Synthesis and structure of mixed Ba₁₂F₁₉Cl_δBr_5-δ Crystals

Article

Reference

Synthesis and structure of mixed Ba

₁₂

F

₁₉

Cl

_δ

Br

_5-δ

Crystals

KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans

Abstract

In the structure Ba12F19Cl5 [hexagonal space group P2m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba12F19ClBr5- with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75 mol% BaF2 and 25 mol% BaX2 with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl-/Br- population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).

KUBEL, Frank, HAGEMANN, Hans-Rudolf, BILL, Hans. Synthesis and structure of mixed Ba

₁₂

F

19

Cl

_δ

Br

_5-δ

Crystals. Zeitschrift für anorganische und allgemeine Chemie , 1996, vol. 622, no. 8, p. 1374-1380

DOI : 10.1002/zaac.19966220816

Available at:

http://archive-ouverte.unige.ch/unige:3536

Disclaimer: layout of this document may differ from the published version.

1 / 1

Z . anorg. allg. Chem. 622 (1996) 1374-1380

Zeitschrift fur anorganische und allgemeine Chemie

0 Johann Ambrosius Barth 1996

Synthesis and Structure of Mixed Ba,,F,,CI,Br,

^-

, ^Crystals

Frank Kubel, Hans Hagemann, and Hans Bill"

Geneva/Switzerland, DCpartement de Chimie Physique de 1'UniversitC de Geneve Received October 2nd, 1995 and December Sth, 1995, respectively.

Abstract. In the structure Ba,2F19C1S [hexagonal space group P62m] the two chlorides on the sites Cl(1) and Cl(2) can par- tially be replaced by bromide ions. Single crystals of the type BalzF19ClsBrs-s with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75molVo BaF, and 25 mol% BaX, with X = C1, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods.

The refined total Cl-/Br- population ratio in the crystals is close to the one of the flux. The lattice parameters and in- teratomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).

Keywords: Mixed barium halides; crystal growth; X-ray diffrac- tion; Raman spectroscopy

Synthese und Struktur von Ba,,F,,CI,Br,-,-Mischkristallen

Inhaltsiibersich!. In der Struktur Ba12F&1, [hexagonale Raumgruppe P62mI kann Chlorid auf den Positionen Cl(1) und Cl(2) teilweise durch Bromidionen ersetzt werden. Einkristalle des Typs Ba,2F,9ClsBrs-a rnit einem Chlorid zu Bromid Verhaltnis von maximal 2 : 3 wurden aus einer Schmelze von 75 mol% BaF2 und 25 mol% BaX, (X = C1, Br) erhalten. Die Kristallqualitat nimmt rnit zunehmendem Bromidgehalt ab.

Strukturdaten von funf ausgewahlten Einkristallen rnit unterschiedlichem Chlorid zu Bromid Verhaltnis wurden rnit

Hilfe von Rontgen-Einkristallbeugungsmethoden bestimmt.

Die verfeinerten Besetzungswerte der Halogene stimmen gut rnit dem C1 zu Br Verhaltnis der Schmelze iiberein. Die Gitter- parameter und interatomaren Abstande andern sich, wenn C1- durch das groRere Br- ersetzt wird. Die Verfeinerungen zeigen eine statistische Verteilung der Halogene auf beiden Git- terplatzen, wobei allerdings C1- auf der Position Cl(1) bevor- zugt durch Br- ersetzt wird.

1 Introduction

We have previously reported the synthesis and characterization of the new structure type of hexagonal symmetry with the chemical composition of Ba,,F,,CI, [I]. Crystals of this type are potential host materials for optical applications, such as photochemical hole burning or as phosphors. Crystallochemical aspects of the struc- ture, as for instance the observed short Ba-F distance, allowed to predict a high probability to replace chloride by bromide. This short distance could be released by in- cluding a bigger anion.

The present structure of Bat2Fl9Cl5 contains three dif- ferent cation sites. All of them are expected to be substituted when rare earth [2] or transition metal ions [3]

are introduced as dopants.

With the aim to further develop similar materials, we started to explore the range of possible solid solutions beginning from Ba12FlyC1,. In this paper, we report on the partial exchange of chloride by bromide that has been realized up to a 60% substitution.

2 Results Synthesis

The compounds were synthesized in graphite crucibles under argon atmosphere by slow cooling (1 -2 "C/min) of a melt of composition Ba2F,X, X = C1,Br (soaking for 30min at 1300 "C, followed by the cooling procedure). These conditions proved to be favorable for the preparation of Bal,F19C1, [I], because it lowered the melting point of the mixture sufficiently to allow the crystallisation of the compound. In a typical run, 2 g of BaF,, 1.31 g BaFCl and 1.08 g BaFBr (to yield the com- position Ba,2F,9C13Br2) were placed in a graphite crucible, and then heated in a furnace under an argon atmosphere. All crystals were doped with (nominally

-

1-2070) Sm2+ prepared by reducing SmF, in situ by the graphite crucible. Some addi- tional BalZF,9C16BrS-d samples doped with Eu" were also prepared for further investigation by ESR and optical spec- troscopy. All crystals were transparent indicating much lower doping concentrations than the nominal ones. Needle shaped crystals with hexagonal habit grew mainly in the central cavity created by the retraction of the flux on cooling, but crystals were also found elsewhere in the mass. The crystals showed

F. Kubel et al.. Svnthesis and Structure of Mixed Ba12F,,Cl,Br5-, Crystals 1375 maximum dimensions of 0.3 x0.3 x 2 mm3. New modifications

of the synthesis (by flux and Czochralski techniques, respective- ly) allowed now to obtain needle shaped samples of typical dimensions of 0.5x0.5 x 15 mm3 and bulk material of dimen- sions up to 4 x 4 x 2 m m 3 . Smaller amounts of tetragonal Matlockite (BaF(C1, Br)) and dendrite shaped cubic barium fluoride crystals were also obtained. The different crystals can be distinguished by optical microscopy under polarized light.

The nominal composition of Ba,,F,,Cl,Br,-, was varied be- tween 6 = 0 and 5 in the following steps: 6 = 0.0,0.75, 2.0, 2.5, 3.75, 4.5 and 5.0. The two first preparations did not yield any needle shaped crystals with the Ba12F,,C15 type structure; only a mixture of BaF, dendrites and BaFX, X = C1, Br were found. Individual needles with hexagonal habit were selected from the other five batches for single crystal X-ray diffraction, Raman and luminescence measurements. Several attempts to prepare BaI2Fl9Br5 from the melt with varying BaF2 to BaFBr ratios were made; no crystals with this new composition could be obtained.

Single crystal X-ray diffraction

Details of the data collection and refinement conditions [4] are given in Table 1. In all cases at least half a sphere in the reciprocal space was measured up to (sinO)/L = 0.6. The ab- solute structure factor [ 5 ] was refined; all crystals were found to be polar single domains. Table 2 gives the refined heavy halogen composition, the lattice parameters, the standardized [6] atomic positional and vibrational parameters of the five crystal struc- tures with decreasing bromide concentration. No deviation from the Ba,,F,,Cl, structure type was found.

Table 1 Crystal data and experimental details (e.s.d.s in brackets)

3 Discussion

The quality of the refinements allow to determine the relative occupation of the heavy halogen sites. It is im- portant to note, that the refined bromide/chloride population is close to the nominal one. Lattice constants of several crystals within one synthesis batch were deter- mined and found to be equal within three standard devia- tions. Raman measurements (see below) confirm the homogeneity of the samples.

The present data set allows to analyze a variety of structural trends as a function of bromide content.

Lattice constants: the hexagonal lattice parameters a and c vary approximately linearly with increasing bromide content, as can be seen in Fig. 1 a, b. An in- teresting correlation between the occupation of the posi- tion Cl(1) which is part of the inner column [l] with the hexagonal c axis is found. This linear behavior is shown in Fig. 2. The knowledge of the lattice parameters allows thus to determine by extrapolation (semiquantitatively) not only the overall chloride to bromide ratio but also the individual site occupation.

Interatomic distances: selected interatomic distances between barium and halides are given in Figure 3. It can be seen, that all bonding distances ( = sum of Shannon radii [7]: bond length = 270 pm for Ba-F and = 340 pm for Ba-Cl/Br) and most nonbonding distances up to 370 pm increase with augmenting bromide concentration.

Interestingly, one nonbonding distance between Ba(2) and F(4) decreases significantly from 364.1 (7) to

Crystal system and space group Diffractometer type

Wavelength Method Temperature Program used Absorption correction Z

Number of parameters refined

hexagonal, P62m (No. 189) STOE

MoK& 71.073 pm o/lO-scan 293 K XTAL [4]

Analytical 1 47 6 of Ba12F19ClaBrS-S

Nrer measured Nref independent Absorption coefficient TmJT,,,

R,,,

Nref with F > 3o(F)

Rob W)

Rw = .+I)

Goodness of fit (all) Abs. structure param. [ 5 ] e-mln,max density values

2 5515 547 19.72 0.23/0.37 0.033 308 0.01 7( 17) 0.01 7 2.66 0.08

-0.9/1.3

2.5 9066 870 18.84 0.25/0.39 0.037 474 0.020(21) 0.018 2.60 0.97

- 1.4/1.8

3.0 7893 541 18.58 0.14/0.27 0.036 303 0.018(18) 0.017 2.70 1 .oo

- 1.2/2.5

3.75 3315 757 17.44 0.24/0.37 0.041 417 0.025(26) 0.024") 1.85") 0.87

- 2.0/1.8

4.5 4074 714 17.40 0.27/0.34 0.017 406 0.0 19( 1 8) 0.027 4.7 0.94

-3.5/3.6

") Excluding 4 reflections strongly affected by extinction

1376 Z. anorg. allg. Chem. 622 (1996) Table 2 Lattice parameters, unit cell volume [in p m / x 106pm3],

atomic positions, isotropic vibration ( x 100) parameters and heavy halogen site population as a function of d. E.S.D’s are given in parenthesis

6 Composition a = b ^C V

2.0 Ba,zF,9C1z.o,,7,Brz.9,,7,: 1414.30( 14) 433.62(3) 75 1.14( 16) 2.5 Ba,zF,9C12.44(43Br2.s6(43: 1412.17(7) 432.24(4) 746.52(9) 3.0 BalzF19C13.0,(4)Br1.99~4): 1413.28( 14) 43 1.87(3) 747.03( 16) 3.75 Ba12F19C13.89(4)BrI.,1(4,: 141 1.52(6) 430.19(4) 742.28(8) 4.5 BalfF19C14.36(4)Br0.64(4): 141 1.90(10) 429.33(4) 741.19(12) 5.0 BalzF19Cls: 1408.48(14) 427.33(5) 734.1 7(17)

2.0 Ba(1) 0.16803(6) 0.47232(6) 0.5 1.43(3) 2.5 0.16807(4) 0.47269(4) 0.5 1.27(2) 3.0 0.16808(4) 0.47321(3) 0.5 1.46(2) 3.75 0.16795(5) 0.47396(4) 0.5 1.27(2) 4.5 0.16783(4) 0.47438(4) 0.5 1.34(2) 5.0 0.16765(3) 0.47514(3) 0.5 1.31(1)

2.0 Cl(1) 0.1910(1) x 0.5 1.8(1) 0.33(2) 0.67(2) 2.5 0.1906(1) x 0.5 1.74(6) 0.42(1) 0.58(1) 3.0 0.1902(1) ^X 0.5 1.90(8) 0.53(1) 0.47(1) 3.75 0.1893(2) x 0.5 1.73(9) 0.71(1) 0.29(1) 4.5 0.1892(2) X 0.5 2.0(1) 0.80(1) 0.20(1) 5.0 0.1884(2) x 0.5 1.94(8)

2.0 F(l) 0.3668(6) 0.4907(6) 0.5 1.8(3) 2.5 0.3660(4) 0.4904(4) 0.5 1.7(2) 3.0 0.3655(4) 0.4898(4) 0.5 1.9(2) 3.75 0.3654(6) 0.4894(6) 0.5 1.8(2) 4.5 0.3639(4) 0.4880(4) 0.5 1.7(2) 5.0 0.3634(4) 0.4881(4) 0.5 1.9(2) 2.0 F(2) 0.1843(6) 0.3758(6) 0 2.1(3) 2.5 0.1839(4) 0.3755(4) 0 1.7(2) 3 .O 0.1835(5) 0.3748(5) 0 1.9(2) 3.75 0.1 833(6) 0.3757(5) 0 1.7(3) 4.5 0.1824(4) 0.3752(4) 0 1.8(3) 5.0 0.1825(4) 0.3749(4) 0 1.7(2) 2.0 F(3) 0 0.2780(8) 0.5 1.5(3)

2.5 0 0.2773(5) 0.5 1.4(2)

3.0 0 0.2775(5) 0.5 1.6(2)

3.75 0 0.2787(7) 0.5 1.6(3) 4.5 0 0.2785(5) 0.5 1.5(3) 5 .O ⁰ 0.2786(4) 0.5 1.6(2)

Table 2 (Continued)

6 x/a Y/b ^Z/C Uiso POP POP

(x100) (Cl) (Br) 2.0 Ba(3) 0.37861(7) x

2.5 0.37765(5) x

3.0 0.37663(5) x

3.75 0.37513(6) x 5 .O 0.37267(4) x

4.5 0.37425(5) X

2.0 Cl(2) 0.333 0.667

2.5 0.333 0.667

3 .O 0.333 0.667

3.75 0.333 0.667

4.5 0.333 0.667

5.0 0.333 0.667

2.0 F(5) 0 0

2.5 0 0

3.0 0 0

3.75 0 0

4.5 0 0

5 .O 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1.40(3) 1.25(2) 1.46(3) 1.26(3) 1.34(3) 1.29(2)

1.6(1) 0.51(2) 0.49(2) 1.46(7) 0.59(1) 0.41(1) 1.63(9) 0.71(1) 0.29(1) 1.3(1) 0.88(2) 0.12(2) 1 .I (1) 0.980(9) 0.020(9) 1.73(7)

2.1(5) 2.1(3) 2.4(4) 2.2(4) 2.6(5) 2.2(3)

Table 3 Selected interatomic distances in pm as a function of refined chlorine occupation 6 for Ba,2F,9CldBrS-S

6’ nominal:

6 refined:

Bal-C12 x 2 Bal-F1 Bal-F2 x 2 Bal-F3 Bal-F4 x 2 Bal-F1 Ba2-Cl1 x 4 Ba2-F2 Ba2-F3 x 2 Ba2-FS Ba2-F4 Ba3-Cl1 x 2 Ba3-F1 x 4 Ba3-F2 x 2 Ba3-F4

2.00 336.08( 5 ) 269.0(9) 263.2(6) 258.3(7) 305.4(2) 264.(1)

2.01 (7)

340.1(2) 270.1(6) 258.2(6) 252.9( 1) 359.(1) 342.6(1) 274.0(6) 273.(1) 267.( 1)

4.50 4.36(4) 33 3.09(3) 267.8(7) 262.7(4) 259.0(4) 303.9(2) 265.(1) 336.9(2) 268.5(4) 257.2(4) 251.51(7) 364.1(7) 338.1 (2) 272.8(4) 271.5(7) 267.9(7) 2.0 Ba(2) 0.17883(7) 0 0 1.35(4)

2.5 0.17865(4) 0 0 1.17(2)

3 .O 0.17851(4) 0 0 1.38(3) 3.75 0.17824(5) 0 0 1.20(3)

4.5 0.17814(5) 0 0 1.30(3)

5.0 0.17792(4) 0 0 1.28(2)

2.0 F(4) 0 0.4325(8) 0 2.9(4)

2.5 0 0.4334(6) 0 2.7(3)

3.0 0 0.4351(6) 0 2.7(3)

3.75 0 0.4356(7) 0 2.4(3)

4.5 0 0.4360(5) 0 3.0(4)

5.0 0 0.4370(4) 0 2.1(2)

358.7(1.2) pm (Figure 3). The relatively large distance between Ba(1) and F(4) ( - 305 pm, see Figure 3) can be explained by the propeller network structure shown in Figure4. The atom F(4) is shared by two adjacent pro- pellers forming thus a bridge between two Ba(1) atoms.

In addition, the atom F(4) is a ‘capping’ atom to the an- ticube of Ba(2) which is formed by four equidistant Cl(1) ions, two equidistant F(3) ions and two further F(2,5) ions. The dilatation of the two structural units, which can be described by a column formed by three blade sharing

F. Kubel et al., Synthesis and Structure of Mixed Bal2FI9Cl,Br5-, Crystals 1377

1420 4

a

^bhex

=

8

4 4 0 1 ‘hex

420

4257

^{2.0 2.5 3.0 3.5 4.0 4.5 5.0}

refined halogen occupation of Bal2Fl9CI,Br5-, Fig. l a Lattice constants a,bh, and chex. of Bal2FI9ClSBr5-, as a function of the chemical composition 6. The connecting line may be considered as a guide for the eyes. The size of the sym- bols is larger than the corresponding e.s.d.’s.

435

-

E

Q

c

i=. 430

u) X m

.-

X a

5

425

I I I I

2 3 4 5

refined halogen occupation of Ba,,F,9C1,Br5,

Fig. 1 b Unit cell volume as a function of 6. The connecting line may be considered as aguide for the eyes.

Fig. 2

tion on the lattice parameter chex

Influence of the occupation of the Cl/Br(l) posi-

0.2 0.4 0.6 0.8

refined occupation of position CI(1)

1378 Z. anorg. allg. Chem. 622 (1996) propellers and the big cavity formed by a propellertype

network [I], is found to be different. It should be noted that the distance Ba(2)-F(4) - drawn in bold lines in Figure4 - forms the link from the inner column

360

5

C

.- 340 C m

v) -0

G-

³²⁰

m

0

$

³⁰⁰

a,

I=

-0

3

280 0 a,

c

.-

Ls.

m

.-

CI 2

CI .-

- 2

260

B a( 2)- F (4)

;

Ba( 1 )-F(4)

v 'I 'I v

Ba-F

8

A ⁰

I I I ' I ' I ' 1 1 - x

A A A A

2.0 2.5 3.0 3.5 4.0 4.5

refined halogen occupation of Ba12F,&I,Br5,

Fig. 3 Interatomic Ba-Cl/Br and Ba-F distances as a func- tion of composition 6

to the outer matrix. The three positions Cl(1) which are replaced preferentially by bromide ions (see Table 2) form the propeller-axes of the inner column. Replacing these positions may release the shortest Ba(2)-F(5) distance.

In contrast to this latter distance, the other short Ba( 1,3)-F bonds remain approximately constant (see Figure 5 ) . Figure 6 shows the strong change of the bond- ing distances between Ba and the mixed halide site. This is expected, when an ion with bigger ionic radius (rg,!,annon = 181 pm and r!iannon = 196 pm, [7]) is incor- porated into the structure.

Atomic displacements: the atomic displacements do not change significantly, however the displacement of the site Cl/Br(l) shows a tendency to decrease (see table2).

As the atomic displacement can be related to vibrational amplitudes (phonon spectrum) as well as available space on the cristallographic site, this behaviour can be explain- ed by the difference in atomic weight between Br and C1 and the larger ionic radius of bromide.

Raman and Luminescence spectra

Partially polarized spectra of individual crystal needles of varying composition are shown in Figure 7. The similari- ty of the different spectra underlines the structural resemblance of these compounds. We have identified in the previous paper 111 the 335 cm-' band with a vibra- tion involving the shortest Ba-F bond in the crystal.

This frequency shifts continuously (linearly within ex- perimental error) to lower values with increasing bromide content, in agreement with the corresponding lengthen- ing of this bond as shown by the X-ray diffraction data.

It is interesting to note, that also the other Raman bands

Fig. 4 Drawing of the structure of Ba12F19C16BrS-S along the hexagonal cher axis. The Ba(2)-F(4) distances (see also Fig. 3) which separate the inner colums from the outer matrix are drawn by bold lines

1379 F. Kubel et al., Synthesis and Structure of Mixed Ba12F19C1SBrS-d Crystals

t

C 265\

z

v)

8

c

Ba( 1)-F(3)

m

5

²⁵⁵

v)

Ba (2)- F (5)

(at -255 and 200 cm-') with the same E ' symmetry as the 335cm-' band shift more than the E " bands at

-280, 235 and 190 cm-' (see Figure 7).

We attempted to estimate the spread of composition within one growth batch by taking Raman spectra at various places of the crystalline mass. The observed Br mole fraction does not deviate by more than

+

/ - 5 % from a nominal mole fraction of 40%. Raman spectra of the bromide rich batches with 6 ⁼ 0.0 and 0.75 did not show any spectral features similar to those shown in Figure 7, but rather the Raman spectrum of BaFBr/CI and BaF,.

The emission spectra from different samples of the 'D,-'Fo Sm" transition at room temperature are shown in Figure8. As there are three different Ba sites in the crystal, one would expect three 5D,-7F, emission bands, one per site. In Ba,,F,,CI, : Sm", there are indeed 3 bands centered at

-

14525, 14540 and 14610 cm-' respectively.

The last one is very weak compared to the two others.

Figure 8 shows that the relative intensity of the 14525 cm-' band increases strongly with increasing bromide concentration. In addition, a blue shift of the emission bands is observed upon substitution of chloride by bromide ions, similar to our results obtained on the Sm2+ doped mixed SrFCI,Br,

-,

crystals [8]. The spec- trum of Sm2' in Ba,2F,9C12,5Br2,5 (Figure 8) shows signifi-

345

- Ea

c 340

v) Q) 0 c

m

v)

U OJ

.-

Y

c

.-

-

5

m u 335

a,

0 a,

c

- %

330

Ba(3)-Cl/Br( 1)

__j__j___i

Ba(2)-CI/Br(l)

\ +

Ba( l)-CI/Br(2)

A

l r l ~ l ~ l ~ ~ ~ l ~ r

2.0 2.5 3.0 3.5 4.0 4.5 5.0

refined halogen occupation of

Ba,,F

,9C16Br5-6

Fig. 6 Short Ba-CVBr distances as a function of heavy halogen occupation 6

1.8

* a

0' I

150 200 250 300 350 400

Raman Shifi [cm-11

Fig. 7 Partially polarized (ab '

+

ac) room temperature Raman spectra of BalZF19C16BrS-6

cant inhomogeneous broadening of the bands. This is an essential condition for spectral hole burning.

Preliminary measurements showed that it is indeed possible to do photochemical hole burning at 77 K in these samarium-doped mixed crystals [9]. Spectroscopic studies on Eu2+ doped samples are also under way.

1380 Z. anorg. allg. Chem. 622 (19961 References

‘ - 1 I

[I] l? Kubel, H. Hagemann, H. Bill, Z. anorg. allg. Chem. 622

[2] E Kubel, H. Hagemann, H. Bill, Mat. Res. Bull. 30 (1995) [3] D. Nicollin, H. Bill, J . Phys. C: Solid State Phys. 11 (1978) [ 4 ] S. D. Hall, H. D. Flack, .lM. Stewart, Editors of Xta13.2:

[5] H. D. Flack, Acta Crystallogr. A39 (1983) 876

[6] L. M. Gelato, E. Parthe, J. Appl. Crystallogr. 20 (1987) 139 [7] R. D. Shannon, Acta Crystallogr. A32 (1976) 751

[8] R . Jaaniso, H. Hagemann, H. Bill, J. Chem Phys. 101 [9] R. Jaaniso et al., to be published

(1996) 343 405 4803

Users Manual (1 992)

(1994) 10323

1.445 1.45 1.455 1.46 1.465 1.47

1 .44 0

Emission Frequency [cm-11 x 10‘

Fig. 8 SDo-7F0 Emission spectra of Sm” in Ba,2F19CldBrS-d at room temperature, excited with the 488 nm argon laser line The financial support of the Suiss National Science Foundation and of the “Optique” priority program of the Board of the Suiss Federal Institutes of Technology is gratefully acknowledg- ed. The authors thank Mr. Didier Frauchiger for technical help.

Authors’ address:

PD Dr. F. Kubel, Dr. H. Hagemann, Prof. Dr. H. Bill DCpartement de Chimie Physique

UniversitC de Genkve 30 Quai E. Ansermet

CH-1211 Geneve 4/Switzerland