Chem. Mater. 1990,2, 315-319
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Resonance Raman Evidence for Electron and Hole Defect Asymmetry in the Quasi-One-Dimensional Mixed-Valence Solid [Pt11(en)2][Pt1V(en)2C12][C104]4 (en = ethylenediamine) R. J. Donohoe, C. D. Tait, and B. I. Swanson* Inorganic and Structural Chemistry Group, INC-4, M S - C345, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received November 20, 1989 Resonance Raman (RR) spectroscopy of [Pt"(en),] [PtW(en),Cl2][C1O4I4provides both electronic and vibrational information about intrinsic local gap defect states. Selective enhancement of weaker RR features when tuning through the A and B band defect absorptions below the charge-transfer band edge serves to resolve the two absorption maxima into five distinct electronic transitions. The observed defect vibrational frequencies include a peak at 263 cm-l, which is enhanced in two of the ranges, and a peak at 287 cm-l, enhanced in two other ranges. Together with predictions from theoretical calculations and a previous study of photoinduced defects, the results herein allow assignment of the two electronic transitions associated with enhancement of the 263-cm-l band to electron polaron defect gap-to-gap and gap-to-band transitions and the two associated with 287-cm-l band to corresponding transitions due to hole polaron defects. The above results require significant electronic asymmetry for electron versus hole polarons, in contrast to the half-filled single-bandmany-bodymodels, which predict no asymmetry. The observed electron/hole polaron asymmetry can be explained via interaction of the metal band with the filled halide band in a threequarter-fiied, two-band model. Alternatively,preexisting defects may preferentially trap one type of polaron and thereby remove symmetry. of these chain-axis-polarized transitions via electronic absorption spectroscopy has been reported by several In particular, two absorption features, designated the A and B bands, have been observed immediately below the band edge of the intervalence charge-transfer absorption (IVCT) of [Pt*l(en)z] [Ptlv(en),C1,] [C1O4I4 (hereafter referred to as PtC1; en = eth~lenediamine).~*~?~ However, the characterization of the defects responsible for midgap absorption features is not straightforward the A band has been tentatively attributed both to kinkg and to p ~ l a r o n ~ defects. -~ Strong evidence for the polaron assignment was provided by Kurita et al. when they reported that irradiation of the PtCl system at low temperatures with energies above the IVCT edge caused metastable increases in both the A and B band intensities and the EPR ~ i g n a l . Thus, ~ in addition to absorption spectroscopy, other spectroscopic and physical measurements are required to firmly identify defects. Resonance Raman spectroscopy of HMMC systems has been used extensively by our group to probe defect states. The observed frequencies and RR band structures can be correlated with theoretical or semiempirical predictions to aid in assignments. In addition, the excitation profiles for vibrational features associated with particular defect states can yield information on the location and band widths of the defect electronic transitions. Thus, the RR experiment can provide both vibrational and electronic structural characterization of defects. RR spectroscopy within the IVCT may indirectly reveal defects by observation of structure and dispersion in the optic mode associated with the IVCT band. However, the tunability of Raman excitation sources to the red of the band edge, where isolated transitions due to defects are observed, is the strongest point of the defect RR experiments. Herein we report the results of RR experiments that probe the absorption region to the red of the PtCl band edge where several distinct vibrations due to intrinsic defects are ob-
Introduction Halogen-bridged mixed-valence metal chains (HMMC) are low-dimensionalcharge density wave (CDW) systems1-" that exhibit a wide variety of defect states. Intrinsic levels of defects are present in all crystals, while increased concentrations of certain types of defects can be generated via photoexcitation,"s application of high pressure^,^ or impurity doping.6J0 Defects play a crucial role in the determination of macroscopic properties such as conductivity and magnetic susceptibility as well as microscopic properties, which are reflected in spectroscopic measurements. Because defects have such profound effects on the properties of low-dimensional solids, investigation of the gap states generated by these various types of sites is important for the development of many-body models to describe charge and structural fluctuation in HMMC materials. Associated with the many possible types of defect sites (polarons, bipolarons, kinks, and excitons) are a profusion of electronic transitions with energies below that of the charge-transfer band edge.3J1-14 The direct observation (1)Ueta, M.; Kanzaki, H.; Kobayashi, K.; Toyazawa, Y.; Hanamura, E. In Excitonic Processes in Solids. Springer Ser. Solid State Sci. 1986, 60.
(2)Onodera, Y. J. Phys. SOC.Jpn. 1987,56, 250-259. (3)Baeriswyl, D.; Bishop, A. R. J. Phys. C.: Solid State Phys. 1988, 21, 339-356. (4)Nasu, K.; Mishima, A. Reu. Solid-state Sci. 1988,2, 539. (5)(a) Kurita, S.;Haruki, M.; Miyagawa, K. J. Phys. SOC.Jpn. 1988, 57, 1789-1796. (b) Kurita, S.;Haruki, M. Synth. Met. 1989,29, F129-Fl Rfi. (6)Matauahita, N.; Norimichi, K.; Ban, T.;Tsujikawa, I. J.Phys. SOC. Jpn. 1987,56, 3808-3811. (7)Donohoe, R. J.; Ekberg, S. A.; Tait, C. D.; Swanson, B. I. Solid State Commun. 1989,71,49-52. (8) Donohoe, R. J.; Dyer, R. B.; Swanson, B. I. Solid State Commun. 1990, 73, 521-525. (9)Kuroda, N.; Sakai, M.; Nishina, Y.; Tanaka, M.; Kurita, S. Phys. Rev. Lett. 1987,58, 2122-2125. (10)Haruki, M.;Kurita, S. Phys. Reu. B 1989,39, 5706-5712. (11)Conradson, S.D.; Stroud, M. A,; Zietlow, M. H.; Swanson, B. I.; Baeriswyl, D.; Bishop, A. R. Solid State Commun. 1988, 65, 723-729. (12)Bishop, A. R.;Gammel, J. T.; Phillpot, S. R. Synth. Met. 1989. 29, F151-Fl59.
0897-4756/90/2802-0315$02.50/0
(13)Mishima, A.;Nasu, K. Phys. Reu. B 1989, 39, 5758-5762. (14)Mishima, A.; Nasu, K. Phys. Reu. B 1989,39, 5763-5766.
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served. This work also serves as a basis for the analysis of photoinduced and pressure-induced defects.
Experimental Section Synthesis of PtCl was achieved by previously reported methods.7J5 Single-crystal samples were selected for RR spectroscopy from several different batches of crystals and yielded similar resulta, although the relative intensities of some weak RR features did vary slightly from sample t o sample. The single-crystal absorption data of the samples used for RR measurements were essentially identical with the literature results.16 The samples were mounted on a copper block coupled to the tip of an Air Products Displex cryostat, which was used to maintain sample temperature a t approximately 15 K during data acquisition. The laser excitation was provided by a Spectra Physics Model 171 Kr+ laser, a Spectra Physics Model 2025 Ar+ laser, an Ar+-pumped Coherent Model 599 dye laser with rhodamine 6G, or an Ar+pumped Spectra Physics Model 3900 Tksapphire solid-state laser, which provides tunable excitation between approximately 1.7 and 1.2 eV. Focused laser intensity at the sample was in all cases less than 5 mW (excellent signals from single-crystal samples were obtained with less than 1 mW for excitations to the red of the IVCT). The scattered light was coupled either into a J Y Model UlOOO scanning double monochromator equipped with a thermoelectrically cooled RCA C31034-A photomultiplier tube (PMT, visible and far-red experiments) or into a SPEX Model 1877D triple spectrograph equipped with a Photometrics Incorporated PM512 prime grade charge-coupled device detector (CCD, far-red and near-infrared experiments).
Results The intense RR feature associated with the u1 mode of PtC1, which is due to symmetric displacement of the halides around the PtIV sites, has been previously reported."J8 The work of Kurita et al. is especially noteworthy for the careful analysis of the fine structure and dispersion observed in this RR feature. Dispersion and fine structure are also observed for the u1 band in the PtI analogue.l8 For PtBr, selective enhancement of the finestructure components in the u1 feature has been observed while tuning the excitation energy through the IVCT absorption.l8 The structure and enhancement behavior of u1 for the various PtX systems may have its origin in distinct correlation lengths of the MX chain.18J9 Defects (apart from chain ends) may also be the source of this result.20 While the unusual intensity pattern and dispersion of the v1 band may provide indirect evidence for defect states in HMMC systems, direct observation of these defects via RR spectroscopy can now be reported. Figure 1 displays the 1.40-eV excitation RR spectrum of PtCl from 200 to 330 cm-' Raman shift obtained with the Ti:sapphire laser and CCD detector as well as the RR spectra from 200 to 650 cm-' obtained with Kr+, rhodamine 6G dye, or Ar+ lasers and PMT detector (data obtained with excitations from 1.65 to 2.50 eV). The spectra are displayed with the u1 feature off scale so that the weaker features can be readily observed. The excitation energies in Figure 1vary from the red extreme of the A absorption band, through the A and B absorption maxima (1.65 and 1.97 eV, re(15)Basolo, F.; Bailar, J. C., Jr.; Tarr, B. R. J. Am. Chem. SOC.1950, 72,2433-2438. (16)Tanaka, M.;Kurita, S.; Kojima, T.; Yamada, Y. Chem. Phys. 1984, 91,257-265. (17)Clark, R.J. H.; Turtle, P. C. J . Chem. SOC.,Dalton Trans. 1978, 1622-1627. - __ - _ . .
(18)Tanaka, M.;Kurita, S. J . Phys. C: Solid State Phys. 1986,19, 3019-3028. (19)Conradson, S.D.; Dallinger, R. F.; Swanson, B. I.; Clark, R. J. H.; Croud, V. B. Chem. Phys. Lett. 1987,135,463-467. (20) The relative intensities and number of bands in the u1 mode are influenced by doping. See: Tanaka, M.; Kurita, S.; Haruki, M.; Fujisawa, M. Synth. Met. 1987,21,103-108.
574
-.-/I
v, i s t \5?1
165eV
272
263
287 2 1 a ~
v
2 33 e V
v
2 4 1 eV
n
2 50 e V
200
lJJ
I
300
400 cm-,
500
600
Figure 1. Resonance Raman spectra of PtCl obtained with the indicated excitation energies. The spectra are displayed with the peak due to the symmetric stretching mode (vl) off scale in order to facilitate viewing the excitation dependence of the weaker features.
spectively) and into the IVCT absorption (2.41-2.50 eV).16 Because excitation of the PtCl crystals within the IVCT at low temperatures results in formation of metastable absorptions in the red,5 the RR data for a given crystal were collected with red excitation prior to obtaining data within the IVCT. As can be seen in Figure 1, several features yield RR intensity only with selected excitation energies, while others appear in all of the Raman spectra. The latter type are by and large due to ethylenediamine-related vibrations such as the PtN4 umbrella deformation at 215 cm-1.21 Such modes are not expected to exhibit significant resonance enhancement and have been used elsewhere as intensity standards for the generation of excitation pr0fi1es.l~ We have not used this approach here for several reasons. First, in PtCl these modes are especially weak, and error generated by signal/noise variations is potentially significant. Also, we observe some relative intensity and band-shape changes among these nonenhanced modes. Obtaining excitation profiles of the weak defect modes via the traditional approach of supporting a powder of HMMC materials within a nonresonance vehicle such as Na2S04 results in loss of resolution in the Raman peaks and alteration of the levels and nature of defects present. As opposed to bona fide excitation profiles, we shall report the range of excitation energies that yield observable intensity for these weak features, referred to hereafter as the enhancement range. The first enhancement range is observed with near-infrared excitations (1.29-1.44 eV) and is associated with enhancement of an asymmetric peak at 263 cm-', as seen in the spectrum acquired with 1.40-eV excitation displayed (21)Campbell, J. R.;Clark, R. J. H.; Turtle, P. C. Inorg. Chem. 1978, 17, 3622-3628.
Electron and Hole Defect Asymmetry
Chem. Mater., Vol. 2, No. 3, 1990 317
W
I 2.4
2.3
2.2
2.1
2.0
1.9
1.0
1.7
1.6
1.5
1.4
eV
Figure 2. Absorption spectrum of PtCl between 1.4 and 2.5 eV. The inset absorption, from 1.1 to 1.4 eV, was obtained on the same sample with a different detector (see text). The ranges of the distinct electronic transitions as detected by selective resonance Raman enhancement of defect vibrations are superimposed.
in Figure 1. The intensity of this feature varies from sample to sample and is quite weak in some crystals. Because published single-crystal absorption results16 do not indicate absorbance beyond 1.40 eV, enhancement of the 263-cm-' peak beyond 1.40 eV seems surprising. To investigate this, we acquired single-crystal absorption data for PtCl at low temperatures. These data are shown in Figure 2. Like the previously published results, no A-band absorption intensity is observed from approximately 1.45 to 1.40 eV. However, at least for our own absorption data, this result appears to be an artifact of the spectrometer response. The single-crystal absorption data are collected in a single-beam mode, and we observe very limited response for the spectrometer in the 1.45-1.40-eV range. Our spectrometer switches detectors at 1.4 eV, and the single-beam energy is much higher on the low-energy side of that change. The inset in Figure 2 displays the absorption spectrum of PtCl at low temperatures immediately to the red of the detector switch. All of our samples exhibited an absorption tail in this region, although the intensity of this absorption relative to the A band is difficult to gauge. (The apparent shoulder in the inset may be due to an overtone of the stretching mode of physisorbed water: 3 X 3400 cm-'). Thus, enhancement of the 263-cm-l mode out to 1.29-eV excitation is reasonable. It is important to note that, like the A and B absorption bands: the 263-cm-l Raman feature observed with 1.41-eV excitation gains intensity after photolysis of the PtCl chain within the IVCT absorption at low temperatures.8 The next enhancement range, which partly overlaps the first, extends from 1.40 to approximately 1.75 eV and is coincident with the A-band absorption maximum. The exact edges of this range are difficult to establish because nonenhanced Raman features underlie those that gain intensity. This range is associated with the RR pattern that contains the 287-cm-' peak as seen in the 1.65-eV spectrum (Figure 1). This pattern, which is chain-axis polarized, contains at least six vibrational features that are found (by curve fitting) at 278.3,280.8, 283.5,286.6, 290.7, and 293.8 cm-l. However, the six peaks do not exhibit the same excitation wavelength dependence. Indeed, the RR band at 278.3 cm-' is observed in all the spectra and thus does not appear to be reasonantly enhanced. Data obtained with the tunable Ti:sapphire laser (not shown) indicate that the 287- and 291/294-cm-l peaks appear to gain
intensity concomitantly. These peaks dominate the 270300-cm-' portion of the RR spectrum upon excitation from 1.69 to 1.40 eV. One of the most striking observations associated with this resonance-enhanced structure is that the 287-cm-' peak displays overtone intensity at 574 cm-I in the RR spectrum acquired with 1.65-eV excitation (see Figure 1). Thus, this peak is enhanced via a Franck-Condon mechanism upon excitation near the A band absorption maximum. No overtone intensity is observed with excitations to the red of approximately 1.44 eV. The 291- or 294-cm-' peak may also exhibit overtone intensity, although in this case there would be a coincidence of this overtone with a non-resonantly enhanced 581-cm-l mode, which has been assigned as a Pt-N stretching mode.21 Examination of the relative intensities of the 581- and 215-cm-' peaks, both assigned as nonenhanced ethylenediamine modes, indicates that the 581-cm-' peak is comparatively much more intense with 1.65 eV excitation. The 1.40-1.75-eV enhancement range and observation of Franck-Condon scattering suggest that the resonantly enhanced peaks observed between 280 and 294 cm-' with 1.65-eV excitation are associated with the A-band electronic transition. This suggestion is also supported by the observation of concomitant relative intensity increase for this vibrational pattern and the A-absorption band upon photolysis.s The observation of an overtone at 574 cm-' demonstrates that the potential well associated with the 287-cm-' mode is quite harmonic. Upon excitation on the high-energy side of the A band, a new chain-axis-polarized RR band at 272 cm-I with a unique enhancement range is observed. This band is seen in Figure 1 in the RR spectrum obtained with 1.83-eV excitation. There is very weak intensity for this band in the spectrum acquired with 1.92-eV excitation, while data collected by using excitation from the Ti:sapphire laser indicate that the red boundary of observation is at approximately 1.69 eV. No distinct overtone is observed. Certain absorption data from Kurita et allo clearly indicate a high-energy shoulder on the A band that coincides with this enhancement range. Whether or not this band follows the A band in gaining intensity upon photolysis is not clear. The next enhancement range, from 1.92 to 2.10 eV, marks the reappearance of the 263-cm-' peak, first observed in the 1.29-1.44-eV range. Data obtained with a rhodamine 6G dye laser demonstrate that this peak is identical in both band shape and frequency with that previously described. In the present range, the 263-cm-l peak is observed most intensely with 1.97-eV excitation (see Figure 1). With this excitation energy, the 263-cm-' feature displays weak overtone intensity at 526 cm-'. The maximum resonance enhancement for this peak coincides with the B-band absorption, which may originate either from the midgap-to-band or band-to-midgap excitation of a polaronic defect.'J2 The RR spectra obtained from 2.18 to 2.47 eV establish a fourth enhancement range. (The blue edge of this range may be an artifact caused by self-absorption due to the onset of the IVCT.) The key feature of this fourth enhancement range is the reappearance of a peak at 287 cm-'. Unlike the 287-cm-l RR band previously described (1.65eV excitation), this band displays no apparent overtone intensity. Also, while the 287-cm-' peak was associated with peaks at 291 or 294 cm-' in the 1.40-1.75-eV range, in this excitation range only a shoulder at 290 cm-' is observed. In addition to the reemergence of the 287-cm-l feature, several poorly resolved sets of peaks, which are centered at approximately 482, and 464, and 789 cm-', show intensity with these excitation energies. The 789-
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318 Chem. Mater., Vol. 2, No. 3, 1990
Table I. Enhancement Ranges Observed in the Resonance Raman of the PtCl Chain upon Excitation to the Red of the IVCT Band Edge enhancement associated vibr range, eV features, cm-' assignt 1 1.29-1.44 263 electron polaron (P-) A band 2 1.41-1.75" 287, 291, 574 hole polaron (Pt) A band 3 1.69-1.92 272 ? 4 1.92-2.10 263, 524 electron polaron (P-)B band 5 2.18-2.47* 287, 480, 890 hole polaron ( P') B band
2.41 eV
Ll 2.60 eV
I!,
2.71 eV
-v
700
800
cm-1
I
900
1000
Figure 3. Resonance Raman spectra of PtCl obtained with the indicated excitation energies.
cm-' peak is shown in Figure 3. This peak may be due to a combination of the v1 band and the 482-cm-' peak. The intensities of the 482-, 464-, and 789-cm-' RR bands seem to be more sample dependent than the bands described to this point. However, single-crystal absorption data reveal no clear differences between samples in this region. Vibrational fundamentals with energies higher than 400 cm-I are more suggestive of ligand bending and stretching modes as opposed to metal-halide stretches. A final type of excitation profile for weak RR features is observed to coincide with the onset of the IVCT band (excitation 12.33 eV). There are a number of peaks (those near 415 and 890 cm-', for example) that appear to gain intensity as the excitation energy is tuned into this absorption. These features can be observed in Figures 1and 3.
Discussion Resonance Raman spectroscopy is especially useful for the characterization of defects in low-dimensional materials because both electronic and vibrational information can be extracted and used in tandem to identify defect states. The present work serves to confirm the intrinsic presence of multiple types of defects in PtC1. Five distinct enhancement ranges are observed while probing the region immediately to the red of the IVCT band edge. Thus, selective Raman enhancement serves to resolve the A and B absorption features into five separate electronic transitions. This result is depicted in Figure 2, where the five separate enhancement ranges are superimposed over the A and B bands. The range of the separate profiles and the RR bands associated with each are summarized in Table I. The assignment of observed vibrational features to particular defects is contingent on other measurements as well as consideration of the expected effects of creation of such defects on vibrational energies. We have previously associated the modes at 263 and 287 cm-' with electron and hole polaron defects (P-and P+),respectively.8 These
"The edges for this range are approximate. bThe high-energy edge of this range may be an artifact due to self-absorption.
assignments are primarily based on the effects of lowtemperature photolysis within the IVCT of PtC1. Photolysis causes increases in the EPR signal strength, A, B, and C absorption feature intensities, and the relative intensities of the 263- and 287-cm-' RR band^.^^'^^ Because photoinduced polaronic defects presumably come about via an excitonic intermediate, creation of equal amounts of electron and hole sites is probable. Creation of an electron polaron is expected to have a more dramatic effect on the Pt-halide bond strength.8 Thus, while both 263- and 287-cm-' bands gain intensity upon photolysis, the softer 263-cm-' mode is assigned to the electron polaron and the 287-cm-' band to the hole site. As will now be discussed, the ordering of the electronic transitions associated with enhancement of the 263- and 287-cm-I bands lend further support to their assignment to electron and hole polarons, respectively. Four of the five observed enhancement ranges can be grouped into two sets of two. The appearance of the 263-cm-' peak in the first and fourth ranges suggests that these two ranges demarcate electronic transitions due to electron polarons. On the other hand, the second and fifth ranges entail enhancement of the 287-cm-' peak, which we have assigned to hole polaron defects. Provided that the assignments are correct, these results reveal that the electronic transitions of the electron and hole polarons occur at different energies. This electronic asymmetry is not expected in calculations based on a single half-filled metal band.14 However, the three-quarter-filled two-band model developed by Bishop et a1.12 for these materials predicts asymmetry for the electronic properties of the electron and hole polarons. In this model, the asymmetry arises due to mixing of the occupied metal and halide u levels and is enhanced by inclusion of nonzero Hubbard U (on-sitepairing energy) terms.12 The model predicts two redlnear-infrared transitions for each type of polaron, which are the A (midgap to midgap) and B (band to midgap or midgap to band) transitions, along with midinfrared transitions (C bands) one of which we have observed in photolyzed samples of PtC1.' The electronic asymmetry leads to the following predicted energy ordering for the polaronic transitions: A(P-) < A(P+)< B(P-) < B(P+). Thus, the 1.29-1.44- and 1.92-2.10-eV ranges, characterized by the soft mode at 263 cm-', are assigned to the A and B transitions of the electron polaron while the 1.40-1.75- and 2.18-2.47-eV ranges, associated with a peak at 287 cm-', are assigned to the analogous transitions for hole polarons. This assignment is indicated in Figure 2. The correlation between these separate ranges and the so-called A and B bands as labeled in the single-crystal absorption studies indicates that these experimentally observed absorption maxima are in fact related to distinct defect types, namely, hole and electron polarons, respectively. This is consistent with the conclusion of Kurita et al.,5although they did not specifically assign either band to a particular defect type.
Electrf and Pole Defect Asymmetry The agreement between the observed results for the electron and hole polaron electronic absorptions and the many-body calculations employing a three-quarter-filled two-band model is excellent. We stress, however, that these calculations are based on self-trapping of electron and hole polarons on an otherwise undisturbed lattice. An alternative explanation for the electronic asymmetry between the two polaron types is that one or both are trapped on preexisting defect sites. Several experimental observations are consistent with this idea. In particular, the fact that metastable levels of the defects cannot be generated by photolysis at room temperatures while the levels generated at low temperatures take several hours to vanish upon attaining room temperature may be indicative of the importance of preexisting defects. Also, the EPR signal shape,22 which gains intensity upon photolysis at low temperature^,^ does not exhibit hyperfine structure due to a bridging halide. This result may suggest the importance of adjacent reduced sites, which could be expected to trap hole polarons. Thus, the external pinning potential associated with a preexisting defect that preferentially traps one type of polaron could be the source of the observed electronic asymmetry. To further examine this possibility, experimental and theoretical studies are underway to probe and model potential preexisting defects and the associated kinetics of polaron trapping. The remaining unassigned electronic transition is that located from 1.69 to 1.92 eV and is associated with the 272-cm-l vibrational feature. The assignment of this enhancement range to a particular type of defect state is difficult. One possibility is that a transition from a u halide to a hole polaron level may fall in this energy range. Valence defects, such as bipolarons, kinks, or structurally related defects such as chain ends and missing or extra halides or counterions may also generate gap states at this energy. In this regard, electronic absorption due to electron bipolarons is predicted in this region. Another possibility for this transition is suggested by the fact that the 272-cm-' frequency is identical with that observed for the symmetric C1-PdTv-C1stretch in the Pd analogue of PtCl.= (22) Kawamori, A,; Aoki, R.; Yamashita, M. J . Phys. C : Solid State Phys. 1985,5487-5499. (23) Clark, R.J. H. In Advances in Infrared and Raman Spectroscop y ; Clark, R. J. H., Heater, R. E., Eds.;Wiley: New York, 1984; Vol. 11, pp 95-132.
Chem. Mater., Vol. 2, No. 3, 1990 319 Thus, this feature may be due to Pd impurities present in the Pt metal starting materials2* The band edge for the PdCl complex is located to the red of that of PtC1;23 however, it is not clear that a highly oxidized Pd'" site would be stable within a PtCl chain. In this regard, the mixed-metal chain with PdUand PtIVhas been prepared while the existence of such a chain with the oxidation states reversed has not been convincingly demonstrated.= The assignment of the enhancement ranges to defect state transitions is summarized in Table I.
Conclusions Resonance Raman spectroscopy aids in the characterization of the vibrational and electronic properties of intrinsic defects in PtC1. Defect electronic transitions located to the low-energy side of the IVCT absorption are directly manifested by resonance enhancement of modes that are softer than the v1 mode of the unperturbed chain. There are five distinct transitions in the 1.3-2.5-eV region. Two of these are assigned as the A and B transitions due to electron polaron defects and two as the corresponding transitions for hole polarons. These results suggest that the electronic properties of the electron and hole polarons are dissimilar, which is not predicted by half-filled oneband models. Such asymmetry in the polaron defects could be explained by inclusion of the halide band in calculations of the defect transitions. Alternatively, asymmetric trapping of one type of polaron may give rise to the observed asymmetry. A fifth transition is not readily assigned to a particular type of defect. Acknowledgment. This work was performed at Los Alamos National Laboratory under the auspices of the U.S. Department of Energy, Basic Energy Science, Materials Science Division. C.D.T. gratefully acknowledges the support of the Director's Fellowship Program. We acknowledge the support of the Center for Material Sciences, Energy Research and Development Center at Los Alamos National Laboratory. We are especially indebted to S. Kurita, M. Haruki, A. R. Bishop, and J. T. Gammel for enlightening discussions and for making their results available prior to publication. (24) Krigas, T.;Rogers, M. T.J. Chem. Phys. 1971,55, 3035-3040.
