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(26) Clifford, R. P.; Gowenlock, B. G.; Johnson, C. A. F.; Stevenson, J. J . Organomet. Chem. 1972, 34, 53. (27) Taylor, J. E.; Milazzo, T. S.J. Phys. Chem. 1978, 82, 847. (28) Bernard, C.; Deniel, Y.; Jaquot, A.; Vay, P.; Ducarroir, M. J . Less-Common Met. 1975, 40, 165. (29) Aubreton, J. Laboratoire de Thermodynamique et Plasma, Limoges, personal communication. See also: Yoon, K. H. Ph.D. Doctoral Thesis, Universitd de Limoges, June 30, 1986. (30) Kingon, A. J.; Lutz, L. J.; Liaw, P.; Davis, R. F. J . Am. Ceram. Soc. 1983, 66, 558. (31) Helm, D. F.; Mack, E. J. Am. Chem. Soc. 1937,50,60. (32) Ye, C.; Suto, M.; Lee, L. C. J . Chem. Phys. 1988, 89, 2797. (33) Langlais, F.;Prebende, C.; Couderc, J. P. J. Cryst. Growth 1991,113, 606.
(34) Prebende, C.; Langlais, F.; Naslain, R.; Bernard, C. To be published in J . Electrochem. Soc. (35) Fuyuki, T.; Allain, B.; Perrin, J. J . Appl. Phys. 1990, 68, 3322. (36) Hirschfelder, J. 0.; Curtis, C. F.; Byron Bird, R. Molecular Theory of Gases and Liquids; Wiley: New York, 1967, Chapters VI1 and VIII. (37) Mourits, F. M.; Rummens, F. H.Can. J . Chem. 1977, 55, 3007. (38) Stinespring,C. D.; Wormhoudt, J. C. J. Cryst. Growth 1988,87,481. (39) Annen, K. D.; Stinwpring, C. D.; Kuczmarski, M. A.; Powell, J. A. J . Vac. Sci. Technol., A 1990, 8, 2970. (40) Allendorf, M. D.; Kee, R. J. J . Electrochem. Soc. 1991, 138, 841. (41) Ball, D. F.; Goggin, P. L.; McKean, D. C.; Woodward, L. A. Spectrochim. Acta 1960. 16, 1358. (42) Herzberg, G. Infrared and Raman Spectra; Van Nostrand Reinhold: New York, 1945.
Electronic Structure of Nitrogen Square Planar Copper Complexes in Langmuir-Blodgett Films Stiphane Carniato, Henri Roulet, Georges Dufour, Luboratoire de Chimie Physique, Universit; P. et M . Curie, 11, Rue P. et M.Curie, 75231 Paris Cedex 05, France
Serge Palacin, Andre Barraud, Service de Chimie MolBculaire. Dgpartement de Recherche sur I’Etat Condens;, les Atomes et les MolCcules, Centre d’Etudes de Saclay, 91 191 Gif sur Yvette Cedex, France
Philippe Millii, and IrZne Nemer* Service des Photons, Atomes et MolZcules, Dgpartement de Recherche sur I’Etat Condens:, les Atomes et les MoIZcules, Centre d’Etudes de Saclay, 91191 Gif sur Yvette Cedex, France (Received: November 5, 1991; In Final Form: April 14, 1992)
The Cu 2p and N 1s X-ray photoelectron spectra of nitrogen square planar copper(I1) complex, derived from copper phthalocyanine and especially substituted to produce Langmuir-Blodgett (LB) films,are reported and compared with those of commercial copper phthalocyanine (CuPc) and selected porphyrin compounds. Although the copper atom is found primarily in the Cu(I1) state, we observe the presence of the reduced Cu(1) form, with a great variety of relative intensities, because of a concomitant ability to reduction. In contrast, in the LB films, the copper atom remains in the Cu(I1) state. We discuss this different behavior in terms of the reduction degree of the molecule, the localization of the additional electrons on the metal or the macrocycle, and a different nitrogen geometry around copper.
I. Introduction The electronic structure of copper(I1) compounds in a square planar environment is a matter of active interest because (i) the discovery of high- T, superconducting materials has stimulated a number of studies’ on the electronic properties of copper perovskites, in which the copper atom is found with four oxygen atoms in a square planar arrangement, and (ii) the search for conducting organic two dimensional thin films has favored the choice of substituted phthalocyaninecompounds: in which the copper atom is found in the middle of a nitrogen square plane. The electronic and magnetic properties of these systems have been successfully probed by X-ray photoelectron spectroscopy (XPS). Copper oxides, for example, are considered as model systems for semiconductors. CuO is well establi~hed~-~ to be a charge-transfer antiferromagnetic semiconductor with a small gap (1.4 eV), in which copper is in the Cu(I1) state with an open shell 3d band. In contrast, C u 2 0 is a textbook semiconductor (gap of 2.17 eV), in which the copper atom is in the Cu(1) state, i.e. in a situation closer to Cu metal,3 i.e. with a full 3d band. The Cu 2p s p t r u m 3of CuO shows two doublets separated by about 20 eV due to the spin-orbit splitting. Each doublet consists of one intense “main peak” (E(2p312) = 933.2 eV) and a broad satellite structure at some 9 eV toward high binding energy. According ‘To whom the correspondence should be addressed.
to previous experimental and theoretical studies?-5 the formation of a 2p hole favors a charge transfer from the oxygen ligands onto the open 3d shell of the metal (mainline) and strong 2p,3d electron interaction effects (satellite). In contrast, in Cu20, the 3d copper band is full, and there is no possible coupling of the 3d electrons with the 2p hole and the XPS spectrum is reduced to a single line ( E ( ~ P ~=, 932.4 ~ ) eV) with a very weak satellite. Very similar observations have been made on XPS spectra of high-T, supraconducting materiah6 Other XPS studies of copper(I1) compounds with ligands other than oxygen,’ including amorphous copper phthalocyanine: show that the satelliteshape and its energy separation from the main peak depends significantly on the local symmetry and the nature of the ligand. In this paper, we are considering a copper atom surrounded by four nitrogen atoms in a plane square geometry at the center of phthalocyanine and porphyrins systems (Figure 1). Among them, the so-called CuS18 one is of particular interest because its amphiphilic character favors the formation of LangmuirBlodgett (LB) films.9 The CuS18 molecule is basically a copper phthalocyanine with four pyridine rings in the outskirts of the macrocycle, each pyridinic nitrogen atom being quaternized to attach a carbon chain. The main difference of such LB films with the same compound in the amorphous phase is that the molecule, when processed in LB films, is built-in in a reduced form ( C U S ~ ~ ~In~ any ~ - of ) ?the complexes of Figure 1, the central
0022-3654/92/2096-7072103 .00/0 0 1992 American Chemical Society
Electronic Structure of Square Planar Cu Complexes
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 IO13
c u Si8
,+ .:. ,*
a
c u 2p l
N 1s
Cu TPy Pz -d
*. L
: %..
970
950
930
404
400
396
BINDING ENERGY ( e V )
Figure 2. Cu 2p and N 1s XPS spectra of (a) CuS18, amorphous partially reduced; (b) Cu(TPy)Pz; (c) Cu(TPy)P (d) CuPc; and (e) CUP. cu
Pc
Cu TPy P
c u 2p
Cu S18 L.B.
0 NI
cu P Figure 1. Schematicsof the copper(I1) complexes studied in the present work: CuPc, copper phthalocyanine; CUP,chlorophyllin coppered trisodium salt; Cu(TPy)P, copper 5,10,15,20-tetra(4-pyridyl)prphyrin; Cu(TPy)Pz, copper tetrapyridino[3,4-b3”4’~g:3’’,4’’-1:3”’,4”’-q]porphyrazine; CuSl8, copper tetrao“adecyltetrapyridino[3,4-b:3’,4’-g:3’’,4’’1:3”,4”’-q]porphyrazinium bromide.
copper atom is knowdoto be in the Cu(I1) state, including CuS18 in the amorphous phase. According to previous characterization, especially ESR? CuS18 in LB films surprisingly keeps the same Cu(I1) state, although two additional electrons attach themselves to the molecule and should put the copper in the Cu(1) state. The question in this paper is aimed at understanding this problem, especially the localization of the supplementary electrons on the molecule. We have performed XPS experiments in the region of N 1s and Cu 2p lines. Having in mind that the nitrogen square planar complexes should be very similar to their oxide counterpart, we expect to probe the copper oxidation state by direct analysis of main line and satellite intensities.
II. Experimental Section The X-ray photoelectron spectroscopy measurements have been obtained in a KRATOS DS800 apparatus. The X-ray source is a nonmonochromatized Mg Ka12 source (hu = 1253.6 eV). The hemispherical analyzer is operated in the fixed pass energy mode, with an energy resolution set at 0.86 eV, as measured on the Ag 3dS,2line (fwhm). We have taken the C 1s line of carbon contamination at 284.5 eV as the reference energy, because of charge effects. On each original spectrum, we have performed a Shirley type of background substraction. All XPS spectra are presented on a scale with 0 as the Fermi level. We have purchased CuPc from Fluka and CUPfrom Aldrich. We have synthetized Cu(TPy)P as described in /2/, Cu(TPy)Pz and CuS18 as described in ref 9. The LB films have been elaborated using the method published previously.’l The subphase is Millipore Q-grade water, unless noted otherwise, and all experiments are conducted at 20-22 OC in a nitrogen atmosphere. The silicon substrates are cleaned by the usual rigorous procedure. The transfer is achieved at the constant surface pressure of 35 ”am-’ using the classical vertical dipping method. Transfer
BINDING ENERGY ( e V )
Figure 3. Cu 2p and N 1s XPS spectra for a series of LB films with increasing number of monolayers (ML).
speed generally ranges between 0.5 and 2 cm-min-I. The coated substrates are blown between each dip with dry nitrogen. Under these conditions, a perfect Y type deposition is observed and the transfer ratio is found to be close to unity either for upstrokes or for downstrokes. All compounds other than LB films are used in an amorphous phase. The powders are compacted and deposited on the stainless steel sample holder with silver colloid glue. In the case of LB films, they are prepared directly on silicon thin plates which are then attached to the sample holder. Special care has been taken on sample handling and storage. We have made different measurements on freshly made samples in order to avoid oxidation by air. The reduction effects seen upon irradiation on specific compounds have been of special interest in the present study. 111. XPS Results and Discussion
(A) Geaerpl characteristicsof CoPperTetram Complexes. The Cu 2p and N Is spectra of the series of amorphous Cu-tetraza complexes (Figure 1) are reported in Figure 2. The spectra of CuS18 in LB films are reported in Figure 3. All spectra have been obtained with typically 60-min recording time. Binding energies for the main lines and satellites are reported in Table I together with the results of our deconvolution procedure when several components are present. From the position of the main lines in CuPc, we have clearly the reference for the Cu(I1) oxi-
Carniato et al.
7074 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992
TABLE I: Experimental Binding Energies and Relative Intensities of Copper(1) Contribution after Deconvolution, in the Copper 2p and Nitrogen 1s XPS Spectra of Amorphous Copper Complexes and LB Films Cu 2p (eV) A c d N Is (eV) Cu(I1) CW) (AiCuil+C”(ll)+lat.))~ 41 compounds (main line) (”/.I a b (4 + Ab)* CuS18 (amorphous phase, partially reduced) Cu(TPy)Pz Cu(TPy)P CuPc CUP
934.8 934.8 934.8 934.8 934.8
CuS18 cusia cusia cusia CuS18
934.8 934.8 934.8 934.8 934.8
(3 ML) (20 ML) (45 ML) (loo ML) (amorphous phase unreduced)
932.4 932.5 932.5 932.5 932.8
50 42 53 11 11
LB Films 932.6 932.6 932.6 932.6
18
a 6 20 18
398.3 398.3 398.3 398.3 398.3
400.0
0.5
398.2 398.2 398.2 398.2 398.3
399.5 399.5 399.5 399.5 400.1
0.53 0.53 0.53 0.55 0.44
“Ratio of peak areas obtained after deconvolution into Cu(1) and Cu(I1) main lines. *Ratio of peak areas obtained after deconvolution into two N Is components (see text).
dation state. This is also seen by the presence of the satellite structure observed some 9 eV above. This is very close to the CuO behavior (see Introduction). A very similar observation is made for porphyrin (Figure 2e). Surprisingly, on the other complexes of Figure 2a-2, the main peaks (2p3/2and 2p1/2)are broadened and even split toward lower binding energy. Meanwhile, the relative intensity of the satellite with respect to the integrated main peak diminishes in the cases of CuS18, Cu(TPy)Pz, and Cu(TPy)P. We interpret these observations as the presence of two oxidation states of copper, Cu(1) and Cu(I1). In other words, for specific compounds, the molecular sample is partly reduced at the copper site, probably because of irradiation in the apparatus. In amorphous CuS18, fresh samples show essentially a Cu(I1) signal and the Cu(1) one appears gradually. In LB films, the spectra are dominated by the Cu(I1) component, but there is always a significant part of Cu(1) signal. The N 1s line shape is not so straightforward because in the series of complexes there are several nitrogen atoms in different chemical bonding situations which could be responsible for significant chemical shifts. Indeed, in Figure 2 we observe a N 1s line with different shapes. In prophyrin, CuPc, and Cu(TPy)Pz, one observes a quasi-Gaussian signal centered a t 398.3 eV with a small asymmetry on the high binding energy side. In these latter compounds, there are respectively one, two, and three “kinds” of nitrogen atoms. They appear as a single line without any chemical shift probably because they are in normal trivalent state in any of the positions of the macrocycle. Indeed, in other N-trivalent molecules such as pyridine, the N 1s XPS line is located at 398.0-eV binding energy.12 The asymmetry already observed in CuPc compounds13is due to a satellite line originating from a .n .n* valence excitation associated with the core ionization, as discussed in ref 13. In Cu(TPy)P, the N 1s line is clearly broader and a t least two lines contribute to it. We believe that there is a contribution of two classes of nitrogen atoms which are totally decoupled. The low binding energy component is likely due to the peripheral nitrogen atoms, whereas the other is due to those of the macrocycle. In CuS18, the N 1s line is composed of two components. This is readily interpreted by associating the quaternized atom to the high binding energy component (400 eV) and the main signal (398.3 eV) to the atoms belonging to the main part of the macrocycle. The splitting of trivalent and tetravalent nitrogens is found to be +1.7 eV in the direction with the chemical shift of +2.6 eV found in small nitrogen substituted compounds.I3 In the LB film case, the N 1s line keeps a similar shape as for CuS18 in the amorphous phase with a small shift of the maxima (398.3 and 399.6 eV) toward lower binding energy and a reduction of the splitting from 1.7 to 1.4 eV together with a reduction of the total width. This change is indeed small considering the width of each peak, but it is systematically found independently of the number of layers. This point is discussed in the following section. (B) Reduction Effects. The interpretation, offered above, of line splitting in the Cu 2p XPS spectrum in some of the complexes (Figure 2) in terms of a reduction ability of the molecule at the
-
Amorphous
- I
Cu
S18
.. .. d
,:
r‘
Chemically reducad
..C
BINDING ENERGY ( eV )
F i e 4. C u 2p and N 1s XPS spectra of unreduced amorphous CuS18: (a) fresh sample; (b) irradiated sample (90 min); (c) chemically reduced sample.
copper site is now examined in detail on the basis of additional measurements on the CuS 18 amorphous species. We have performed the measurements on freshly made samples (Figure 4a) and on other samples exposed to X-ray irradiation in the XPS apparatus during 1-2 h (Figure 4b). For less irradiation exposure, we obtain a variety of spectra in intermediate situations between the spectra of Figure 4a,b, with an increasing contribution of the Cu(1) peak with irradiation time, together with a parallel reduction of the satellite intensity measured with respect to the integrated intensity of the main line. These observations confirm that irradiated CuS18 amorphous samples are increasingly reduced at the copper site, giving the superposition of two spectra, one being the “normalmCu(I1) and the other the “reduced” Cu(1). We have made similar observations for the Cu(TPy)Pz molecule, but the irradiation effects are less pronounced compared to the CuSl8 amorphous species. In contrast, the CuPc samples stay remarkably stable upon irradiation. Those irradiation effects have been observed before in other copper compounds. In copper oxide, CuO, the reduction of copper giving Cu20 (Cu(1)) has been observed14 but after some 15 h of X-ray exposure. The most likely interpretation is that secondary slow electrons that are efficiently produced after any inner shell e x ~ i t a t i o nattach ’~ themselves to the molecule, as long as the electron affinity is large enough. Indeed the small gap of copper oxides favors the phenomenon. In order to confirm the reduction process, we have performed a
Electronic Structure of Square Planar Cu Complexes chemically induced reduction of the CuSl8 amorphous species, using sodium dithionite in a CHC13/H20mixture, in the presence of a phase-transfer agent. The spectrum is shown in Figure 4c. Here the main line reduces to a single Cu(1) contribution with a very small satellite as normally expected. This confirms that the CuS18 amorphous species is easily reduced at the copper site. The information extracted from the N 1s spectra, also reported in Figure 4, is less striking. We see a small narrowing of the line for the irradiated sample, in contrast to the chemically reduced sample. If the extra electrons partly attach themselves to the quaternized nitrogen atoms, we should observe a merging of the quaternized nitrogen atom signal into the main signal. This is perhaps the case of the irradiated sample. However, it is impossible in the case of the irradiated sample to correlate this observation to either &(I) or Cu(I1). In the case of the chemically reduced amorphous sample (Figure 4c), we are in a pure Cu(1) case and the N 1s signal shows a normal quaternized nitrogen contribution, well separated from the main peak. films (Figure 3). We turn now on the behavior of Those species like CuPc have a remarkably stable Cu 2p XPS spectrum with a dominant Cu(I1) character. We do observe a Cu(1) signal which varies with the number of layers from 6 to 20% (Cu(I)/(Cu(II) + Cu(1) + sat.) ratio but this ratio is constant over irradiation time. In the first place, we kn0w~3'~J'that CuS18 is spontaneously reduced by water at the water interface, during the monolayer formation. This spontaneous reduction arises from the combination of local field effects, the dimerization of the macrocycle at the water interface and the outstanding accepting properties of the S18 macrocycle." The final reduced form is perfectly stable within the LB films and is consistent with a two-electron reduction of the macrocycle. One then understand that the LB film cannot accept easily extra electrons, and this explains its stability upon further reduction and the observation of a persistent dominant Cu(I1) signal. The observation of a significant remaining Cu(1) peak may be partly related with the well-known paramagnetic impurity which gives rise to an ESR signal on diamagnetic phthalocyanines in the solid state18 but accounts for less than 10% of the whole material. The problem is now to analyze the reason for this contrast between the two species. The necessary condition to have a Cu(I1) state in LB films is that the antibonding 3 d + 2 copper orbital is half-occupied, as in the normal amorphous CuS18 molecule. It is known that this orbital is not a good electron acceptor because this orbital is strongly localized on the copper site leading to a large bielectronic repulsion. If one discusses the electronic structure of the negative ion species in terms of the relative orbital energy of the LUMO of the unreduced ground-state species, then it is likely that the two-electron reduction proceeds by occupation of the eg(?r*)orbitals. These orbitals which are delocalized on the macrocycle are the vacant orbitals of lowest energy, as shown by Henriksson et al.19 who has calculated the ground state of CuPc using a semiempirical method. In the reduced amorphous species, the observation of Cu(1) in the series of amorphous compounds except CuPc shows that the copper site plays an active part in the reduction process. We suggest that this form is reduced through a three-electron process, following the work of Giraudaw et aLzoon copper tetracyanotetraphenylporphyrin(CN,(TPP)Cu). Indeed, the addition of a third electron is not likely in the doubly occupied e,(?r*) orbital because of electrostatic repulsion, favoring the double occupation of the copper 3dX2-,,zorbital. Alternatively, if we consider that the reduced amorphous CuS18 species has only two extra electrons, then one should invoke a distortion around the copper site compared to the LB film. This induces a relative energy change of the 3d9-9 and e,(.*) orbitals. Indeed, the energy of the antibonding orbital 3d9-9 is very sensitive to the Cu-N distances2'According to ref 16, the CuS18 molecules face each other as dimers and the distance between copper atoms of neighboring molecules is close to 5 A. In the amorphous species the Cu-Cu distance is probably much shorter since, in 0-CuPc, it amounts to 3.25 A.22 Therefore there is a significant interaction between neighboring molecules in the amorphous phase which probably disappears in LB films. We conclude that in LB films,
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 7075 the molecule is planar, whereas, in the amorphous phase, there is a distortion around the copper site. The fact that CuPc is not reduced easily as compared to the isoelectronic species Cu(TPy)Pz is likely due to the structure of the macrocycle. Indeed, electrochemical results23 and band calculations24have shown an energy lowering of the e,(?r*) orbital in the Cu(TPy)Pz compound as compared to CuPc. Finally, on the basis of the above discussion, we reject the previous interpretation of Healy et al.25on related copper complexes who observed also a splitting of the copper 2p main line but interpreted it as an evidence of a copper atom in the Cu(II1) state.
IV. Conclusions XPS measurementson nitrogen square planar complexes, either in the normal amorphous phases or in reduced LB films, offer a direct probe of the copper oxidation state. In the substituted Cu-phthalocyanine CuS18, when in the LB form (reduced), the copper atom is found primarily in the Cu(I1) state, the extra electrons being located on the macrocycle. In contrast, in the reduced amorphous CuS18 species, the metal is in the Cu(1) form showing clearly that the extra electron is found unambiguously on copper. This markedly different behavior is tentatively interpreted in terms of a different number of added electrons or perhaps in terms of an intramolecular distortion at the copper site, probably less pronounced in LB films compared to amorphous species. Further X-ray photoabsorption experiments near the Cu 2p and N 1s edges, which will allow a probe of the lowest unoccupied orbitals, and the Cu-Cu distance are in progress to confirm the present interpretation. RegiPtry NO. CuS18, 104574-70-1; Cu(TPy)Pz, 15275-52-2; CU(TPy)P, 14518-23-1; CUPC,147-14-8; CUP,11006-34-1.
References and Notes (1) Gourieux, T.; Krill, G.;Manter, M.; Ravet, M. F.; Menny, A.; Tolentino, H.; Fontaine, A. Phys. Rev. B 1988, 37, 7516. (2) Ruaudel-Teixier, A.; Barraud, A.; Belbeoch, B.; Roulliay, M. Thin Solid Films 1983, 39, 33. MacArdle, C. B.; Ruaudel-Teixier, A. Thin Solid Films 1985. 133. 93. (3) Ghijkn, Tjeng L. H.; Van Elf, J.; Eshes, H.; Westerink, J.; Sawatzky, T. A,; Czyzyk, M. T. Phys. Rev. B 1988, 38, 11322. (4) Okada, K.; Kotani, A. J . Phys. Soc. Jpn. 1989, 58, 2578 (51 Larsson, S. Chem. Phvs. Leii. 1977. 48. 596. (6) Roulet, H.; Dufour, G.: Cheene, A,; Rochet, F.; Carlier, C. Appl. Surf. Sci. 1991. 47. 173. (7) Van d& Laan, G.;Westra, C.; Haas, C.; Sawatzky, T. A. Phys. Rev. E 1981, 23,4369. (8) Frost. D. C.; Mc Dowell. C. A.; TaDDina. . _- R. L. J. Eleciron Soecirosc. Relai. Phenom. 1975, 6, 347. (9) Palacin, S.;Ruaudel-Teixier, A.; Barraud, A. J . Phys. Chem. 1986,90, 6237. (10) Lin, W. C. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol IV, p 358. Skorobogaty, A.; Lancashire, R.; Smith, T. D.; Pilbrow, J. R.; Sinclair, G.R. J . Chem. Soc., Faraday Trans. 2 1983,79, 1123. (11) Barraud, A.; Leloup, J. French Patent No. 83 19 770. (12) Siegbahn, K.; et al. Aiomic Molecular and Solid Siate Structure Studied by Means of Electron Spectroscopy; Nova Acta Regiae Societatis Scientiarum Upsaliensis, Series IV, Vol 20; Uppsala, Sweden, 1967. (13) Niwa, Y.; Kobayashi, H.; Tsuchiya, T. J . Chem. Phys. 1973,60,799. (14) Navaks, V. T.; Prins, R. Solid Siaie Commun. 1971, 9, 1975. (1 5) Nenner, I.; Monn, P.; Lablanquie, P. Comments Ai. Mol. Phys. 1988, 22, 51. (16) Porteu, F.; Palacin, S.; Ruaudel Teixier, A.; Barraud, A. J . Phys. Chem. 1991,95,7438. (17) Palacin, S.; Barraud, A. Colloids Surf. 1991, 52, 123. (18) Lin, W. C.; Lau, P. W. J . Am. Chem. Soc. 1976,98, 1447. Harbour, J . R.; Loufty, R. 0.J . Phys. Chem. Solids 1982, 43, 513. Boas, J . F. Ausi. J . Chem. 1987, 40, 557. (19) Henriksson, A.; Roos, B.; S u n d h m , M. Theor. Chim. Acta (Berlin) 1972, 27, 303. (20) Giraudaux, A.; Louati, A.; Gross, M.; Callot, H. J.; Hanson, L. K.; Rhodes, R. K.; Kadish, K. M. Inorg. Chem. 1982, 21, 1581. (21) Schiiffer, A. M.; Goutermann, M.; Davidson, E. Theor. Chim. Acta (Berlin) 1973, 30, 9. (22) Brown, J. C. J . Chem. Soc. A 1968, 2488. (23) Van Wieberge, B.; Yang, M. 2.;Sauvage, F.; de Backer, M. G.; Chapput, A. Specirochim. Acia 1986, 42A, 1133. (24) Canadell, E.; Alvarez, S.Inorg. Chem. 1984, 23, 573. (25) Healy, P. C.; Myhra, S.; Stewart, A. M. Jpn. J . Appl. Phys. 1987, 1 1 , L1884. ~
