Far infrared reflectivity measurements on quasi one dimensional halogen bridged mixed valence compounds

Synthetic Metals, 29 (1989) FI37-F144

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FAR INFRARED REFLECTIVITY MEASUREMENTS ON QUASI ONE DIMENSIONAL HALOGEN BRIDGED MIXED VALENCE COMPOUNDS

L. Degiorgi and P. Wachter Laboratorium fiir Festkhrperphysik, ETH - Zfirich, 8093 Zfirich (Switzerland) M. Haruki and S. Kurita Yokohama National University, Lab. of Applied Physics, Faculty of Engineering, Hodogaya, Yokohama 240 (Japan)

ABSTRACT We have performed reflectivity measurements on one dimensional halogen-bridged mixed-valence compounds, regarding in particular the far infrared energy range. The IR-active phonon modes are investigated and a tentative assignment is proposed.

INTRODUCTION The compounds of the type [M(en)2][M(en)2X~](C104)4 with M = Pt or Pd, and X = C1, Br and I and en = ethylenediamine, hereafter to MX chain abbreviated, are members of a large family of quasi one-dimensional halogen-bridged mixed-valence insulators. Their skeletal structure consists of linear chains of alternated metal-halogen ions, where the halogen X ions are closer to the M IV than to the M II ions [1]. These and similar systems, as e.g. Wolffram's red salt, were intensively investigated with optical methods in the past. Refiectivity measurements in the visible and UV region and resonance Raman scattering investigations (also with pressure dependence) were mostly performed [2-5] on these compounds. Concerning the reflectivity measurements, the corresponding spectra are dominated by peaks at 2.72, 1.95 and 1.37 eV for Pt-C1, - B r and - I ions, respectively, when measuring with light polarized parallel to the chain axis [3]. This strong absorption is ascribed to the charge-transfer (CT) transition from the dz2 orbital of M II to the dz2 orbital of M IV, which corresponds to a transition across the energy gap formed at the edge of the folded Brillouin zone, caused by the displacement of X ions form the midpoints. Thus the MX chains are considered as a realized Peierls system, where the displacement of the halogen X-ions results in an appearance of a commensurate charge density wave (CDW) with a period of twice the M-M separation. Tight binding calculations, based on the one electron band model, well confirm the above results [6-7]. 0379-6779/89/$3.50

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Besides, these systems are, from a theoretical point of view, good prototype materials, which are suitably described by the Peierls Hubbard (PH) model [8]. Among the further theoretical investigations, we will only underline, for the sake of brevity, the work of Baeriswyl and Bishop [9], essentially based on the PH-model. They have calculated the ground state of a MX-chain and the configurations of many excited and metastable states, as e.g. exciton, polaron and bipolaron, kink or soliton. Furthermore the model considers the elastic forces, in the harmonic approximation, between neighboring atoms, making then possible a connection between the electronic properties related to the CT-band and the lattice-dynamical behavior of these MX-chains. Therefore the motivation of our work resides in the study of the vibrational modes and the aim of the present paper is to present our first measurements in the near and far infrared (FIR) (i.e. the typical energy range for IR-active phonon modes) for the Pt-C1 and P t - I compounds. BXPERIMENT AND RESULTS The optical refiectivity (R(v)) with light polarized perpendicular and parallel to the chain axis (b) of large single crystals (0.5 x 1 x 2mm) of Pt-C1 and P t - I is measured in the energy range from 12 eV down to 1 meV at 300 K and 6 K, using four spectrometers. In the FIR a Fourier spectrometer is employed with a He cooled Ge bolometer as detector.

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The single crystals were obtained from recrystallization of powder samples. The powdered materials were synthesized by the procedures described in the literature [6]. The recrystallization was performed in dilute HC104 solution and crystals of various shapes, such as needles or platelets, are grown depending on the HC104 concentration. The I-compound-recrystallization was performed in Japan by two of us (M.H. and S.K.), the Pt-C1 compound was recrystallized in Ziirich by E. Jilek. Fig. 1 presents R(w) for both I - and C1 compounds at 300 K with light polarized parallel and perpendicular to the chain axis in the energy range between 0.1 and 3.6 eV. We can recognize the well known CT band structure, polarized along the chain axis. The perfect agreement with other previous and similar measurements [3,6] confirms the good quality of our single crystals. The FIR energy region spectra, however, are shown in Fig. 2,3 for Pt-C1 and in Fig. 4,5,6 and 7 for Pt-I, for both investigated polarizations and temperatures. We limit our attention to the FIR and visible energy range, since the other spectral regions do not present any particular interesting features for the purpose of our discussion. DISCUSSION In the visible region, besides the well known CT-band, seems to be of particular interest the broad structure at 1. eV for the Pt-C1 compound. Following the model of Baeriswyl and Bishop [9], we assign the former structure to the optical excitation, due to the presence of a soliton. A similar structure was found by photoinduced absorption experiment [10] and by absorption measurements under hydrostatic pressure at ~ 1.5 eV [11], where the observed midgap absorption band structure is ascribed to a soliton excitation, corresponding to kinks of the CDW. However, it would be more appropriate to consider our soliton structure as an intrinsic defect instead as a metastable state. This is well accounted for by the fact that the intensity of the CT-band in R(x) is lower than in the literature (where the midgap state is not detected at normal condition, e.g. Wada et al. [3]). Infact the existence of a soliton kink defect seems to suppress the CT absorption band, as shown in polyacetylene by Suzuki et al. [12]. The FIR measurements are instead characterized by many modes in both polarization directions. In our discussion we will concentrate us only on the modes that clearly show a polarization dependence along the chain axis. Such modes at 300 K are summarized in Tab. 1. It should be noted that the four phonon modes in P t - I are identified among the other structures, comparing the shape and the relative intensity with the corresponding four structures detected in Pt-C1. TABLE 1 FIR-modes at 300 K for Pt-C1 and Pt-I

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We think that the other modes in P t - I are related to the internal vibrational states of the (en)-complexes and the interchain (C104)-molecules. These four phonon modes are qualitatively very similar to the four modes detected in BaBil-xPbxO3 [13]. Eventhough the former system has a three dimensional structure it is, from the dynamical point of view, totally equivalent to our P t - X systems and it is then not surprising that the same IR-active structures appear. (For more group theoretical details we refer the reader to ref. [13]). From the CDW ground state follows the unit cell p t I I I + t i - X - - p t I I I - 6 - X - (where o<6<1 describes the charge disproportion), which contains four atoms. Consequently this will give at least two IR phonon modes, one Raman active vibration together with one acoustic mode which is inactive. The expected IR phonon modes correspond to the stretching mode, where the Pt-ions move against the X-ions, and to the zone boundary acoustic mode as a result of Brillouin zone folding. Since the reduced mass of the last mode is equal to the Pt-ion mass, we expect this mode to have the lower frequency (vl). We assign also to the stretching mode the structures at 175 cm -1 (v2) and at 350 cm -1 (z,4) for P t - I and Pt---Cl, respectively. As it will be developed in more detail in a forthcoming publication [14], we find a good agreement between the experimental frequencies and the calculated normal modes of a one dimensional Bravais lattice in the harmonic approximation, where we used for the spring force constant (describing the interaction between neighbouring ions) the value obtained within the model of Baeriswyl and Bishop [9].

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In the approach described above we have simplified the vibrational analysis, neglecting the interaction between the Pt ions and the ions of the (en)---complexes along the chain. Taking into account such nearest neighbour dynamical interactions, one finds new phonon modes, essentially due to the enlarged number of ions in the unit cell [13]. These are the so called bending modes, one of which is IR-active. The remaining two structures are then ascribed to the IR bending mode and to a vibrational state of external type, where also the (C104)molecules along the chains are involved. Finally we remark that with decreasing temperature the mode structures are more pronounced and show the typical splitting effect, sometimes combined with a light energy shift. Furthermore, some structures grow up with decreasing temperature from precursor broad bumps detected at room temperature. This could be an evidence of structural phase transitions at low temperature, where internal distortions can induce new IR-active phonons [14]. CONCLUSIONS

We have presented the whole FIR spectra of P t - X compounds (with X = C1 and I), identifying the IR-a~tive phonon modes along the chain. Further investigations on P t - B r and mixed halogen compounds are in progress, in order to consistently study the effect of the d-orbital states extension of the Pt-ions on the phonon modes.

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ACKNOWLEDGEMENTS The authors are grateful to Dr. D. Baeriswyl for stimulating discussions and to E. Jilek, J. Mfiller and H.P. Staub for technical assistance. REFERENCES 1 R.J.H. Clark, in R.J.H. Clark and R.E. Hester (ed.), Advances in Infrared and Raman SeDctroscopy. Wiley Heyden, 1984, Ch. 3, p. 95 2 M. Tanaka, S. Kurita, M. Fujiawa and S. Matsumoto, J. Phys. Soc. Jap. 54 (1985) 3632 3 Y. Wada, T. Mitani, M. Yamashita and T. Koda, J. Phys. S0c. Jap. 54 (1985) 3143 4 H. Tanino and K. Kobayashi, J. Phys. Soc. Jap. 52 (1983) 1446 5 H. Tanino, N. Koshizuka, K. Kobayashi, M. Yamashita and K. Hoh, J. Phys. Soc. Jap. 5._44(1985) 483 6 M. Tanaka, S. Kurita, T. Kojima and Y. Yamada, Chem. Phys. 91 (1984) 257 7 M.-H. Whangbo and M.J. Foshee, Inorg. Chem. 20 (1981) 113 8 K. Nasu, J. Ph~/s. Soc. Jap. 52 (1983) 3865 9 D. Baeriswyl and A.R. Bishop, J. Phys. C 21 (1988) 339 10 S. Kurita, M. Haruki, K. Miyagawa, J. Phys. Soc. Jap, to be published 11 N. Kuroda, M. Sakai, Y. Nishina, M. Tanaka and S. Kurita, Phys. Rev. Lett. 58 (1987) 2122 12 N. Suzuki, M. Ozaki, S. Etemad, A.J. Heeger and A.G. MacDiarmid, Phys. Rev. Lett. 4..55(1980) 1209 13 S. Uchida, S. Tajina, A. Masahi, S. Sugai, K. Kitazawa and S. Tanaka, J. Phys. Soc. Jap. 54 (1985) 4395 14 L. Degiorgi, P. Wachter, M. Haruki and S. Kurita, in preparation