. The peak at 0 ppm corresponds to that of TMS and the one at 1.2 ppm to CH2. Figure 3. 13C NMR spectra of undoped and doped deuterated triglycine sulphate The application of nuclear magnetic resonance in NMR spectroscopy, with respect to hydrogen nuclei within the molecule of a substance, is used to determine the structure of its molecules, is proton (1H) NMR. The isotope of hydrogen, deuterium will not give any peak as its spin is zero. NMR spectra are recorded in solution with a small amount of 0.1% Figure 4. 1H NMR spectra of pure and dopant substituted DTGS 31 P nuclear magnetic resonance is conceptually the same as proton (1H) NMR as both have a nuclear spin of ½, which makes the spectra relatively easy to interpret. 31P NMR is an excellent technique for understanding the structure of American Journal of Materials Science 2015, 5(3A): 48-54 53 organic and metal coordination complexes containing phosphorous. While 1H NMR spectra use tetra methyl silane as the reference, the chemical shift in 31P is referenced with respect to 85% phosphoric acid (δ=0), which is used as an external standard due to its reactivity. Chemical shift in 31P NMR depends on the concentration of the sample, the solvent used, and the presence of other compounds. This is because the external standard does not take into account the bulk properties of the sample. 31P spectra are recorded with all proton signal decoupled 31P-{1H}, giving rise to a single, sharp signal per unique 31P molecule, which depends on the electron environment of each phosphorous nucleus. The 31P NMR spectra recorded for the DTGSP and DTGSP-LA are shown in figure 5. The 85% H3PO4 signal will appear as part of the sample NMR spectrum and thus be set to 0 ppm. In the recorded spectrum, peak appearance at -3.647 ppm at a downfield of -133.42 Hz is indicative of O=P (OR)3 bond in the sample which again splits into -3.53ppm and -3.73ppm at the down field of -129.43 Hz and -136.6 Hz due to the change in the electronic environment due to the addition of 2 gms of L-alanine. spectrum, we confirm the co-existence of PO43- and L-alanine in DTGS crystal, which does not change the crystal structure of symmetries of groups in the unit cell with a possibility of change in the bond length and bond angles, which in turn constitute a change in the pyroelectric figure of merit compared with TGS. FTIR and FT-Raman studies reveal that no major shift or additional peaks occur in their respective spectra, which rules out the possibility of formation of any other complex compound in alanine doped DTGSP crystal. The spontaneous polarization reversal in TGS is due to the proton transfer between glycine and glycinium ions. This switching mechanism can be confirmed by the existence of a dipolar ion in the unit cell. Vibrational spectra of the samples under the present study indicate the presence of the zwitterions, which supports Hoshino's theory for the spontaneous polarization reversal. NMR spectroscopic analysis confirms the formation different functional group in the compound. ACKNOWLEDGEMENTS The authors wish to thank Brian D. Thomas, chemistry department, AAMU, Normal Alabama, USA for his assistance in the recording of vibrational and NMR spectra of the samples. The author, AKB gratefully acknowledges support for this work through the National Science Foundation grant # EPSCoR R-II-3 (EPS-1158862) and DHS grant # 2010-ST-062-000034. Some work was accomplished with the partial support of NSF APEX project and DUE1238192. REFERENCES Figure 5. 31P NMR spectra of substituted and un-substituted deuterated triglycine sulphate 4. Conclusions FTIR studies show that in all the spectra there is broadening of the band due to OH stretch of COOH and NH of –NH3+, and some of the bands are either narrowed or broadened in the spectra of the mixed crystals, which is a clear evidence of the presence of amino acid in the doped crystal. Based on the broadening of the peak in FTIR [1] M. Banan, R. B. Lal, A. K. Batra, and M. D. Aggarwal, Effect of Poling on the Morphology and Growth Rate of TGS Crystals, Cryst. Res. Tech., vol. 24, No. 3, p. K53, 1989. [2] N. Nakatani, Ferroelectric Domain Structure and Internal Bias Field in TGS Crystals, Jpn.. J. Appl. Phys., vol. 30, No. 12A, pp. 3445–3449, 1991. [3] L. Yang, A.K Batra, and R. B. Lal, Growth and Characterization of Triglycine Sulfate (TGS) Crystals Grown by Cooled Sting Technique, Ferroelectrics, vol. 118, Nos. 1–4, pp. 85–91, 1991. [4] M. Banan, R. B. Lal, and A.K Batra, Modified Triglycine Sulfate (TGS) Single Crystals for Pyroelectric Infrared Detector Applications, J. Mater. Sci., vol. 27, pp. 2291–2297, 1992. [5] D. Zhao-De, A New Method of Growth Ferroelectrics Crystal, Ferroelectrics, vol. 39, pp. 1237–1239, 1981. [6] K. L. Bye, and E. T. Keve, Structural Inhibition of Ferroelectric Switching in Triglycine Sulfate X-Ray Treatment, Ferroelectrics, vol. 4, pp. 87–95, 1972. 54 Arun K. J. et al.: Vibrational Spectral Studies of Pure and Doped DTGS Crystals [7] G. M. Loiacono, and J. P. Dougherty, Final Technical Report (contract no DAAK70–77– C–0098) Submitted to Night vision and Electrooptics Laboratories, Fort Belvoir, VA, 1978. [8] M. H. Lee, R. Guo, and A. S. Bhalla, Pyroelectric Sensors, J. Electroceramics, vol. 2, pp. 229–242, 1998. [9] J. Chang, A. K. Batra, and R. B. Lal, Growth and Properties of Urea-Doped Triglycine Sulfate (UrTGS) Crystals, J. Cryst. Growth, vol. 158, pp. 284–288, 1996. [10] J. Novotny, L. Zelinka,; and F. Moracev, Broadband Infrared Detectors on the Basis of PATGS/Pt(Iv) Single Crystals, Sensors and Actuators A, vol. 119, pp. 300–304, 2005. [11] A. K. Batra, M. D. Aggarwal, and R. B. Lal, Growth and Characterization of Doped DTGS Crystals for Infrared Sensing Devices, Mat. Lett., vol. 57, pp. 3943–3948, 2003. [12] G. Su, Y. He, H. Yao, et al., A New Pyroelectric Crystal L-Lycine-Doped TGS (LLTGS), J. Cryst. Growth, vol. 209, pp. 220–222, 2000. [13] K. Meera, R. Muralidharan, A. K. Tripathi, et al., Growth of Thiourea-Doped TGS Crystals and Their Characterization, J. Cryst. Growth, vol. 260, pp. 414–421, 2004. [14] A. Shaulov, W. A. Smith, and N. Y. Rao, Advantages of Obliquely Cut TGS Crystals in Pyroelectric Applications, Ferroelectrics, vol. 38, pp. 967–970, 1981. [15] A. Shaulov, and W. A. Smith, Optimum Cuts of Monoclinic m Crystals for Pyroelectric Detectors, Ferroelectrics, vol. 49, pp. 223–228, 1984. [16] A. Shaulov, Improved Figure of Merit in Obliquely Cut Pyroelectric Crystals, Appl. Phys. Lett., vol. 39, No. 2, pp. 180–183, 1981. [17] P. W. Whipps, and K. L. Bye, Polycrystalline Triglycine Sulfate-Selenate Materials for Pyroelectric Applications, Ferroelectrics, vol. 7, pp. 183–185, 1974. [18] K. L. Bye, P. W. Whipps, and, E. T. Keve, High Internal Bias Fields in TGS (L-alanine), Ferroelectrics, vol. 4, pp. 253–256, 1972. [19] A. Hadni, R. Thomas, and C. Erhard, An Unusual Type of Epitaxial Growth, Phys. Status Solidi (a), vol. 39, pp. 419–424, 2006. [20] S. Hoshino, Y. Okaya, R. Pepinsky, Low frequency Raman spectra of triglycine selenate, Phys. Rev. vol. 115, 323-327, 1959.

... pages left unread,continue reading

Document pages: 7 pages

Please select stars to rate!

         

0 comments Sign in to leave a comment.

    Data loading, please wait...
×