The region of existence of CuInS2

Mat. Res. B u l l Vol. 14, pp. 237-240° 1979. Printed in the USA. 0025-5408/79/020237. 452.00/0 Copyright (c) Pergamon P r e s s , Ltd.

THE REGION OF EXISTENCE

OF CuInS 2

A.W.Verhe~en,L.J.Giling and J.Bloem Catholic University of N~megen RIM Laboratory of Solid State Chemistry Toernooiveld,N~megen,the Netherlands

(Received December 1, 1978; Communicated by S. Amelinckx)

ABSTRACT It is shown from quenching experiments followed by X-ray analysis that the region of existence of C u I n ~ is very small and in fact is limited to O
Introduction CuInS2 is one of the chalcopyrites originally orepared by Hahn et al (1).Since then,this compound was also identified to occur in nature,as the mineral Roqu~site (2).Presently,chalcopyrite compounds are widely investigated for their semiconducting properties and for their non-linear optical properties,which originate from their non-centrosymmetrical structure.In this laboratory we are mainly interested in the defectchemistry of these compounds. For the investigation of the defectchemistry of a compound it is necessary to be able to prepare the compound in a well defined and reproducable way.This requires a complete knowledge of the phase relations in the system.For a number of compounds with the chalcopyrite structure,the phase relations in the pseudo binary phase diagrams have been determined (3).Inspection of the published phase diagrams reveals that the region of existence of an I-III-VI2 chalcopyrite invariably is located on the III2-VI3 side in the pseudo binary phase diagram.This observation can be accounted for on the basis of the structural relationship between the chalcopyrite and III2-VI3 structures,which both posses 4 electrons per building unit,whereas Cu2S possesses only 2.66 electrons per building unit (4).

237

238

A.W.

VERHEIJEN,

et al.

Vol. 14, No. 2

The authors know of only one direct observation on CuInS 2 with respect to its homogeneity region as is reported by Kasper (5),who merely stated that for CuInS2 the stoichiometry variations are smaller than for CuGaS2,of which the existence region extends to x-values 0~x~0.13 in the general formula C u 1 _ x G a 1 + x / 3 S 2 . T h i s lack of experimental evidence cannot be filled with theoretical a r g u m e n t s , a l t h o u g h according to Palatnik the width of the homogeneity region should be inversily proportional to the tetrahedral distortion l(2-c/a)l.The validity of this rule seems dubious however. For instance,on account of their tetrahedral distortion,a larger region of existence should be predicted for CulnSe2 than for CuGaSe2,( l(2-c/a)l =0.00 and 0.04 resp.),whereas inspection of the e x p e r i m e n t a l l y determined phase diagrams show that the reverse is true (4,6).In addition it would predict a large homogeneity region for C u I n S 2 , b e c a u s e for this compound the tetrahedral distortion l(2-c/a)l =0.00 So based on Kasper's original observation together with the structural relationship between Cu!nS2 and In2S3,it will be expected that CuInS2 will have a finite composition range, which will be smaller than that of CuGaS2.The purpose of this note is to give e x p e r i m e n t a l data on this subject. E x p e r i m e n t a l t Results In order to obtain an impression of the phase relations in the Cu2S-In2S3 s y s t e m , s a m p l e s of different compositions were melted and quenched to room t e m p e r a t u r e . T h e ingots were then chemically analysed by standard methods and powder diffraction patterns were recorded using the D e b ~ e - S c h e r r e r t e c h n i q u e . T h e resulting relations between lattice constants and ingot c o m p o s i t i o n are plotted in figure 1. Addition of In2S3 to the s t o i c h i o m e t r i c composition (corresponding to x = O . O O ) , i m m e d i a t e l y appeared to produce an a d d i t i o n a l set of lines in the d i f f r a c t i o n p a t t e r n , s u p e r imposed on those belonging to the chalcopyrite structure.These extra lines are already present at the limit of observation at x = o ~ O 5 . I t was found that these a d d i t i o n a l lines could be assigned to the spinel phase CuInSS8oThis compound has been prepared by Flahaut and co-workers (7)oFor Cu/In ratios ~ I / 5 , t h e chalcopyrite lines vanish and only the spinel phase survives° Figure 1 shows that Culn5S8 is present up to the boundary of the chalcopyrite single phase r e g i o n , p o s s i b l y the x-ray diffraction sensitivity is the limiting factor in further narrowing of the chalcopyrite single phase region° On the Cu2S rich side in the diagram immediately coppersulphide reflections are d e t e c t e d , i n d i c a t i n g that also on this side the allowed variation is very small.(In figure 1,the chalcopyrite single phase region is drawn symmetrically around the s t o i c h i o m e t r i c c o m p o s i t i o n ; h o w e v e r chemical arguments only support an extension of the homogeneity region into the In rich side and not into the Cu rich side.) For the two compounds CuInS 2 and CuIn5S 8 also the optical absorption spectra are recorded (fig.2).Fron these spectra it can be deduced that the optical absorption edge of both compounds is quite similar (CulnS 2 E =Io44 eV;CuIn5S ~ E =1.38 eV).

Vol. 14, No. 3

CuInS 2

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The relation between sample composition and lattice constants in the Cu2S-In2S 3 system.The spinel data,taken from (7),agree well with our observations.

Because of this similarity and the possibility of small composi tion variation it is quite likely that the presence of micro precipitates of CuIn5S 8 in the chalcopyrite CuInS 2 will influence the optical p r o p e r t i e s , e s p e c i a l l y in f l u o r e s c e n c e ~ o f CulnS 2 crystals. Additional information on the phase relations in the system can be obtained by differential thermal analysis on samples with a different c o m p o s i t i o n . P r e l i m i n a r y investigation with DTA on a CuInS 2 sample with near s t o i c h i o m e t r i c composition revealed three transitions at 990 C,1050 C and 1090 C.The highest temperature signal is thought to represent the liquidus temperature, while the 1050 C signal then represents an eutectic temperature or if the region of existence is very small this signal could indicate a solid state t r a n s i t i o n . A s s u m i n g that the 1050 C signal indicates a eutectic t e m p e r a t u r e , t h e n the 990 C signal indicates a solid state t r a n s f o r m a t i o n , p r o b a b l y from the room temperature chalcopyrite structure to the sphalerite structure.This knowledge indicates that,to obtain large crystals of CuInS2, slow cooling of the melt through this critical temperature region is necessary.

Conclusion

and Summary

X-ray and chemical analysis of samples with variable composition in the Cu2S-In2S3 system,show that CuInS 2 has a small region of existence which is confined between O~x~O.05 in the general formula C u 1 _ x I n l + x / 3 S 2 . F u r t h e r m o r e , a preliminary DTA i n v e s t i g a t i o n indicated the presence of a solid state phase t r a n s f o r m a t i o n , w h i c h interferes with crystal growth from the melt.ln fact,ingots of CuInS2 although prepared at a low cooling rate,1 C h r - ' , c o n t a i n e d only crystals of millimeter size,and as yet it is not possible to grow large crystals from

240

A . W . VERHEIJEN, et al.

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Vol. 14, No. 2

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FIG. 2 The optical a b s o r p t i o n of the temperature.

edge

of CuInS 2 and Culn5S 8 as a function

the m e l t . H o w e v e r , f u r t h e r progress in this respect ted,because now the critical temperature at which formation occurs in this compound is known.

can be expecphase trans-

References I . H.Hahn,G.Frank,U.Klinger,A.r~eyer

and G o S t o r g e r , Z o A n o r g . C h e m .

71,1s3,(19s3) 2. P.Picot

and

R.Pierrot,Bull.Soc.franc.P~in~r.Crist.,LXXXVI,7,

C1963) 3. J.L.Shay and J . H . W e r n i c k , T e r n a r y Ohalcopyrite S e m i c o n d u c t o r s : G r o w t h , E l e c t r o n i c properties and A p p l i c a t i o n , P e r g a mon Press,Oxford (I 975). 4. L.S.Palatnik and E . I . R o g a c h e v a , S o v . P h y s . - D o k l a d y , 12,503 (1967). 5. H.M.Kasper in:R.S.Roth and S . J . S c h n e i d e r jr.,Solid C h e m i s t r y , N B S Special Publicetion 364 (1972). 6. L.S.Palatnik

and E . K . B e l o v a , N e o r g a n . M a t e r . , 3 , 2 1 9 4

7. J . F l a h a u t , L . D o m a n g e , M . G u i t t a r d , M . O u r m i t c h i Kom,Bull.Soc.Chim.France,28,2382 (1961) .

State (1967).

and J.Kam Su