Dynamics of Single Selenium Chains Confined in One-Dimensional ...

VOLUME 82, NUMBER 9

PHYSICAL REVIEW LETTERS

1 MARCH 1999

Dynamics of Single Selenium Chains Confined in One-Dimensional Nanochannels of AlPO4 -5: Temperature Dependencies of the First- and Second-Order Raman Spectra Vladimir V. Poborchii,1, * Alexander V. Kolobov,2, * Jürgen Caro,3 Victor V. Zhuravlev,4 and Kazunobu Tanaka2 1

Joint Research Center for Atom Technology-Angstrom Technology Partnership, 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan 2 Joint Research Center for Atom Technology-National Institute for Advanced Interdisciplinary Research, 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan 3 Institute for Applied Chemistry, Rudower Chaussee 5, 12489 Berlin, Germany 4 Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia (Received 30 March 1998) The temperature dependence of the Raman spectrum of single helical Se chains in nanochannels of AlPO4 -5 was studied. Dramatic changes in the spectra due to structure transformations in this one-dimensional system are observed. The chains are found to be ordered at 77 K. Reversible weak disordering of the chains (mainly due to the large-amplitude torsional vibrations) occurs with increasing temperature. At temperatures above ,340 K, a phase transition accompanied with a structural relaxation of the chains to the strongly disordered state with unfixed dihedral angles (“torsional melting”) is observed. [S0031-9007(99)08571-3] PACS numbers: 78.30.Hv, 63.22. + m, 64.60.Cn

Phase transitions in nearly one-dimensional systems are widely studied theoretically. However, shortage of the experimental studies takes place. One of the reasons for this is a difficulty to find a suitable object for experiments. In this Letter, we report results of experimental study of phase transformations in a nearly one-dimensional system, namely, in single helical Se chains confined in the channels of a molecular sieve. Selenium chains incorporated into nanochannels of zeolites or other molecular sieves on the one hand are an excellent example of one-dimensional systems, and, on the other hand, they are molecular units of bulk trigonal and amorphous selenium which are responsible for interesting properties of these solids. Thus, studying the properties of zeolite-confined Se chains is important for the investigation of general properties of one-dimensional systems and for an understanding of the properties of bulk selenium. The technique of incorporation of selenium into zeolite pores is well developed [1–12]. Zeolites are wide-band gap insulators displaying high transmission for the light in a wide spectral range (1–6 eV). Raman activity of zeolites is very weak compared to the activity of the zeoliteconfined selenium [2,3,6,10–12] and so Raman scattering is a fruitful tool for characterization of Se chains in zeolites. AlPO4 -5 (we shall use another notation of AlPO4 -5, namely, AFI) is a molecular sieve with nearly cylindrical channels with diameter ,7.3 Å [Fig. 1(a)]. Details of the AFI structure are described in Ref. [13]. AFI is fruitfully used as a good one-dimensional channel model object for studying some properties of channel-confined systems, such as phase transitions [14], molecular diffusion [15], etc. Selenium incorporated into AFI was studied in several works [7–11] and arguments supporting one-dimensional Se chain formation in the AFI channels were shown. Well 0031-9007y99y82(9)y1955(4)$15.00

argued identification of Se species in AFI-Se crystals (AFI containing selenium) was done in Ref. [11]. It was shown that two types of Se species are stabilized in AFI channels, namely, helical Se chains and Se8 rings [Fig. 1(b)]. Both species were found to be perfectly oriented. Se8 rings are oriented by a fourfold axis along the c axis (channel direction) of AFI. Orientation of chains along the c axis is obvious. Moreover, a small portion of Se in the form close

FIG. 1. Cross section of the AFI crystal structure in the plane perpendicular to the c axis (a). Schematic image of Se chain and Se8 ring incorporated into the AFI channel ( b). An approximate dependence of the internal rotational potential of the Se chain vs the dihedral angle, the minimums at 90± and 270± being associated with trans and cis fragments, correspondingly (c).

© 1999 The American Physical Society

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to crystalline trigonal Se was found in the mesochannels organized due to partial destruction of the interchannel walls in AFI. It can be concluded from the data [7–11] that Se chains stabilized in AFI channels are a good nearly onedimensional object showing very weak influence of the matrix. Interchain interaction in AFI-Se is also weak because the spacing between the centers of Se chains in adjacent channels (,13.7 Å) is much larger than the van der Waals diameter of the Se chain (,5.97 Å for geometry corresponding to trigonal Se [16] or ,6.26 Å for that of the single Se chain calculated in Ref. [17]). However, it cannot be completely excluded and, probably, some pore-pore correlation effects [14] can be observed at low temperatures. In this Letter, we report results of a study of vibrational spectra and a phase state of single Se chains confined in the AFI nanochannels in a wide temperature range. We present temperature dependencies of the first-order and second-order Raman spectra of AFI-Se (AFI containing selenium) which display lattice dynamics and structure transformations of one-dimensional Se chains in AFI channels. Synthesis of our AlPO4 -5 crystals is described in Ref. [18]. Crystals used for the measurements possessed a hexagonal prism shape with sizes 120 150 mm in length and 30 50 mm in width. The unit cell framework formula is Al12 P12 O48 , the space group is P6ymcc, and the unit cell parameters are a ­ 13.7 Å and c ­ 8.4 Å, the channels being directed along the c axis (Fig. 1). The AlPO4 -5 framework displays a high stability up to the temperature of 1223 K [19]. Details of AFI-Se sample preparation are given in Ref. [11]. Raman spectra were measured using a Renishaw spectrometer equipped with a microscope. A 632.8 nm line of a He-Ne laser was used as a light source. A Raman signal was detected by a coupled charge device camera. Spectral slit width was ,1.5 cm21 . Spectra were studied in the backscattering geometry for two configurations of light polarization with both incident and scattered light polarized parallel to each other and parallel to the c axis (cc configuration) or a axis (aa configuration) of the AFI crystal. Temperature dependencies were studied using a special thermoregulating device for microscopes. Spectra were registered during ,1 2 min for each temperature after exposure of the sample during ,10 min to the corresponding temperature. Raman spectra of the AFI-Se single crystal obtained at different temperatures (at heating) for aa and cc configurations of light polarization are presented in Fig. 2. Spectra for cc configuration [Fig. 2 (top)] are more than 1 order of magnitude stronger than those for aa configuration [Fig. 2 (bottom)] in accordance with a very anisotropic structure of AFI-confined selenium. There is a number of bands in the bond-stretching mode region of the first-order Raman spectrum s200 300 cm21 d. We mark these bands with letters “a,” “b,” “c,” “d,” and “e.” The bands marked 1956

1 MARCH 1999

FIG. 2. Raman spectra of AFI-Se single crystal for cc configuration (top) and aa configuration ( bottom) at different temperatures at heating. The amplitudes of the d bands at different temperatures are roughly normalized for cc configuration and those of d and e bands for aa configuration. The scale of the absolute intensities for cc configuration is more than 1 order of magnitude higher than that for aa configuration.

with letters a, c, and d were observed at room temperature in Ref. [11] at 237, 259, and 268 cm21 , respectively, and have been assigned to the symmetric bond-stretching mode of single Se chains (the c band active for cc configuration), to the symmetric bond-stretching mode of Se8 rings (the d band active for cc and aa configurations), and to the symmetric bond-stretching mode of trigonallike Se incorporated into mesochannels (the a band active for cc configuration). As has been mentioned above, the Raman spectra of zeolites are very weak. The strongest AFI framework band active for aa configuration is seen as a weak feature at ,500 cm21 in Fig. 2 (bottom). At low temperatures, Se incorporated into mesochannels shows a decrease in the intensity of the band of the crystallinelike component (a band) and an increase in the intensity of the band of the amorphouslike component (b band) at ,250 cm21 . This behavior is typical for Se incorporated into channels with diameters of several

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PHYSICAL REVIEW LETTERS

nanometers [20]. In this Letter, we do not consider properties of the meso-channel-confined Se and concentrate our attention on the temperature dependencies of the features associated with single Se chains confined in the original AFI channels. At low temperatures, the single Se chain c band is sharp (at 77 K, the band half-width is of ,4.3 cm21 ) and strong compared to other bands in the cc spectrum. At first, we conclude that the chains at low temperatures are quite regular. This follows from the sharpness of the first-order Raman chain band and from the well resolved fine structure of the second-order Raman spectrum [Fig. 2 (top)] associated with the peaks in the vibrational density of states of the Se chain. An increase in the temperature dramatically influences the Se chain spectrum. The chain c band becomes weaker and broader and, at high temperature, only a broad e band at ,262 cm21 , which is actually a superposition of the chain and ring bands, is observed. This broad band is active for both aa and cc configurations (Fig. 2). The fine structure of the second-order spectrum disappears. All these changes in the spectrum indicate that the Se chains are disordering. Because of the disordering, not only the symmetric bond-stretching mode but a continuous vibrational spectrum contributes to the first-order chain spectrum in the range of ,200 280 cm21 . An increase in the intensity of the second-order spectrum (cc configuration) with temperature [Fig. 2 (top)] is probably associated with an increase in anharmonicity of vibrations. At high temperatures, Se atom vibrations along channels can have large amplitudes providing large anharmonicity and high second-order activity in the cc spectrum. At temperatures higher than ,500 K, evaporation of Se from the sample becomes noticeable and so the temperature range of the data presented in Fig. 2 is terminated at ,498 K. Temperature dependencies of the Raman shift and halfwidth of the Se chain band c in the range 77–323 K are shown in Fig. 3. The dependence of the Raman shift [Fig. 3(a)] is roughly linear with the coefficient ,20.013 cm21 yK. The temperature dependence of the chain band half-width [Fig. 3(b)] is nonlinear suggesting an activation mechanism for the chain disordering. This dependence can be approximated using a standard expression for an activation process with an activation energy U: w ­ w0 1 w1 exps2UykT d ,

(1)

where w is a chain band half-width, w0 is a chain band half-width at 0 K, and w1 is a fitting parameter. In Fig. 3(b), a curve calculated using the values w0 ­ 4.2 cm21 , w1 ­ 234 cm21 , and U ­ 0.12 eV is shown; it is in very good agreement with experimental data. Let us discuss the physical sense of the observed activation dependence. A helical Se chain possesses a soft structural parameter, namely, a dihedral angle [17,21]. The probability of generation of torsional vibrations with very

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FIG. 3. Temperature dependencies of the Raman shift (a) and half-width ( b) of the symmetric bond-stretching-mode band of single Se chain in AFI-Se. Experimental data are shown as scatters. A linear fitting is shown as a solid line for the dependence of the Raman shift (a) and an exponential fitting [expression (1) in the text] is shown as a solid line for the dependence of the half-width ( b).

large amplitudes and probability of transformation of trans configurations to cis configurations [Fig. 1(c)] are proportional to ,exps2Ur ykT d, where Ur is a value of the potential barrier for the dihedral angle rotation [Fig. 1(c)]. The calculated value of Ur , 0.15 eV [17] is very close to the value of the activation energy U , 0.12 eV determined from the temperature dependence of the chain band broadening. Taking into account reversibility of the temperature dependence of the AFI-Se spectrum at temperatures below ,340 K, we can conclude that the chain band broadening is associated with excitation of large-amplitude torsional vibrations and may be reversible single cis-fragment generation (single cis fragments are unstable in a strongly oriented Se chain). We consider disordering occurring in this temperature range as a weak disordering. It should be noted that a good correspondence between the experimentally determined activation energy and the internal rotational potential barrier of the helical Se chain is an important argument for assignment of the observed disordering to the properties of single chains without 1957

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PHYSICAL REVIEW LETTERS

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be considered as an example of behavior of a nearly onedimensional system confined in a nanochannel. Because of the simplicity of this system, it can be treated as a good one-dimensional model system for both experimental and theoretical studies. Moreover, important information for bulk Se can be obtained because the ordered Se chains in AFI-Se at low temperatures are similar to the structural units of trigonal Se and the strongly disordered chains are similar to the structural units of liquid and amorphous Se. This work, partly supported by NEDO, was performed in the Joint Research Center for Atom Technology (JRCAT) under the joint research agreement between the National Institute for Advanced Interdisciplinary Research (NAIR) and the Angstrom Technology Partnership (ATP). FIG. 4. Evolution of the Raman spectrum of the AFI-Se single crystal for cc configuration at a fixed temperature of 348 K. A top spectrum is obtained for a sample cooled down to 298 K after exposure to 348 K during ,60 min.

any noticeable contribution of possible weak interchain interaction. At higher temperatures, a phase transition accompanied with a structural relaxation of the chain from the weakly to strongly disordered state is observed. In Fig. 4, evolution of the cc spectrum of AFI-Se with time at a fixed temperature of 348 K is shown, the relaxation time being ,30 50 min. Increasing temperature leads to a decrease in the relaxation time. The strong disordering is probably associated with the generation of many dihedral angle rotations; as a result the chains relax to the structure with unfixed dihedral angles containing both trans and cis fragments. We can say that a “torsional melting” takes place. Recovery of the chain structure from the strongly disordered state to the weakly disordered state is also a relaxation process. The relaxation time of this process at room temperature varies from several hours to several days depending on the sample. We should note that no indication of appearance of the short chains due to intensive chain breaks was observed in the studied temperature range. To summarize, the temperature dependencies of the Raman spectra of one-dimensional Se chains incorporated into AFI channels show transformations of the chains from the ordered state at low temperatures to weakly and strongly disordered states at higher temperatures. A phase transition from a weakly to a strongly disordered state is accompanied by structural relaxation of the chain. The observed structure transformations in single Se chains can

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*On leave from Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia. [1] V. N. Bogomolov, E. L. Lutsenko, V. P. Petranovskii, and S. V. Kholodkevich, JETP Lett. 23, 482 (1976). [2] V. N. Bogomolov, V. V. Poborchii, S. V. Kholodkevich, and S. I. Shagin, JETP Lett. 38, 532 (1983). [3] V. N. Bogomolov, V. V. Poborchii, S. G. Romanov, and S. I. Shagin, J. Phys. C 18, L313 (1985). [4] K. Tamura et al., J. Phys. Soc. Jpn. 55, 528 (1986). [5] O. Terasaki et al., Nature (London) 330, 58 (1987). [6] V. V. Poborchii, Chem. Phys. Lett. 251, 230 (1996). [7] J. B. Parise et al., Inorg. Chem. 27, 221 (1988). [8] T. Kodaira et al., Trans. Mater. Res. Soc. Jpn. 20, 470 (1996). [9] Z. K. Tang et al., Solid State Commun. 101, 333 (1997). [10] Z. K. Tang and M. M. T. Loy, Appl. Phys. Lett. 70, 34 (1997). [11] V. V. Poborchii et al., Chem. Phys. Lett. 280, 17 (1997). [12] P. Armand et al., Phys. Rev. Lett. 79, 2061 (1997). [13] J. M. Bennet et al., ACS Symp. Series 218, 109 (1983). [14] R. Radhakrishnan and K. E. Gubbins, Phys. Rev. Lett. 79, 2847 (1997). [15] D. S. Sholl and K. A. Fichthorn, Phys. Rev. Lett. 79, 3569 (1997). [16] R. M. Martin, G. Lucovsky, and K. Helliwell, Phys. Rev. B 13, 1383 (1976). [17] A. Ikawa and H. Fukutome, J. Phys. Soc. Jpn. 58, 4517 (1989). [18] I. Girnus et al., Zeolites 15, 33 (1995). [19] A. Stein, B. Wehrle, and M. Jansen, Zeolites 13, 291 (1993). [20] V. V. Poborchii, A. V. Kolobov, V. V. Zhuravlev, and K. Tanaka (to be published). [21] M. Garcia-Hernandez et al., Phys. Rev. B 48, 149 (1993).