Arsenic sulfide clusters

Solid State Communications, Printed in Great Britain.

Vol.47,No.2,

pp.lll-ll4,

1983.

0038-1098/83 $3.00 + .00 Pergamon Press Ltd.

ARSENIC SULFIDE CLUSTERS T.P. Martin Max-Planck-Institut fEu- Festktrperforschung 7000 Stuttgart 80, Fed. Rep. Germany (Received 31 March 1983 by M. Cardona)

As 2 3S vapor. . quenched in He gas is observed to contain clusters of varying composltlon and size. The most stable cluster with composition (As2S3)n is As6S 9. Sulfur-rich clusters are apparently formed by bonding a fourth sulfur atom to As. We conjecture that these clusters reflect the local structure of AspS 3 glass.

Introduction

a pressure of 5 x 10 -6 mbar and containing a quedrupole mass spectrometer. The clusters were ionized with 70 eV electrons. The qundrupole was operated with a mass resolution of 8 a ~ over a range of 0 - 2000 amu.

The determination of the structure of glasses is a difficult task. Although x-rs~- diffraction and extended x-ray absorption fine structure spectra reflect short range order, inf~,rmation can be extracted from the spectra only indirectly. Structural models must be assumed, the corresponding spectra calculated, and a comparison made with experiment. If the comparison is favorable, the model m~y reflect to some degree the real local structure. In this note we suggest that a study of the composition of condensed clusters by means of mass spectroscopy m ~ yield complementary information helpful in building structural models of glasses. The composition of small clusters can be unambiguously determined from their mass. The mass distribution of the clusters indicates their relative stability and hence probably the local composition and even structure of glasses. For an initial study we have chosen the system arsenic sulfide which can condense into many different forms. AshS h (realgar) and As4S q (dimorphite) are molecular-crystals containing ~ highly stable 8 and 7 atom clusters, respectively, weakly bonded with one another. As2S3, on the other hand, crystallizes not with a cluster structure but with a layer structure I (orpiment). Arxe-nic sulfide with this composition condenses also into a glassy state, the structure of which is by no means unique. As2S 3 glass prepared from the melt is thought to co~tain almost exclusively As-S bonding 2 . Vapor deposited As2S 3 seems to contain a large fraction of homopolar bonds 3,~.

Results

and Discussion

Figure I is a mass spectrum of clusters formed by the evaporation of As2S ~ glass. The spectrum contains many lines but ~hese can be assigned without ambiL,~ity to clusters containing a well defined number of arsenic and sulfur atoms. Clusters with composition AshS , AScSx, As8S x and AsIoS x have been labeled exp~icitYy. Notice that clusters with certain compositions have particularly strong lines in the mass spec+ + + 4" ~ + trum; AshS_ , As-Sh, As-S-, As-S , As-S , As I S.-

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that these clusters necessarily occur with a high probability in the beam of neutral clusters. Relatively high energy (70 eV) electrons have been used for ionization. This leads to high sensltlvity but also to a high degree of fragmentation on ionization. It is valid to assume, however, that a strong line in the mass spectrum indicates the existence of a highly stable cluster of the same mass, be it parent cluster or ionized fragment. That is, the f r a ~ e n t a t i o n , far from being a hindrance, allows us to examine the stability of a larger variety of clusters than is found in the beam of parent clusters. In order to reduce confusion in analyzing this complicated spectrum, the peak values for ASmS x clusters have been plotted in Fig.2. Consider first the series AshS x. AshS 3 and As4S h appear %,ith very high intensities. We would therefore conclude that these clusters are highly stable. This conclusion is borne out by the fact that these two clusters, and only these two, occur as closed molecular units among the crystalline forms of arsenic sulfide. The peak corresponding to AshS 6 is unusually weak in this series. We conclude that this cluster is unstable, again a conclu~:ion supported by the fact that AshS6, or equivalently As2S3, crystallizes with a layer structure and not lnto molecular units. The apparent instability of the AshS 6 molecule means

Experiment The technique used to produce clusters is based on that described by Sattler et al 5. As2S ~ glass was evaporated from a heated quartz crucible placed in a liquid nitrogen cooled chamber filled with I mbar of He gas. Clusters and small particles condense out of the high temperature vapor on contact with the cold He gas. The clusters and gas flow through a small 3 mm diameter hole into a differentially pumped intermediate chamber and then enter as a collimated beam the final vacuum chamber having Iii

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that it will not be a unit of local stability in As2S 3 glass. This result is rather surprising conszdering the high degree of stability of the As~O 6 molecule. It is a building block in two modifications of arsenic oxide, arsenol[te and claudetite. The closed, tightly bonded AshO 6 units are weakly bonded w~th one another to form cubic and monoclinic lattices. We have examined the clusters contained in the quenched vapor of AshO 6. The mass spectrum is shown in Fig.3. The most intense lines belong to As406, As8016, As12018, As1602h and As200_~O. It is i ~ e d i a t e l y clear £hat the stable AshO~-units are also building blocks in the f o ~ a t i o n of clusters. Except for As h , AshS 3 and AshS 4 there does not appear to be much direct information available concerning the structure of As4S x clusters. However, the clusters P~$5' PKST' PhS8 and P4SI0 have all been examined using the techniques of X-ray and electron diffraction ~. Based on this information, the probable structures of AsKS x clusters are shown in Fig.h. They are all derived from the As h tetrahedron. At first sulfur is incorporated as a bridge between As atoms. When AsKS 6 is reached all bridging sites have been filled. Additional sulfur can be incorporated in a cluster either as a terminal atom on four-fold coordinated As or by forming S-S bonds at bridging sites. The arsenic atom has three hp electrons ave/fable for bonding and a normally inactive lone pair of Ks electrons i.e., with a coordination of 3 the octet rule is fulfilled. The sulfur atum with six electrons fills its octet with a coordination of two. Therefore, we expect that 8any cluster composed of only threefold coordinated arsenic and two-fold coordinated sulfur has a good chance of being highly stable. The first four clusters in Fig.h (ASK, AshS3, AshSh, AshS 6) satisfy this bonding scheme.

2

Vol. 47, No. 2

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also the largest value of x possible without allowing for the possibility of S-S bonds. Therefore, we conclude that the AshS x clusters s.re probably built out of three- and four-fold coordinated As with bridging and terminal S atoms. As-As and As-S bonding is present but not S-S. Clusters containing an odd number of As atoms are most likely frs~ments produced in the ionization process. Since one arsenic bond is always left unsatisfied, it is rather difficult to imagine a highly stable cluster of this type. However, the mass spectrum clearly shows strong peaks for As5S 7, As7S 9, As7S13 and As3S 4. Actually, there is no contradiction here when we remember that the clusters are positively charged. The electron most easily removed is a non-bonding sulfur p-electron. Sulfur acquires a valence of three, i.e. it becomes As-like. Therefore, the charged clusters in question can be compared to the corresponding neutral clusters As6S6, AsgS6, As8S]2 and AshS3, all of which are highly stable. Conclusions Fig.4

Probable structure of AShS x clusters.

However, arsenic can also be four-fold coordinated. The exact nature of this type of bonding is not well understood. Apparently the fourth bond cannot be thought of as a polar donor G-bond. Some degree of n-back-bonding involving As d-orbitals must be present. However, it is convenient to think of the arsenic atom shining its pair of 4s electrons with a terminal sulfur atom which then needs no more bonds to satisfy the octet rule. The last four clusters in Fig.3 ( A s 4 ~ , As4S 3, As4S 9, As4S10) contain four-fold coordinated As. No AShS x clusters were observed in our mass spectrum for x larger than ~0. This is

Clusters condensed out of the vapor Over As2S3 have a composition which varies from arsenlc-rich AS2nS2n to sulfur-rich AS2nShn+2. The arsenic-rich clusters probably contain As-ps/rs linked together with sulfur bridges. The sulfur-rich clusters most likely conts/n h-fold coordinated As including one terminal S atom. The cluster AshS 6 is unusualy weak in the mass spectrum and the As6S 9 cluster particularly strong. Not only clusters but also glasses are formed by the rapid cooling 8.nd condensation of hot vapor. Therefore, it is not unreasonable to consider the possibility that both condensed forms have a similar structure. Some of the first structural studies on As2S 3 glass indicated

114

ARSENIC SULFIDE CLUSTERS

a dense -random packing of AshS 6 molecular units 7. Our mass spectra show that this cluster is not stable and therefore probably plays no important role in the structure of As2S 3 glass. The first stable building block with composition As2nS3n is the molecule As6S 9. In addition, As-As bonding is

Vol. 47, No. 2

ccmpensated not by excess sulfur in the form of S-S bonds but by terminal sulfur atoms on t-fold coordinated As at(xzs. Acknowledgement - Several helpful conversations with H.G. yon Schnering are gratefully acknowledged.

References I) SSfITH, B.A., COWLAM, N. and SHAMAH, A.M. Phil. Mag. 13 39, 111 (1979). 2) AFL!NG, A.J., 12ADBETTER, A.J., and WRIGHT, A.C., J. Non-Crystalline Solids 23, 369 (1977). 3) DANIEL, M.F., LEADBETTER, A.J., WRIGHT, A.C. and SINCLAIR, R.N., J. Non-Crystalline Solids 32, 271 (~979). 4) NES~LNICH, R.J., CONNELL, G.A.N., HAYES, T.M. and STREET, R.A., Phys. Rev. B 18, 6900 (1978). 5) SATTLER, K., MUHLBACH, J., RECKNAGEL, E., Phys. Rev. Lett. hS, 821 (1980). 6) CORBRIDGE, D.E.C., "Phosphorus" (Elsevier, Amsterdam, 1978). 7) DE NEUFVILLE, J.P., MOSS, S.C. and OVSHINSKY, S.R., J. Non-Crystalline Solids, 13, 19] (197h).