EXAFS-STUDY OF THE Zn2+ COORDINATION IN AQUEOUS HALIDE SOLUTIONS

HAL Id: jpa-00226058

https://hal.archives-ouvertes.fr/jpa-00226058

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EXAFS-STUDY OF THE Zn2+ COORDINATION IN AQUEOUS HALIDE SOLUTIONS

P. Dreier, P. Rabe

To cite this version:

P. Dreier, P. Rabe. EXAFS-STUDY OF THE Zn2+ COORDINATION IN AQUEOUS HALIDE SOLUTIONS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-809-C8-812.

�10.1051/jphyscol:19868155�. �jpa-00226058�

EXAFS-STUDY OF THE

zn2+

COORDINATION IN AQUEOUS HALIDE SOLUTIONS

P. DREIER and P. RABE*

Institut fiir Experimentalphysik, Universitat Kiel, Olshausenstrasse 40, D-2300 Kiel 1, F.R.G.

*~achhochschule Ostfriesland, Fachbereich

Naturwissenschaftliche Technik, Constantiaplatz 4, 0-2970 Emden, F.R.G.

Abstract

Aqueous solutions o f ZnC12, ZnBr2 and ZnI i n the concentration range from 2 0.1 molal up t o saturation have been studied by EXAFS experiments a t t h e zinc K-edge. EXAFS spectra o f the c r y s t a l l i n e compounds o f ZnC12, ZnBr2, Zn12 and ZnS04*7H20 have been used as references. I n the low concentration range up t o 0.5 molal a complete hydration o f the zinc ions by 6

-

7 water molecules i s observed. A t high concentrations above 5 molal t h e mean l o c a l coordination around the zinc ions consists o f h a l i d e ions and water molecules.

The s t r u c t u r e o f e l e c t r o l y t i c solutions a t high concentrations and especially the behaviour o f t h e metal1 i o n between i o n i c hydration (e.g. SrC12) and forma- t i o n o f anionic complexes (e.g. ZnC12, cdc12) has received considerable

i n t e r e s t (1). Coordination numbers p l a y a dominant r o l e i n t h e i n t e r p r e t a t i o n o f t h e s t r u c t u r a l data but t h e i r determination i n an EXAFS-analysis demands r e l i a b l e reference data. So we s t a r t e d our analysis w i t h a measurement o f EXAFS-spectra from c r y s t a l l i n e zinc halides.

The absorption experiments have been performed w i t h a focussing monochromator (2) using a r o t a t i n g anode x-ray generator. The measurement o f the reference spectra o f c r y s t a l l i n e ZnC12, ZnBr2 and Zn12 i s made d i f f i c u l t by t h e very hygroscopic nature o f these compounds. The f a c t t h a t four anhydrous modifications and (below 40'~) a t l e a s t f i v e modifications o f hydrates o f ZnC12 e x i s t ,

requires a c l e a r i d e n t i f i c a t i o n o f t h e reference compound. We have constructed a special sample c e l l f o r sample preparation, characterization and subsequent absorption experiments. The samples are prepared by the c r y s t a l l i z a t i o n o f a t h i n f i l m o f the aqueous s o l u t i o n on polyimide f o i l . The sample c e l l i s provided w i t h windows t o enable d i f f r a c t i o n experiments i n r e f l e c t i o n geometry i n t h e angular range

oO<

28<120~. The d i f f r a c t i o n spectra have been compared t o

spectra calculated from the known c r y s t a l s t r u c t u r e o f t h e anhydrous zinc halides (3), (4), (5). By v a r i a t i o n o f the growth parameters homogeneous f i l m s o f anhydrous ZnC12, ZnBr2 and Zn12 could be produced. I n these compounds t h e zinc atoms are surrounded t e t r a h e d r a l l y by 4 h a l i d e atoms. A f t e r the characterization the absorp-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868155

C8-810 JOURNAL

DE

PHYSIQUE

t i o n experiments have been performed using another p a i r o f windows i n the sample c e l l . As reference spectrum f o r t h e Zn-H20 coordination a spectrum o f c r y s t a l l i n e ZnS04*7H20 was used. I n t h i s compound zinc i s surrounded by 6 H20 molecules.

The f i n e structures o f the c r y s t a l l i n e reference compounds ZnC12 ZnBr2, ZnI, and ZnSO,*7H0O are shown i n f i q . 1. The spectrum o f ZnC1, shows a

sinGle o s z i l l ~ t i o ~ w i t h a

-

L

monotonically f a l l i n g envelope

0.25f-~

O e l ( ^{" ' ' ' ' ' I ' ' ' '}

1

produced by the four c h l o r i n e

x

(k) neighbours o f the zinc atoms.

The spectrum o f ZnBr, shows 0 an o s z i l l a t i o n w i t h

2

maximum i n the envelope a t -&-I t y p i c a l l y f o r medium sized backscatterers as bromine.

The absorption spectrum o f

x

^(k)

Zn12 has a poorer q u a l i t y

due t o t h e high background 0 0

I I

absorption o f iodine and the

Zn12

.

low amplitude o f - t h e o s z i l -

l a t i o n ' a t k >58-'. The

u

s t r u c t u r e a t 98-I o r i g i - 0

5

10 0

5

10

nates from the c h a r a c t e r i s t i c

WLB2 l i n e i n the emission k

/ A-'

^k

/ A-'

spectrum o f the x-ray genera- Fig. l EXAFS-spectra o f the c r y s t a l l i n e t o r . So t h i s reference s ectrum reference compounds.

can only be used up t o 9Ed.

The reference spectrun o f ZnS0;7H20 shows the c o n t r i b u t i o n o f several s h e l l s w i t h l i g h t backscatterers. The s c a t t e r i n g phases and the backscatterer amplitudes have been extracted by the Fouriertransform

-

backtransform method (6). These data are used i n the analysis o f the absorption spectra o f the aqueous solutions.

Fig. 2a shows the k-weighted f i n e structures o f the absorption spectra o f the aqueous ZnBr2-solutions. For comparison the reference spectra f o r t h e Zn-Br coordination and the Zn-H 0 coordination are included. A t the concentration 0.1 m the f i n e s t r u c t u r e i s very s i m i l a r t o t h e Zn-0 reference spectrum. This 2 indicates t h a t a t low concentrations the zinc ion i s coordinated by water molecules.

At t h h i hest concentration (20 m) the spectrum i n the l a r g e wavenumber range

(> 6a-')

i: s i m i l a r t o the Zn-Elr reference spectrum w i t h a smaller amplitude.

I n t h e low wavenumber p a r t (< &-I) i t resembles the Zn-0 reference spectrum.

This comparison shows t h a t a t high concentrations the zinc ions are surrounded by bromine ions

and

water molecules. The other spectra show how t h e f i n e struc- t u r e gradually changes w i t h concentration. The k-weighted EXAFS-spectra of the Zn12 s o l u t i o n are displayed i n f i g . 26. As i n the case o f t h e ZnBr2 solutions the spectrum o f the d i l u t e s o l u t i o n p o i n t s t o a water-like environment o f the zinc ions. With increasing concentration d e t a i l s of t h e Zn-I reference spectrum appear i n the f i n e s t r u c t u r e o f the solutions i n d i c a t i n g a mixed coordination o f the z i n c ions w i t h water molecules and i o d i n e ions. I n t h e case o f t h e ZnC12 spectra ( f i g . 2c) the concentration dependence i s n o t so pronounced. This i s due t o the small difference between t h e Zn-Cl and t h e Zn-HzO reference spectra.

Nevertheless a c h a r a c t e r i s t i c i n d i c a t i o n o f a chan e i n the environment o f the zinc ions i s t h e s h i f t o f t h e f i r s t peak from 3.8

1-'

i n t h e 0.1 m s o l u t i o n t o 4.1

8-I

i n t h e 30 m solution, For a numerical evaluation the spectra have

0 5 10 15

k

/

A*' ^{Fig. 2a}

Zn-I Ref

m

k

/

A-' _Fig.

_2b

Fig.

2

EXAFS spectra of aqueous solutions of ZnC12, ZnBr2 and

Zn12.

Fourier-filtered reference spectra are in- cluded for comparison.

k

/

A-'

Fig. 2c

C8-812 JOURNAL DE PHYSIQUE

been Fwriertransformed. The f i r s t coordination s h e l l s due t o water and halides are too close t o each other t o be separated i n r e a l space, so t h a t they are commonly backtransformed. A least-squares f i t procedure using the phase s h i f t s and the backscattering amplitudes from the reference spectra enables t h e determina- t i o n o f s t r u c t u r a l parameters. The parameters used i n the f i t procedure were the coordination numbers N, the interatomic distances r, t h e Debye-Waller f a c t o r s and a s h i f t AEo i n the zero of the k-scale. Values o f A Eo were very small;

t y p i c a l l y a few eV. A t the conce'ntmtions 0.5 m and 0.1 m the c o n t r i b u t i o n o f the zinc-halide coordination i s t o o weak t o be analysed and one s h e l l was s u f f i c i - ent f o r a good fit. The r e s u l t s o f t h e f i t s f o r r and N are shown i n tab. 1.

Tab.1 S t r u c t u r a l parameters o f the aqueous zinc h a l i d e solutions determined by

-

a two s h e l l f i t . No, Ncl, NT : number o f water molecules, chlorine, bromine and iodine ions i n the zinc environment. rQ, rC1: rBr, rI : Distance from zinc ions t o water, chlorine, bromine and iodine neighbours ( i n Angstroms).

The most i n t e r e s t i n g r e s u l t o f the analysis i s the concentration dependence o f the coordination numbers which appears i n a l l solutions i n a s i m i l a r manner.

I n the-high concentration range the zinc ions are surrounded by 2-3 h a l i d e ions and 2-3 water molecules. I n t h e range from 5 m t o 0.5 m the h a l i d e ions disappear from the zinc environment and the number o f water molecules r i s e s t o about 7. The change o f the zinc coordination takes place i n a concentration range where 1-2 hydration s h e l l s can be b u i l t up around each ion. I n a l l solutions the Zn-H20 distance enlarges w i t h f a l l i n g concentration. Zn-Cl distance and Zn-Br distance do not vary systematically w i t h concentration whereas the Zn-I distance tends t o increase a t l m concentration. I t should be emphasized t h a t t h e distances i n the s o l u t i o n do not d i f f e r strongly from the corresponding distances i n the c r y s t a l l i n e reference compounds.

2

Zn12

No '-0 N~ r~

-- -- -- --

-- -- -- --

2.7 2.02 3.2 2.59 3.1 2.02 2.9 2.61 6.4 2.03 1.0 2.65 6.4 2.05 0.5 2.65 7.4 2.05

-- ^-- -- -- -- --

6 2.03 4 2.62

This work was p a r t i a l l y supported by the Deutsche Fwschungsgemeinschaft.

ZnBr2

No ro %r ' ~ r

-- -- -- --

2.4 2.00 1.9 2.38 3.1 2.03 1.9 2.38 3.2 2.05 2.0 2.38 4.3 2.05 1.4 2.38 5.9 2.06 0.3 2.38

6.3 2.05

-- --

7.0 2.05

-- --

6 2.03 4 2.41

3011 2(h 10m 5m 2m l m 0.5m O.lm Ref

References:

1) P.V. Giaquinta, M.P. Tosi arld N.H. March, Phys.Chem.Liq.

3,

1 (1983) J.E. Enderby and G.W. Neilson, Adv. Phys.

9,

323 (1980)

2) P. Dreier and P. Eabe, Rev.Sci .Instrum.

57,

214 (1986) 3) H.R. Oswald and H. Jaggi, Helv.Chim.Acta

5,

72 (1960) 4) H.R. Oswald, Helv.Chim. Acta

43,

77 (1960)

5) P.H. Fourcroy, D. Carre and 3. Rivet, Acta Cryst.

834,

3160 (1978)

6) G. Martens,

P.

Rabe, N. Schwent~ier and A. Werner, Phys. Rev.

817,

1481 (1978) znC12

No ro r ~ l

2.0 1.99 2.5 2.31 2.5 2.01 2.6 2.34 3.2 2.03 2.4 2.30 2.8 2.02 2.2 2.32 4.2 2.03 1.6 2.31 4.9 2.02 0.9 2.34 5.7 2.05

-- ^--

7.1 2.07

-- --

6 2.03 4 2.30