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Temperature dependence of infrared emissivity properties of (La0.8Sr0.2)1−xMnO3
Journal of Alloys and Compounds 474 (2009) 375–377
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Temperature dependence of infrared emissivity properties of (La0.8 Sr0.2 )1−x MnO3 Xingmei Shen ∗ , Guoyue Xu ∗∗ , Chunming Shao, Chuanwei Cheng College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Yu Dao Street 29, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 19 June 2008 Accepted 21 June 2008 Available online 29 July 2008 Keywords: Ceramics Solid-state reaction Optical properties Light absorption and reflection
a b s t r a c t Non-stoichiometric compounds of (La0.8 Sr0.2 )1−x MnO3 (0.1 ≤ x ≤ 0.4) were prepared by standard solidstate reaction method. The structure, infrared absorption and normal emissivity (εN ) properties of the compounds were systematically investigated. The X-ray powder diffraction (XRD) results indicate that the structure of the samples is distorted rhombohedral but not cubic. The mechanism of new infrared absorption occurring around 719–922 cm−1 is proposed. The εN of the samples in the 8–14 m waveband increases with increasing temperature and changes significantly in the temperature range 288–313 K due to the metal-insulator transition. Moreover, the εN value for x = 0.2 is higher than that for the other samples, owing to two strong infrared absorption peaks at 719 cm−1 and 856 cm−1 . The εN has no drastic change with increasing temperature in the 3–5 m waveband. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Variable-emissivity materials have received considerable attention due to their potential applications in thermal control devices of modern spacecraft [1], optical switching element [2] and smart window [3,4]. Perovskite-type La1−x Srx MnO3 is a very interesting material that has been studied. It has been reported that La1−x Srx MnO3 compounds at certain doping levels undergo metalinsulator (MI) phase transition near transition temperature (TMI ) [5–8]. The above-mentioned phenomenon has been explained by means of double-exchange interaction between Mn3+ and Mn4+ , and electron-phonon interaction relating to Jahn–Teller-type lattice distortion of the MnO6 octahedra [9–11]. Recently, G. Tang et al. [12] have reported that, owing to MI phase transition indicated by electrical resistivity and infrared reflection, La1−x Srx MnO3 compounds exhibit temperature-dependent infrared emissivity properties. In the vicinity of Sr doping level x = 0.2 (La0.8 Sr0.2 MnO3 ), the infrared emissivity shows a remarkable variation. The influence of A-site cation on properties of lanthanum manganites were studied by G. Venkataiah et al. [13]. Apart from A-site cation, the size variance factor also influences the double-exchange interaction. In fact, if the content in A-site for La1−x Srx MnO3 is deficient, the metal-insulator transition will also occur [14–17]. However, studies on the effect of deficient levels on infrared emissivity properties
∗ Corresponding author. Tel.: +86 25 84892903; fax: +86 25 84892951. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Shen), [email protected] (G. Xu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.085
of non-stoichiometric manganese oxides have not been reported yet. In this work, we investigate the structure, infrared absorption, temperature dependent infrared emissivity properties in the 3–5 and 8–14 m wavebands of (La0.8 Sr0.2 )1−x MnO3 (0.1 ≤ x ≤ 0.4) compounds. 2. Experimental procedure Non-stoichiometric samples of (La0.8 Sr0.2 )1−x MnO3 (x = 0.1, 0.2 and 0.4) were prepared by standard solid-state reaction method. La2 O3 , SrCO3 and MnO2 were used as raw materials and La2 O3 was fired in air at 1173 K for 7 h before use. Ethanol was added as a milling medium together with the raw materials. After milling for 12 h, the mixture was air-dried at 353 K to remove the ethanol and calcined at 1273 K, and then pressed into discs with polyvinyl alcohol. The samples were finally sintered at 1473 K for 24 h. The structure of the samples was characterized by Bruker D8 X-ray powder diffraction (XRD) using Cu K␣ radiation ( = 0.15405 nm) operated at 40 kV and 40 mA. Infrared absorption (IR) spectra were measured by NEXUS-670 Fourier transform infrared spectrophotometer. The infrared normal emissivity (εN ) in the 3–5 and 8–14 m wavebands was measured by the IR-2 infrared emissometer from 288 K to 373 K.
3. Results and discussion Fig. 1 shows the XRD patterns of (La0.8 Sr0.2 )1−x MnO3 (x = 0.1, 0.2 and 0.4) samples, respectively. All of the samples exhibit the characteristic peaks of the perovskite structure. Note that diffraction peaks for 2 = 32.4◦ , 40.2◦ , 52.7◦ , 58.4◦ , 67.7◦ and 77.2◦ split into two peaks respectively for x = 0.1, 0.2 and 0.4 samples. It can be seen clearly from the enlarged pattern (as shown in Fig. 1 inset) that the (1 1 0) peak splits into (1 1 0) and (1 0 4) peaks. The results indicate that the structure of the samples is distorted rhombohedral but not cubic.
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X. Shen et al. / Journal of Alloys and Compounds 474 (2009) 375–377
Fig. 3. Schematic diagram of infrared absorption.
Fig. 1. X-ray diffraction patterns of (La0.8 Sr0.2 )1−x MnO3 samples. The inset shows enlarged patterns for 2 = 30–35◦ .
Fig. 2 shows the infrared absorption spectra of the samples with different deficient levels. The peaks appearing at 611 cm−1 (x = 0.1, 0.4) and 580 cm−1 (x = 0.2) can be attributed to the Mn–O stretching vibration. Usually, there are three phonon modes in cubic perovskite: La site external mode, Mn–O–Mn bending mode and Mn O stretching mode [18,19]. However, in our samples, new absorption peaks at 920 cm−1 (x = 0.1, 0.4), 719 and 856 cm−1 (x = 0.2) were observed in the spectra, because the lattices of the samples are not ideally cubic but distorted. This is consistent with the XRD results shown in Fig. 1. Spontaneous polarization is expected to exist in rhombohedral samples along cubic axis. When a sample is irradiated by infrared light at a certain frequency, forces acting on dipoles in the periodic field lead to the increase or decrease of dipole moments (), as can be seen in Fig. 3. The vibrating dipole moments result in the infrared absorption. Therefore, we suggest that the new infrared absorption is due to the coupling effect between infrared light and spontaneous polarization of rhombohedral sample. Fig. 4 shows the temperature dependence of infrared emissivity (εN ) of (La0.8 Sr0.2 )1−x MnO3 in the 8–14 m waveband. From the figure, it can been seen that εN of the samples increases with increasing temperature and changes significantly in the temperature range 288–313 K due to the metal-insulator transition [12]. For
Fig. 2. The infrared absorption spectra of (La0.8 Sr0.2 )1−x MnO3 samples with different deficient levels.
x = 0.1 sample, the εN changes below 313 K and remains constant above 313 K. A similar result is observed for x = 0.4 sample. Note that the εN value is much higher for x = 0.2 than that for the other samples. According to Kirchhoff law concerning the equality of the emissivity and absorptance, this may be attributed to two strong absorption peaks in the 8–14 m waveband appearing at 13.9 m (719 cm−1 ) and 11.7 m (856 cm−1 ), as shown in Fig. 2. Absorption peaks for x = 0.1 and 0.4 appearing at 10.8 m (920 cm−1 ) are weaker than that for x = 0.2 sample. The vacancy levels have effects on the Mn4+ /Mn3+ ratio [20]. When the vacancy level in A-site is low (x = 0.1), it would not exert strong effects on the sample. As the vacancy level is increased (x = 0.2), Mn ions would move into A site in order to fill in the hole to maintain steady structure, and then the bonds of Mn–O–Mn change, resulting in a weakening of the doubleexchange interaction and the decrease of the Mn4+ /Mn3+ ratio [21]. When the vacancy level is over 0.2 (x = 0.4), the above-mentioned Mn4+ /Mn3+ ratio continues to decrease. On the other hand, with the vacancy level increasing, polarons relating to electron–phonon interaction may play a leading role, which lead to the increase of carrier [22]. The range of variation for εN (x = 0.2) is close to 0.2 in the temperature range 288–373 K and 0.15 from 288 K to 313 K. The dot curve in the figure shows dependence of the εN on decreasing temperature. The trend is similar to that for increasing temperature. That is to say, εN of the samples can be varied reversibly with the temperature. Fig. 5 shows the temperature dependence of the εN in the 3–5 m waveband. The results indicate that εN of the samples has no drastic change with increasing temperature in the 3–5 m waveband and is lower than that in the 8–14 m waveband. This is
Fig. 4. The temperature dependence of normal emissivity of (La0.8 Sr0.2 )1−x MnO3 samples in the 8–14 m waveband. The dot curve is the dependence of the εN on decreasing temperature.
X. Shen et al. / Journal of Alloys and Compounds 474 (2009) 375–377
377
Acknowledgements We are thankful for the financial support provided by the National Natural Science Foundation of China (grant 90505008) and Weaponry Equipment Pre-research Foundation of China. References
Fig. 5. The temperature dependence of normal emissivity of (La0.8 Sr0.2 )1−x MnO3 samples in the 3–5 m waveband.
because that no absorption peak appears around 2000–3333 cm−1 (3–5 m). 4. Conclusions In summary, non-stoichiometric (La0.8 Sr0.2 )1−x MnO3 (0.1 ≤ x ≤ 0.4) system is one of variable-emissivity compounds. The relationship between temperature and εN of the samples in the 8–14 m waveband is different from that in the 3–5 m waveband. The non-stoichiometric (La0.8 Sr0.2 )1−x MnO3 compounds may have the potential for applications as thermal control material.
[1] K. Shimazaki, S. Tachikawa, A. Ohnishi, Y. Nagasaka, Int. J. Thermophys. 22 (2001) 1549. [2] D. Vernardou, M.E. Pemble, D.W. Sheel, Chem. Vap. Depos. 13 (2007) 158. [3] H. Kakiuchida, P. Jin, M. Okada, M. Tazawa, Jpn. J. Appl. Phys. 46 (2007) 621. [4] C. Piccirillo, R. Binions, I.P. Parkin, Chem. Vap. Depos. 13 (2007) 145. [5] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [6] R. Dhahri, F. Halouni, J. Alloys Compd. 385 (2004) 48. [7] P. Kameli, H. Salamati, A. Aezami, J. Alloys Compd. 450 (2008) 7. [8] S. Zemni, J. Dhahri, et al., J. Alloys Compd. 392 (2005) 55. [9] C. Zener, Phys. Rev. 82 (1951) 403. [10] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [11] A. Dhahri, J. Dhahri, et al., J. Alloys Compd. 450 (2008) 12. [12] G. Tang, Y. Yu, W. Chen, Y. Cao, J. Alloys Compd. 461 (2008) 486. [13] G. Venkataiah, V. Prasad, P. Venugopal Reddy, J. Alloys Compd. 429 (2007) 1. [14] V. Ferris, G. Goglio, L. Brohan, O. Joubert, P. Molinie, M. Ganne, Mater. Res. Bull. 32 (1997) 763. [15] R. Suryanarayanan, V. Gasumyants, N. Ageev, J. Magn. Magn. Mater. 211 (2000) 226. [16] H. Vincent, M. Audier, S. Pignard, G. Dezanneau, J.P. Senateur, J. Solid State Chem. 164 (2002) 177. [17] J.Y. Yu, S.Y. Zhang, et al., Solid State Commun. 142 (2007) 333. [18] T. Arima, Y. Tokura, J. Phys. Soc. Jpn. 64 (1995) 2488. [19] K.H. Kim, J.Y. Gu, et al., Phys. Rev. Lett. 77 (1996) 1877. [20] Y. Nglee, F. Sapina, E. Martinerz Tamayo, et al., J. Mater. Chem. 7 (1997) 1905. [21] A. Maignan, C. Martin, et al., Phys. Rev. B. 60 (1999) 15214. [22] G.M. Zhao, K. Conder, et al., Nature 381 (1996) 676.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Temperature dependence of infrared emissivity properties of (La0.8 Sr0.2 )1−x MnO3 Xingmei Shen ∗ , Guoyue Xu ∗∗ , Chunming Shao, Chuanwei Cheng College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Yu Dao Street 29, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 19 June 2008 Accepted 21 June 2008 Available online 29 July 2008 Keywords: Ceramics Solid-state reaction Optical properties Light absorption and reflection
a b s t r a c t Non-stoichiometric compounds of (La0.8 Sr0.2 )1−x MnO3 (0.1 ≤ x ≤ 0.4) were prepared by standard solidstate reaction method. The structure, infrared absorption and normal emissivity (εN ) properties of the compounds were systematically investigated. The X-ray powder diffraction (XRD) results indicate that the structure of the samples is distorted rhombohedral but not cubic. The mechanism of new infrared absorption occurring around 719–922 cm−1 is proposed. The εN of the samples in the 8–14 m waveband increases with increasing temperature and changes significantly in the temperature range 288–313 K due to the metal-insulator transition. Moreover, the εN value for x = 0.2 is higher than that for the other samples, owing to two strong infrared absorption peaks at 719 cm−1 and 856 cm−1 . The εN has no drastic change with increasing temperature in the 3–5 m waveband. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Variable-emissivity materials have received considerable attention due to their potential applications in thermal control devices of modern spacecraft [1], optical switching element [2] and smart window [3,4]. Perovskite-type La1−x Srx MnO3 is a very interesting material that has been studied. It has been reported that La1−x Srx MnO3 compounds at certain doping levels undergo metalinsulator (MI) phase transition near transition temperature (TMI ) [5–8]. The above-mentioned phenomenon has been explained by means of double-exchange interaction between Mn3+ and Mn4+ , and electron-phonon interaction relating to Jahn–Teller-type lattice distortion of the MnO6 octahedra [9–11]. Recently, G. Tang et al. [12] have reported that, owing to MI phase transition indicated by electrical resistivity and infrared reflection, La1−x Srx MnO3 compounds exhibit temperature-dependent infrared emissivity properties. In the vicinity of Sr doping level x = 0.2 (La0.8 Sr0.2 MnO3 ), the infrared emissivity shows a remarkable variation. The influence of A-site cation on properties of lanthanum manganites were studied by G. Venkataiah et al. [13]. Apart from A-site cation, the size variance factor also influences the double-exchange interaction. In fact, if the content in A-site for La1−x Srx MnO3 is deficient, the metal-insulator transition will also occur [14–17]. However, studies on the effect of deficient levels on infrared emissivity properties
∗ Corresponding author. Tel.: +86 25 84892903; fax: +86 25 84892951. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Shen), [email protected] (G. Xu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.085
of non-stoichiometric manganese oxides have not been reported yet. In this work, we investigate the structure, infrared absorption, temperature dependent infrared emissivity properties in the 3–5 and 8–14 m wavebands of (La0.8 Sr0.2 )1−x MnO3 (0.1 ≤ x ≤ 0.4) compounds. 2. Experimental procedure Non-stoichiometric samples of (La0.8 Sr0.2 )1−x MnO3 (x = 0.1, 0.2 and 0.4) were prepared by standard solid-state reaction method. La2 O3 , SrCO3 and MnO2 were used as raw materials and La2 O3 was fired in air at 1173 K for 7 h before use. Ethanol was added as a milling medium together with the raw materials. After milling for 12 h, the mixture was air-dried at 353 K to remove the ethanol and calcined at 1273 K, and then pressed into discs with polyvinyl alcohol. The samples were finally sintered at 1473 K for 24 h. The structure of the samples was characterized by Bruker D8 X-ray powder diffraction (XRD) using Cu K␣ radiation ( = 0.15405 nm) operated at 40 kV and 40 mA. Infrared absorption (IR) spectra were measured by NEXUS-670 Fourier transform infrared spectrophotometer. The infrared normal emissivity (εN ) in the 3–5 and 8–14 m wavebands was measured by the IR-2 infrared emissometer from 288 K to 373 K.
3. Results and discussion Fig. 1 shows the XRD patterns of (La0.8 Sr0.2 )1−x MnO3 (x = 0.1, 0.2 and 0.4) samples, respectively. All of the samples exhibit the characteristic peaks of the perovskite structure. Note that diffraction peaks for 2 = 32.4◦ , 40.2◦ , 52.7◦ , 58.4◦ , 67.7◦ and 77.2◦ split into two peaks respectively for x = 0.1, 0.2 and 0.4 samples. It can be seen clearly from the enlarged pattern (as shown in Fig. 1 inset) that the (1 1 0) peak splits into (1 1 0) and (1 0 4) peaks. The results indicate that the structure of the samples is distorted rhombohedral but not cubic.
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Fig. 3. Schematic diagram of infrared absorption.
Fig. 1. X-ray diffraction patterns of (La0.8 Sr0.2 )1−x MnO3 samples. The inset shows enlarged patterns for 2 = 30–35◦ .
Fig. 2 shows the infrared absorption spectra of the samples with different deficient levels. The peaks appearing at 611 cm−1 (x = 0.1, 0.4) and 580 cm−1 (x = 0.2) can be attributed to the Mn–O stretching vibration. Usually, there are three phonon modes in cubic perovskite: La site external mode, Mn–O–Mn bending mode and Mn O stretching mode [18,19]. However, in our samples, new absorption peaks at 920 cm−1 (x = 0.1, 0.4), 719 and 856 cm−1 (x = 0.2) were observed in the spectra, because the lattices of the samples are not ideally cubic but distorted. This is consistent with the XRD results shown in Fig. 1. Spontaneous polarization is expected to exist in rhombohedral samples along cubic axis. When a sample is irradiated by infrared light at a certain frequency, forces acting on dipoles in the periodic field lead to the increase or decrease of dipole moments (), as can be seen in Fig. 3. The vibrating dipole moments result in the infrared absorption. Therefore, we suggest that the new infrared absorption is due to the coupling effect between infrared light and spontaneous polarization of rhombohedral sample. Fig. 4 shows the temperature dependence of infrared emissivity (εN ) of (La0.8 Sr0.2 )1−x MnO3 in the 8–14 m waveband. From the figure, it can been seen that εN of the samples increases with increasing temperature and changes significantly in the temperature range 288–313 K due to the metal-insulator transition [12]. For
Fig. 2. The infrared absorption spectra of (La0.8 Sr0.2 )1−x MnO3 samples with different deficient levels.
x = 0.1 sample, the εN changes below 313 K and remains constant above 313 K. A similar result is observed for x = 0.4 sample. Note that the εN value is much higher for x = 0.2 than that for the other samples. According to Kirchhoff law concerning the equality of the emissivity and absorptance, this may be attributed to two strong absorption peaks in the 8–14 m waveband appearing at 13.9 m (719 cm−1 ) and 11.7 m (856 cm−1 ), as shown in Fig. 2. Absorption peaks for x = 0.1 and 0.4 appearing at 10.8 m (920 cm−1 ) are weaker than that for x = 0.2 sample. The vacancy levels have effects on the Mn4+ /Mn3+ ratio [20]. When the vacancy level in A-site is low (x = 0.1), it would not exert strong effects on the sample. As the vacancy level is increased (x = 0.2), Mn ions would move into A site in order to fill in the hole to maintain steady structure, and then the bonds of Mn–O–Mn change, resulting in a weakening of the doubleexchange interaction and the decrease of the Mn4+ /Mn3+ ratio [21]. When the vacancy level is over 0.2 (x = 0.4), the above-mentioned Mn4+ /Mn3+ ratio continues to decrease. On the other hand, with the vacancy level increasing, polarons relating to electron–phonon interaction may play a leading role, which lead to the increase of carrier [22]. The range of variation for εN (x = 0.2) is close to 0.2 in the temperature range 288–373 K and 0.15 from 288 K to 313 K. The dot curve in the figure shows dependence of the εN on decreasing temperature. The trend is similar to that for increasing temperature. That is to say, εN of the samples can be varied reversibly with the temperature. Fig. 5 shows the temperature dependence of the εN in the 3–5 m waveband. The results indicate that εN of the samples has no drastic change with increasing temperature in the 3–5 m waveband and is lower than that in the 8–14 m waveband. This is
Fig. 4. The temperature dependence of normal emissivity of (La0.8 Sr0.2 )1−x MnO3 samples in the 8–14 m waveband. The dot curve is the dependence of the εN on decreasing temperature.
X. Shen et al. / Journal of Alloys and Compounds 474 (2009) 375–377
377
Acknowledgements We are thankful for the financial support provided by the National Natural Science Foundation of China (grant 90505008) and Weaponry Equipment Pre-research Foundation of China. References
Fig. 5. The temperature dependence of normal emissivity of (La0.8 Sr0.2 )1−x MnO3 samples in the 3–5 m waveband.
because that no absorption peak appears around 2000–3333 cm−1 (3–5 m). 4. Conclusions In summary, non-stoichiometric (La0.8 Sr0.2 )1−x MnO3 (0.1 ≤ x ≤ 0.4) system is one of variable-emissivity compounds. The relationship between temperature and εN of the samples in the 8–14 m waveband is different from that in the 3–5 m waveband. The non-stoichiometric (La0.8 Sr0.2 )1−x MnO3 compounds may have the potential for applications as thermal control material.
[1] K. Shimazaki, S. Tachikawa, A. Ohnishi, Y. Nagasaka, Int. J. Thermophys. 22 (2001) 1549. [2] D. Vernardou, M.E. Pemble, D.W. Sheel, Chem. Vap. Depos. 13 (2007) 158. [3] H. Kakiuchida, P. Jin, M. Okada, M. Tazawa, Jpn. J. Appl. Phys. 46 (2007) 621. [4] C. Piccirillo, R. Binions, I.P. Parkin, Chem. Vap. Depos. 13 (2007) 145. [5] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [6] R. Dhahri, F. Halouni, J. Alloys Compd. 385 (2004) 48. [7] P. Kameli, H. Salamati, A. Aezami, J. Alloys Compd. 450 (2008) 7. [8] S. Zemni, J. Dhahri, et al., J. Alloys Compd. 392 (2005) 55. [9] C. Zener, Phys. Rev. 82 (1951) 403. [10] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [11] A. Dhahri, J. Dhahri, et al., J. Alloys Compd. 450 (2008) 12. [12] G. Tang, Y. Yu, W. Chen, Y. Cao, J. Alloys Compd. 461 (2008) 486. [13] G. Venkataiah, V. Prasad, P. Venugopal Reddy, J. Alloys Compd. 429 (2007) 1. [14] V. Ferris, G. Goglio, L. Brohan, O. Joubert, P. Molinie, M. Ganne, Mater. Res. Bull. 32 (1997) 763. [15] R. Suryanarayanan, V. Gasumyants, N. Ageev, J. Magn. Magn. Mater. 211 (2000) 226. [16] H. Vincent, M. Audier, S. Pignard, G. Dezanneau, J.P. Senateur, J. Solid State Chem. 164 (2002) 177. [17] J.Y. Yu, S.Y. Zhang, et al., Solid State Commun. 142 (2007) 333. [18] T. Arima, Y. Tokura, J. Phys. Soc. Jpn. 64 (1995) 2488. [19] K.H. Kim, J.Y. Gu, et al., Phys. Rev. Lett. 77 (1996) 1877. [20] Y. Nglee, F. Sapina, E. Martinerz Tamayo, et al., J. Mater. Chem. 7 (1997) 1905. [21] A. Maignan, C. Martin, et al., Phys. Rev. B. 60 (1999) 15214. [22] G.M. Zhao, K. Conder, et al., Nature 381 (1996) 676.