Interfacial behavior of hydrogen-treated sulphur deficient pyrite (FeS2−x )

Solar Energy Materials 18 (1988) 9-21 North-Holland, Amsterdam

INTERFACIAL BEHAVIOR OF HYDROGEN-TREATED SULPHUR DEFICIENT PYRITE 0FeS2_ . )

N. ALONSO-VANTE, G. CHATZITHEODOROU, S. FIECHTER, N. MGODUKA, I. POULIOS * and H. TRIBUTSCH Hahn-~deitner-lnstitut, Bereich Strahlenchemie, Glienicker Strasse 100, 1000 Berlin 39, Germany Received 10 June 1988; in revised form 26 July 1988 The influence of molecular as well as atomic hydrogen on the interracial properties of pyrite has been investigated. Molecular hydrogen selectively converts FeS2 surfaces other than the {100} to FeS. Atomic hydrogen generated by electrochemical and chemical treatment does not lead to selective attack of the pyrite surface but significantly improves the photocurrent to dark current ratio. This experimental result evidences the role of atomic hydrogen in the crystal lattice of the material ":n increasing the barrier height at the solid-electrolyte junction and decreasing the concentration of electromc states within the forbidden energy region. Atomic hydrogen is believed to associate itself with sulfur atoms adjacent to sulfur vacanc,_'es. It thereby attracts negative charges from iron and transforms the electronically unfavourable FeS states which produce defect levels in the band gap ir~to more favourable FeSH centers.

I. Introduction

Pyrite single crystals produce0 in this laboratory have been studied in photoelectrochemical cells containing different redox species [1]. One of the best systems turned out to be that formed by the junction FeS2-iodide/iodine [2]. Here, the photocorrosion of the material (E s = 0.95 eV) was completely stopped, but unfortunately the output energy of this photocell is siil! t¢o low (2.8~) due to a small photopotential (0.1 to 0.2 V). The low energy conversion output of this me~erial is related to the poor current-voltage characteristics involving a large dark current and a small fill factor. The observation of an improvement of the current-voltage characteristic following cathodic hydrogen evolution is the subject of the present work.

2. Experimental Single crystals of pyrite were grown from its elements using bromine as transport agent, or directly from pyrite powder by chemical vapor transport [3]. Prior to all experiments, the crystals were etched with HF:CH3COOH:HNO3 (1 : 1 : 2) and subsequently rinsed with bidistilled water. The treatment with molecu* Permanent address: Laboratory Physical Chemistry, University of Thessaloniki, Greece.

0165-1633/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

N. Alonso-gante et a L / lnterfacial behavior of hydrogen-treated pyrite

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lar hydrogen was carried out at 400 ° C for 5 rain. Electrodes were then prepared by rubbing the back surfaces of the crystals with I n - G a amalgam followed by silver paste (Scotchcast 3M), making a contact with the copper shaft of Vespel holders and encapsulated with an electrical resin (Scotchcast 3M). The surface of the electrodes was polished with diamond paste (Winter corp.) down to 0.25 /~m, followed by a rinse with bidistilled water. A standard photoelectrochemical apparatus was used for the recording of (photo)current-voltage curves. Illumination was suppfied by a halogen-W lamp (250 W). The cell had one compactment containing the working electrode, the reference electrode Hg/Hg2SO 4 (0.615 V/SHE), a platinum ring as counter electrode, and a bubbler inlet.

3. Results

3.1. Oriented pyrite surface~electrolyte interface Crystallographically orientated F e ~ surfaces were placed in contact with electrolytes to determine their influence on the photoelectrochemical behaviour of this material. Because of the cubic structure of pyrite no significant difference was expected between {100} and {110} or {111} interfaces. An experimental verification seemed to be nevertheless necessary because of observations explained below. In fact, after having obtained for every surface oriented electrode a minimum level of the dark current (see section 3.3), the resulting photocharacteristic showed no big difference, see fig. 1. 40 C'4 |

E (3 < E

30

r-

.m

(J3 c" Q~

~

"1

0 C

2O

a L

.-1 (.} o Q .,c n

10

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0 -0.8

Y i -0.6

I -0.4

i

I -0.2

~.

3

, -0.0

.

I 0.2

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Electrode potential / V (Hg2SO4) Fig. 1. Photocurrent-voltage characteristic obtained when the level of dark currents is minimal for surface oriented pyrite electrodes. (a) {100}; (b) {111};(c) {111), in !-/I~ solution. Illumination ~ 0.5 W cm-2.

N. Alonso.Vante et al. / Inwr.facialbehavior of hydrogen.treatedpyrite

11

3.2. Molecular hydrogen A clear surface orientation effect was however found during treatment of pyrite with molecular hydrogen. The exposure of pyrite single crystals to a molecular hydrogen atmosphere at 400°C during 5 rain affects surfaces other th~n the {100}, as shown by the optical microscope picture in fig. 2. The dark surfaces are clearly visible on the phot¢~raph. They are the consequence of FeS formation, as confirmed by X-ray fluorescence, see table 1. It has to be pointed out that at 400°C pyrite starts to become thermodynamically unstable. The hydrogen treatment procedure was repeated using the same conditions to expose different crystal surfaces of Ires2 which were subsequently placed in contact with an electrolyte. Significant differences of current-voltage curves were observed on mounted surfaces as obtained from the hydrogen treatment. Etching of the darkened (FeS containing) surface resulted in the elimination of the IreS layer, but did not improve its photocharacteristic. The conclusion drawn from these observations was that some intrinsic defects, probably FeS centers, could be built in the bulk of the c~sta]~ (see ~ f i ~ n 3.5). The

e

OA mm

I

Fig. 2. Light microscopic picture of ~ pyrite single crystal after treatment with molecular hydrogen at 400 ° C (5 min). The smooth surface corresponds to the orientation { 100}. Other faces are of index type {hki} (e.g., 111, 210, 321).

12

N. Alonso-Vante et at. / Interracial behavior of hydrogen-treated pyrite

Table 1 Energy dispersive analysis of X-rays of a hydrogen treated pyrite single crystal (cL fig. 2) Surface

{loo} {hkt}

Atom percentage Fe

S

37.5 45.4

62.5 54.6

{100} surfaces, which were not altered by molecular hydrogen at 400°C, did, in contrast, maintain its original (photo)electrochemical quality. 3.3. Atomic liydrogen

In order to study the effect of atomic hydrogen in pyrite, two approaches were undertaken: electrochemical, and chemical treatment. 3. 3.1. Electrochemical treatment Fig. 3 shows the evolution of the current-voltage characteristic of a {100} surface F e ~ electrode in I - / l i solution as a function of hydrogen generation for different periods of electrochemical proton reduction at -1.25 V. Other surface orientations gave similar curves. These curves have been analyzed at - 0 . 3 V, and their respective Id, Iphot (dark current and photocurrent) plotted as a function of time of proton reduction (fig. 4, cf. fig. 3b). For several differently oriented surfaces a decrease by 905 of the dark current after 15 rain of proton reduction is observed which remains at approximately the same level for longer periods of hydrogen evolution. On the other hand, for the same time interval of hydrogen pretreatment, the photocurrent also reached a saturation level. The described experiments clearly show that the hydrogen species is involved in a fa~,om-able inter'facial reaction with FeS2 which has further been explored with the following experiment. The electrochemical hydrogen treatment was performed in absence of I - / I i with 0.5M H2SO4 only. The beneficial effect of the hydrogen evolution treatment on the (photo)electrochemical characteristic of FeS2 turned out to be the same. A difference was found in the Tafel plot, which, however, can be easily understood: the hydrogen evolution reaction can be perturbed by the presence of foreign ions in the electrolyte, as has long been known [4]. The Tafel plot of fig. 5 displays this difference. A gain of 90 mV in the overpotential is observed, for the same current density, when 0.1M I - is added to 0.5M H2SO4. Again, no significant differences for the same reaction with other surface orientations were found. 3. 3. 2. Chemical treatment To confirm the indications of a possible atomic hydrogen interaction with the pyrite surface, the electrodes were exposed ex situ to atomic hydrogen produced chemically in a solution containing 57~ HI and Zn according to the reaction: 2HI 4 Zr,--, Znl 2 + 2Hnasc" Then, as a function of the exposure time of the elec-

N. Aionso.Vante et aL / Interfacial behavior of hydrogen.treated pyrite

13

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250

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t / rain Fig. 3. (a) Some current-voltage characteristics of pyrite { 100} electrode in 4M HI + 2M Cal 2 + 0.05M 12 solution under intermittent illumination (1.06 W cm-2). The decrease and increase of dark current and photocurrent (measured at - 0 . 3 V) as a function of time of proton treatment at - 1.25 V is indicated in (b), respectively. The curves numbered (1), (2) and (3) correspond to 0, 10 and 140 min of hydrogen time treatment, respectively.

trode in this solution, the photocharacteristic of the electrode was recorded in l - / I ~ solution and plotted in fig. 6. This figure shows again a similar behaviour to that obtained in fig. 4.

3. 4. Influence of proton and iodide concentration The concentration of protons is relevant for proton reduction leading to an improvement of the pyrite interface and the I- concentration for the photoelectro-

N. ,41onso-Vante et oL / Interracial behavior of hydrogen-treated pyrite

14

'°I

"

I

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"

b

•~ c-

•

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c c~ $ _

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ar

u o.,

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t/min Fig. 4. Evolution of dark cm-rent (filled symbols) and phot,~current (open symbols) (measured at - 0.3 V) as a function of the time of proton reduction ( - 1.2 V) at surface-oriented pyrite electrodes represented in fig. 1.

chemical quality of the pyrite electrolyte interface. More information was therefore needed about the interracial activity of these ionic species. An experiment was performed in which a dynamic (photo)current-voltage of pyrite was taken in presence of 0.05M I2 + 1M CaI2. The iodide concentration resulting from the dissociation of CaI 2 was sufficiently large to yield a photocurrent density exceeding 60 m A / c m 2, see fig. 7. The measured redox potential of the electrolyte was - 0 . 1 6 V/Hg2SO 4 and the open circuit pb.movoltage amounted to

3

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H2SO4 +0.1M !-.

N. Alonso-Vante et ai. / Interracial behavior of hydrogen-treated pyrite

15

80

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t/rain Fig. 6. Measured evolution of the dark and photocurrent of pyrite at 1 V in I - / I ~ electrolyte (at a fllu~4nation level of 0.7 W em-2), after a given exposure time in a HI ( 5 7 ~ ) + Z n i[2 g) solution; of. fig. 4.

96 mV. Addition to this electrolyte of successively two times 2M HI yielded redox potentials of - 0 . 2 8 and - 0 . 4 4 V, respectively. The photopotential increased thereby to 110 and 134 mV, respectively. Additions of HI had consequently a posittve influence on the energy converting p y r i t e / ( I - / I 3 ) interface. However, the change of redox potential into negative direction could theoretically not increase the photopotential if the added redox species would not counteract the unpinning of the pyrite energy bands. In other words, in order to explain the increase of the photopotential with increasingly negative redox potential of ~he electrolyte, we have to assume that the negative iodide ions are partially neutralising the positive charge accumulating on iron-based surface states. Tlds conclusion is in agreement with recent experimental evidence on a coordination chemical interaction between I Br-, CI- and interfacial iron states in pyrite [5]. The modification of the current-voltage characteristic of pyrite upon addition of HI is shown in fig. 7. In order to verify whether an increase of the proton concentration has an influence on the double layer of pyrite, an experiment was performed in which an excess of H2SO4 was added to the electrolyte. No improvement of the photocurrent-voltage characteristic could be obtained. This observation confirms I - as a surface active species and indicates that there is no specific (chemical) interaction between the proton and the pyrite surface which may lead to an improvement of the photoactivity.

3.5. Crystal sulphur deficiency Based on numerous band structure calculations for cubic pyrite, an empty conduction band and a valence band occupied with paired electrons, are expected

16

IV. Alonso-Vante et al. / Interracial behavior of hydrogen-treated pyrite

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Electrode potential/V (H(J2S04) Fig. 7. Current-voltage behaviour of the illuminated pyrite in O.05M 12 + 1M Cai 2 + x M H I solution. ( ) x = 0; ( . . . . . . ) x = 2; ( . . . . . ) x -- 4. Positive sweep direction under illumination ( - 0.7 W ¢m - 2 ) and negative sweep direction in dark.

Table 2 Correlation between structural features and magnetic properties of different iron sulfide phases FeS2 (pyrite) a)

FeS (tetragonai) b)

FeS (cubic) c~

FeS (hexagonal) d)

Space group

P a3

P 4/nmm

F Fl3m

P 63/mmc

Lattice constants (,~)

ao = 5.417

ao = 5.423

M e - S distance (A) Fe 2 +-coordination Magnetism (expected) (observed) X ej (emu/g)

2.26 Octahedral Diamagnetic Paramagnetic +0.175 x 10 - 6

a o -- 3.768 Co = 5.039 2.30 Tetrahedral Diamagnetic Diamagnetic

ao = 3.438 Co = 5.880 2.47 Trigonal prism Paramagnetic Antiferromagnetic + 12.6 x 10 - 6

a) Refs. [7-9].

b) Refs. [10,121.

o Refs. [11,12].

d) Refs. [8,13].

2.35 Tetrahedral Paramagnetic Paramagnetic

e) Magnetic susceptibility at 300 K . -

N. Alonso.Vante et al. / Interracial behavior of hydrogen-treated pyrite

17

0.26

+i+

O.22 uI . sw 0.20

~, ~ o.18 ~] ~ o.16 •

~1

!

0.14 0.120

•

t

100

200

•

300

T[K] Fig. 8. Magnetic measurements on (a) synthetic samples and on (b) natural samples from ref. [9]. The synthetic samples were measured with a Faraday-magnetometer in the 10 mg balance range at a magnetic field of 10 kOe. Further details are given in reL [20].

[6]. The ideal octahedral coordination of Fe 2+ by S~- provokes a low spin state of the iron 3d level [electronic configuration (t2s)6(es) °] with the highest possible crystal field stabiLLzation energy [7]• However, the brass yellow colour of the compound, the high absorption coefficient, its small van Vleck paramagnetic behaviour and high conductivity speak in favour of a degenerate rather than an ideal senuconducting material. A sulphur deficiency in the S~- sublattice could be the reason for cubic pyrite having properties of an electronically degenerate material as the following consideration shows. A transition from an expected low spin to high spin state becomes probable if the ligartd field is changed modifying or distorting the coordination of the cations, which can be deduced from the metal-ligand distance. Table 2 summarizes the iron-sulfur distance, the lattice constants and the magnetic properties (expected and observed) of some Fe-S compounds. Contrary to all theoretical expectations, pyrite is astonishingly paramagnetic (positive sign of the magnetic susceptibility), fig. 8. The observed paramagnetism is influenced by impurities [9] (curve b in fig. 8). Pyrite, even ~,hen prepared from iron and sulfur of high purity ( > 6N) showed paramagnetism, table 3. To get rid of impurities in crystals grown by chemical vapour transport with halogen, pyrite was crystall:,zed using Te fluxes [16]. No significant improvement of the semiconducting properties has, however, been obtained to date. Sulphur analysis of powders and crystals revealed a deviation from the ideal composition of up to 1.3 wt~ which corresponds to a composition FeS2_ x for x _<0•07, table 4. Pyrite powder from Cerac probably contains elemental sulphur, which evaporates when heated at 200 o C under vacuum. Normally, pyrite powder decomposes at 400 °C into FeS and S as determined by thermogravimetric measurements. Powders from Johnson Matthey Co. and Alpha Ventron might not be pure, containing FeS as a second phase. A compensation of such defects by annealing in sulphur vapour at 500 °C up to 10 days did not change the electronic properties of the material. On account of the rigid pyrite lattice with a degree of Mohs' hardness

18

N. Alonso-trante et al. / Interracial behavior of hydrogen-treated pyrite

Table 3 Impurities in FeS2

Si Ti V Cr Mn Cc Ni Cu Zn Br

FeS2 (natural) (/tg/g)

FeS2 (synthetic) (/tg/g)

Not measured < 79 Not measured 87 < 92 98 < 98 < 106 218 Not measured

134 28 1 1 6 8 <1 <1 <1 350

Impurities in FeS2 (natural) were estimated by X-ray fluorescence in a scanning electron microscope [9]. The impurities in FeS 2 (synthetic) were measured by inductively coupled plasma mass spectrometry [14].

of 6.5, the diffusion coefficient is very small as compared to other doping experience with group V elements. Paramagnetism seems to be an intrinsic characteristic of pyrite materials in nature and synthetic samples grown up to now [6,9,17]. This may be due to the existence of FeS centres in pyrite. Such centres were of the order of 102°/cm 3 in our samples. The replacement of S~- building parts by S 2- causes larger Fe-S distances in the pyrite lattice and produces defect levels in the forbidden region. In the worst case, Fe 2+ ions with octahedral high spin state, i.e., 02s)4(eg) 2, could appear. A further development of pyrite as a semiconducting material might essentially depend on the ability to avoi6 FeS centres during material preparation and crystal growth. This witl requh-e a profound understanding of solid state physical properties of this undesired defect.

Table 4 Chemical analysis of FeS2 (pyrite) samples

Material

Sulphur content (wt~)

FeS2: FeS ratio

x (FeS2_,)

Fe,S2 (theoretical)

53.45

100: 0

0.0

Ventron (powder) JMC (powder) CERAC (powder)

47.92 52.22 52.77

60: 40 90: I0 95 : 5

0.4 0.I 0.05

FeS2 (crystals) FeS2 (nutrient)

52.67 52.63

94: 6 93.6: 6.4

0.06 0.064

The sulphur content was determined by a coulometric method described in ref. [15]. Tile crystals were grown by chemical vapor transport with bromine. The nutrient was prepared from the elements.

N. Alonso.Vante et aL / Interfaciai behavior of hydrogen-treated pyrite

19

4. Discussion

The presented experiments demonstrate that molecular and atomic hydrogen react differently with the pyrite surface. No significant difference can be determined for the interaction of atomic hydrogen with differently oriented pyrite surfaces. Atomic hydrogen is apparently so reactive that the small differences in the chemical bonding of different pyrite surfaces does not influence its chemical pathways. Neither the electrochemical characteristic for hydrogen evolution (fig. 5) nor the hydrogen-atom-induced improvement of photoelectrochemical properties (fig. 1) are dependent on the crystals orientation. In contrast, the thermochemical interaction of molecular hydrogen with pyrite is orientation dependent. Here, significant energetic differences (up to one order of magnitude) of different pyrite surfaces determine reactivity or passivity. The following qualitative explanation can be offered: as known, grow~.h and dissolution velccit3 in the {111} direction, respectively, is larger than in the {100} direction because of permenant presence of kink sites. Atomic hydrogen, produced by electrochemical or chemical reactions, can both react on the pyrite surface and enter into the interior of the material. Insertion of hydrogen into pyrite has been experimentally confirmed [18]. Both the surface and the surface near bulk have, therefore, to be considered as locations for the electronic improvement caused by atomic hydrogen. For the determination of the atomic and electronic nature of the improvement caused by atomic hydrogen the following experimental observations are significant: the improvement of the photocharactecistics with the hydrogen pretreatment depends on the quality of the starting material. There are crystals which exhibit a rapid and dramatic improvement, others which only show sluggish response. However, the tendency is that all pyrite samples significantly improve. A further interesting observation is that photocurrents and photopotentials increase and dark currents decrease. These ~.endencies can only be understood if atomic hydrogen is acting on imperfections in pyrite or on its surface. The most conspicuous imperfections which control the photoelectrochemical properties of FeS2 are FeS centers which can be present both on the interface and in the bulk. They have been determined on pyrite surfaces using XPS techniques and it could be demonstrated that reduction of the concentration of interfacial FeS groups (by anodic photoetching during chlorine evolution) resulted in an improvement of photocurrent densities and a decrease of dark currents [19]. As already indicated above, elimination of one sulfur atom from ideal coordinated Fe 2+ environment modifies the ligand fields in such a way as to shift the t2s levels into the forbidden energy region. It is therefore reasonable to associate the favourable action of atomic hydrogen on pyrite interfaces with an interaction with FeS centers. The following interaction mechanisms of atomic hydrogen with pyrite are imaginable: (a) Atomic hydrogen reacts with interracial FeS. Since atomic hydrogen will associate itself with the sulfur species of FeS, liberation of H2S is possible. Anodically the adjacent Fe center will dissolve as Fe z+ or Fe 3+ and the process thus leads to a purification of ti~e pyrite s~;rf~.:,e from FeS.

20

N. A lonso-Van:e et al. / Interracial behavior of hydrogen-treated pyrite

H+ EC m

FeS

FcS

IH

E

V

FeS2.x

Electrolyte

Fig. 9. Schematic diagram of n-FeS2_x/electrolyte interface, illustrating the different charge transfer processes in dark and under illumination.

(b) Inserted atomic hydrogen will also tend to associate itself with the negatively charged sulfur species. Between ~ - and S 2- it will prefer attachment to the less positive S z- of FeS impurities. Tk,= consequence of such a Fe 8+ SH 6- formation will be that positive charge is drawn from the iron with the result that the I,'gand field splitting will increase. This implies that the FeS defect levels will tend to approach the FeS2 valence band (fig. 9). There are consequently less surface states and near surface states available to support dark electron transfer from redox species in the electrolyte into the conduction band of pyrite. Simultaneously recombination centers for holes and electrons will disappear which favour~ the generation of photocurrents. The photovoltage increases due to a reduction of states which permit the penetration of electrons through the junction barrier. It has to be emphasized that the surface states on FeS2 surfaces in contact with an iodine electrolyte containing solution are complicated by the fact that iodide is forming coordination complexes with iron centers. This means that the reducing agent is involved in the formation of a surface state or modifying existing surface st~.tes. This situation is schematically indicated in fig. 9.

5. Conclusion The passivatior, of FeS centers by electrochemically hydrogen evolution treatment improved the photocharacteristic at the pyrite/electrolyte interface. The main effect is the elimination and (or) displacement of electronic states from within the forbidden energy gap which assist dark or photogenerated electronic charge carriers in bypassing and penetrating the interracial barrier. In this way photoelectrochemical energy conversion is improved.

N. AIonso-Vante et al. / Interracial behavior of hydrogen-treated pyrite

21

Acknowledgement The authors thank Dr. K.-W. Hofmann and D. Jokisch for the electron microscopy and technical assistance, respectively. Thanks are also due to Dr. J. Luck, Dr. J.E. Ruffler (RhSne-Poulenc) for ICP-MS measurements and sulfur analysis and Dr. U. K~bler for magnetic measurements, respectively. This work was in part supported by a grant of the BMFT No. 03E-8375-A.

References [1] [2] [3] [4]

A. Ennaoui and H. Tributsch, J. Electroanal. Chem. 204 (1986) 185. A. Ennaoui, S. Fiechter, W~ Jaegermann and H. Tributsch, J. Electrochem. SOc. 133 (1986) 97. S. Fiechter, J. Mai. A. Ennaoui and W. Szacki, J. Crystal Growth 78 (1986) 438. K.J. Vetter, Electrochemical Kinetics, Theoretical and Experimental Aspects (Academic Press, New York, 1967) p. 565. [5] X.-P. Li, N. Alonso-Vante ~ad H. Tributsch, J. Electroanal. Chem.. 242 (1988) 255. [6] K. Sato, Prosr. Crystral Growth Characterization 11 (1985) 109, and references therein. [7] R.G. Burns, Mineralogical Applications of Crystal Field Theory (Cambridge Univ. Press. Cambridge, 1970) p. 187. {8] R.W.G. Wyckoff, Crystal Structures, Vol. 1 (Wiley New York, 1963) pp. 122, 346. [9] P. Burgardt and M.S. Seel~a, Solid State Commun. 22 (1977) 153. [10] E.F. Bertaut, P. Budet and J. Chappert, Solid State Commun. 3 (1965) 335. [11] R. de M~dicis, Science 170 (1970) 1191. [12] M. Wintenberger, B. Srour, C. Meyer, F. Hartmann-Boutron and Y. Gros, J. Phys. (Paris) 39 (1987) 965. [13] G. Collin, M.F. Gardette and R. Comes, J. Phys. Chem. Solids 48 (1987) 791. [14] J. Luck, Spectroscopy 3 (1988) 27. [15] J.L. Linossier, ~,/1.Fraysse and J.E. Ruffier, Analysis 13 (1985) 238. [16] J. Fleming, J. Crystal Growth 92 (1988) 287. [17] A. Serres, J. Phys. Radium 12 (1953) 689. [18] S.M. Wilhelm, J. Vera and N. Hackermann, J. Eiectrochem. Soc. 130 (1983) 2129. [19] L. Chongyang, Ch. Pettenkofer and H. Tributsch, Surface Sci. 204 (1988) 537. [20] U. K6bler and F. Deloie, Berichte der Kernfosschungsanlage Jiilich Nr. 1305, Jiilich (1976).