The structure of superionic glasses from neutron diffraction and

JOURNAL DE PHYSIQUE IV Colloque CZ, supplkment au Journal de Physique 111, Volume 2, octobre 1992

The structure of superionic glasses from neutron diffraction and reverse Monte Carlo simulations L. BORJESSON,R. L McGREEVY* and J. WICKS* Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden 'Clarendon Laboratory, Parks Road, OX1 3PU Oxford, UK.

We have performed neutron diffraction experiments on metal-halide doped superionic glasses and combined them with Reverse Monte Carlo (RMC) simulations to investigate the microscopic structure and its relation to the ionic conductivity. The experiments reveal a substantial change and the building up of a new type of intermediate range order as silver halide dopant salts are introduced into the glass network, whereas the short range order of the network is unaffected. It is observed as an extra diffraction peak at anomalously low Q-values (0.7-0.8 A - l ) for the doped glasses. The RMC simulations show that the anomalous peak is due to local density fluctuations in the host glass network caused by the requirement to maintain connectivity (i.e. bonding) while decreasing the average density. The simulations indicate separated Agl and host glass networks in the glass. Some features of the radial distribution function of the doped glass may be interpreted as indicating limiting microscopic fractal aspects of the expanded host network. The results are discussed in relation to structure-conductivity models suggested for superionic glasses. I. INTRODUCTION

The high ionic conductivity of superionic glasses makes them interesting for applications, e.g. in solid state micropower suppliers in integrated microelectronic devices 111, as well as for fundamental studies of mass transport in disordered solid materials. Considerable attention has been focused on the mechanism of fast ion diffusion of a subset of ions within an otherwise frozen random structure, i.e. the decoupling of the motions of the conducting ions from those determining the glass transitionl2-41. Typically, the glasses of the highest ionic conductivities, up to Scm-I, are oxide or sulfide based glasses doped with metal halides. Examples of such glasses are (AgZ),(AgP03)1,. (AgZ)x-(Ag20-nB203)1-, (LiZj,-(Li20-nB203)1-x (Z=CL, Br, I). With the salt doping the conductivity increases by several orders of magnitudel2-41. It is therefore of great interest to reveal how the dopant salt is introduced into the glass and how it affects the structure of the host network. Most of the efforts to explain the high ionic conductivity of superionic glassses have involved assumptions of some specific structural arrangements. Two main views can be distinguished; the glass is (1) homogeneous down to a distance scale of a few atomic spacings and the migrating ions, whose concentration is thermally activated, hop through a number of available sites in an immobilized disordered network 131 or (2) inhomogeneous on the scale of 10-100 A and the conduction is a percolative process through pathways built up by the dopant salt 141, for example in the form of interconnected clusters 151. The latter model have especially been discussed for the Agl doped glasses since Agl is the archetypical crystalline fast ion conductor in its a-phase above 147 OC. Apart from some experimental evidence that the dopant salt does not participate in the network formation and leaves the local structure of the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1992211

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host network unchangedl5-81, little is known about the microscopic structure. However, there exists experimental indications of Agl coordinations similar to that of crystalline Agl from Raman scattering/5,6/ and furthermore EXAFS experiments 181 provides evidence of two different environments ( 0 and I) for the silver ions. On the other hand lo9Ag NMR measurements give no indications of two different Ag+ environments/9,10/. One of the main difficulties in the study of glasses and other disordered materials is the production of structural models that agree quantitatively with diffraction data. In this paper we show that unique structural information about superionic glasses, especially of the intermediate range order, can be obtained from neutron diffraction experiments in combination with the recently developed Reverse Monte Carlo (RMC) simulation technique/l I,121. The RMC technique refines an initial structural model by using directly the experimentally determined structure factor and thus no interatomic potential is needed as input. This is an obvious advantage for the complex materials of this investigation since the interatomic potentials are too complicated and not very well known. II NEUTRON DIFFRACTION EXPERIMENTS 11.1 Experimental procedure

Fast ion conducting glasses were prepared using melt quenching according to procedures described before 113,141. The samples, which were in shapes of cylindrical rods with a diameter of 9 mm and a length of 50 mm, were mounted in thin walled vanadium containers. Time-of flight neutron diffraction experiments were performed at the neutron spallation source ISIS, Rutherford-Appleton Laboratory, (U.K). The main advantages of this diffractometer are the very large Q-range (0.2-60 A-1) and the high intensity available. Data were collected simultaneously in seven groups of detectors placed at scattering angles from 5 to 150'. The obtained data were corrected for sample attenuation, multiple scattering and inelasticity effects as well as for container and background scattering from separate runs according to standard procedures I151. 11.2 Results The structure factors, S(Q), of two representative silver borate glasses of different Agl concentrations (x=0.1 and x=0.6) are shown in fig 1(a) and 1(b) for a high and a low Q-range. The different Q-ranges emphasise short range and intermediate range correlations, respectively. We immediately note that S(Q) of the two samples are nearly identical for Q values larger than 3 A-1. This implies that the short range order of the boron-oxygen network is preserved despite the large amount of dopant salt introduced into the host glass of the x=0.6 glass. The observation is verified by the radial distribution function presented in fig. 1(c) from which it is found that the first twoopeaks, representing the closest bond distances of the boron oxyg3n network (first peak at 1.4 A corresponds to the B - 0 bond and the second peak at about 2.5 A contains B-B, 0-0 and Ag-0 contributions), are almost unaffected with respect to the bond-distances and peak areas (coordination numbers) by the large amount of dopant salt. This result is in support of NMR I71 and Raman 161studies of the local boron-oxygen structure. Similar findings have been reported for other metal halide doped oxide-glasses 151. In fig 1(c) a weak peak at about 2.8 A can be observed for the x=0.6 glass which is not present in the data of the x=0.1 glass. We attribute this peak to Ag-l correlations since this distance is close to the Ag-l distance in crystalline Agl. Thus, there seem to exist Ag-l pair distances corresponding to that of crystalline Agl in the highly doped glass. This observation is in support of Raman investigations /5,6/ which have shown that there are contributions to the vibrational spectrum typical of Agl. In the present study we focus on the low Q range in which dramatic differences of the structure factor occur as the Agl concentration increases. The outstanding observation of the low Q region is the appeaIance of a strong and sharp diffraction peak at the anomalously low Q-values of about 0.7-0.8 A-1 when Agl is doped into the glass. No such peak is observed for

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Fig.1. Total structure factors of two (Agl)x-(Ag20-2B203)1-x glasses (x=0.6 and x=0.1) for (a) a high Q- and (b) a low Q-range. In (c) the corresponding radial distribution functions are shown. Upper curves have been shifted vertically by 0.5 ((a) and (b)) and 1.5 (c) units. the undoped glass. Thus, the introduction of Agl into the network causes the building up of con~id~erable intermediate ordering. The corresponding real space Fourier component is about 8-10 A. This may be regarded as the characteristic center-of-mass distance between the correlated structural units. It is then tempting to attribute the new type of intermediate range ordering to small particles of Agl or to density deficits within the expanded host glass network. Before relating the low Q peak to any specific structural ordering effect one should however note that a distinct low-Q diffraction peak, the fir$t sharp diffraction peak (FSDP), has been observed for many amorphous solids at about 1-1.5 A-1 1161. Its origin is currently a matter of controversy and it has been related to a variety of different structural arrangements like layers, chemical ordering, random dense packing of structural units, correlations between locally neutral (charge) units or to ordered density deficits within the glass network. For a recent review see ref. 1171. For covalently bonded network glasses a universal property has been shown 1161: when the momentum transfer of the structure factor is scaled by the shortest bond distance ,.,r the positions of the first sharp diffraction peaks of different glasses fall on a common value QrA.,=2.5 (see fig.2(a)). As seen in the figure the low Q peak of the Agl doped borate glass, when scaled by re-0=1.40 A, occurs at a considerably lower value (QrA.x=l . l ) , which then show that it is of different origin than the FSDP of ordinary network glasses. We conclude that the low Q peak of the present glass represents some structural ordering on a longer length scale than that present in other network glasses.

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x

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Fig.2 (a) Structure factors for some common network glasses and a Agl doped borate glass on a Q scale reduced by the nearest neighbour bond distance ~A.x. (b) Intensity of the anomalous low-Q diffraction peak of the (Agl)x-(Ag20-2B203)v glasses (after subtraction of the host glass intensity) as a function of Agl content. The solid 11nerepresents a linear fit to the data. The prepeak was observed at about the same position (in the range 0.78 - 0.87 A-1) for all the borate glasses with Agl concentrations in the range 0.2 8 A. This is the qualitative behaviour expected for an inhomogeneous structure with a local dimensionality less than 3. While the curve is not straight, which is expected for a real fractal system, a rough average line with a slope of k-1.4 gives a dimension d=3-k=1.6 for the mass scaling. One may therefore speculate that the low Q peak in S(Q) for Agl-AgP03 is being due to cor!elations of limiting 'fractal like' clusters, with an upper characteristic diameter of about 16 A. A slightly larger localization length would be expected for dynamical modes since they usually propagate over regions larger than a single wavelength. A crossover in the vibrational behavior of Agl doped borate glasses is indeed observed in Ramgn and inelastic neutron scattering at frequencies corresponding to a lengthscale of about 25 A 124,251. As noted above the borate glasses show a similar low Q feature in the structure factor when doped with Agl (see fig 1). This reduced dimensionality of the structure for r c 8 A, which is not truly fractal because of the failure to obey a power law over a reasonably long length scale, is easily understood. As the dopant salt is added the decrease in PO3 density causes intra-chain correlations to increase in importance relative to inter-chain correlations, and the overall dimensionality decreases. If one could produce a glass of very high level of Agl doping (the limit in practice is about x=0.6) then the dimensionality of isolated phosphate chains would be 1. This is indeed observed for the AgIAgP03 glass in a certain range beyond the main maximum of pIp (see fig 7). The slope is around -2, i.e the dimensionality is about one. For the understanding of the ionic conductivity the Agl correlations are of great importance. Neutron diffraction is dominated by the scattering from the PO3 network and a structural model based solely on this experiment can not provide detailed information on the Agl structure*. Nevertheless, it can be seen in figure 6 that gn,,(r) exhibits a rather well defined peak at 2.7 A which is close the Ag-I distance in crystalline Agl. In fig 5(b) a slice of the Ag-l structure of the model is shown. Bonds of Ag to neighbours within 5 A are depicted by lines. It can also be at about 2.5 A. Thus there exist noted in fig. 6 that there exists a well defined first peak in g(,r) two different and well defined Ag environments, i.e. 0 and I. This conclusion is then in agreement with that of Tachez et a1 1181, which is also based on neutron diffraction experiments, and that of Dalba et al /a/ which is based on EXAFS data on a Agl doped borate glass. However, it should be noted that lo9Ag NMR experiments do not give any evidence for more than one type of silver ion 19,101. Taken together the various experimental data on the silver coordinations a structural picture evolves in which most silver ions are ionically coordinated to both iodine and oxygen. In the future we propose to combine x-ray diffraction data, which is dominated by Agl, EXAFS (Ag, I and P edges) and neutron diffraction in one RMC simulation to obtain a complete detailed model. In fact x-ray diffraction data, which we have recently measured, and neutron diffraction data have recently been combined in a preliminary RMC simulation of about 5000 atoms. It confirms all the essential conclusions drawn from the model presented here, for example that the low Q peak is due to correlations of the phosphate network. It also provides new and more detailed information about the Ag and I correlations. Especially, there is no evidence of any second coordination shell in the gAgl(r) which then precludes any Agl microdomains. The results will be presented elsewhereM61.

I V CONCLUSION Neutron diffraction experiments on silver-halide doped oxide glasses show a large change of the intermediate range order as the dopant salt is introduced. A new sharp diffaction peak at anomalously low Q appears and indicates a common characteristic intermediate distance of 810 A. A model of Agl-AgP03 created by a RMC simulation on the experimental neutron data show that the low Q peak is due to local density fluctuations in the host glass network caused by the requirement to maintain connectivity while decreasing the average density. The expanded network (by the dopant salt) contains microscopic structyres which may be regarded as having 'fractal like' properties in a small range up to 16 A. These aspects find their dynamical counterparts in previously observed dynarnical localization in the vibrational density of states. The model shows separated Agl and host glass networks indicating the possibility of

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conducting pathways of Agl. However, it does not indicate the existence of Agl microdomains, though two distinctly different Ag environments ( 0 and I) are evident. The question of how Agl is introduced into the host glass network and what its detailed structure is will be adressed in new RMC simulations based not only on neutron diffraction data but also on x-ray diffraction and EXAFS data. ACKNOWLEDGEMENT One of the authors (L.B.) gratefully thank professor A.Fontana for support in the preparation of this paper. This program is supported by the Swedish Research Council for Natural Sciences. REFERENCES I11 Balkanski,M., Physics World 3 (1 I), (1990),29. 121 Angell,C.A., Solid State lonics 18/19, (1986), 72. I31 Ravaine,D., J. Non-Cryst. Solids, 73, (1985),287. I41 Minami,T.,J. Non-Cryst. Solids, 73, (1985),273. I51 Malugani,M.P. and Mercier,R., Solid State lonics 13, (1984), 293. I61 Carini,G., Cutroni,M., Fontana,A., Mariotto,G., and Rocca,F, Phys.Rev. B29, (1984), 3567. I71 Chiodelli,C., Magistris,A.,Villa,M., and Bjorkstam,J.L., J. Non-Cryst. Solids 51, (1982), 143. I81 Dalba,G., Fornasini,P., and Rocca,F., J. Non-Cryst. Solids, 123, (1990),310. 191 Chung,S.H., Jeffrey,K.R., Stevens,J.R. and Borjesson, L., Phys.Rev. B41, (1990), 6154. I 101 Martin,S.W., Materials Chemistry and Physics 23, (1989), 225 1111McGreevy,R.L. and Pusztai,L., Mol. Simul. 1, (1988), 359. I121 Keen,D.A., and McGreevy,R.L., Nature 344, (1990), 423. 1131 Borjesson,L., Phys. Rev. B36, (1987), 4600. 1141 Borjesson,L., Martin,S.W., Torell,L.M, Angell,C.A., Solid State lonics 18/19, (1986), 141. I151Howe,M., Howells,W.S, and McGreevy,R.L., J.Phys: Cond. Matter 1, (1989), 3433 1161Moss,S.C and Price,D.L., in Physics of Disordered Materials, Eds. Adler, D. et al (Plenum, New York, (1985)~.77 1171Elliott,S.R., Phys. Rev. Lett. 67, (1991), 711; Phys. Rev. B (in press) I181Tachez,M., Mercier,R., Malugani,J.P., and Chieux,P., Solid State lonics 25, (1987), 263. I191Rousselot,C., Tachez,M., Malugani,J.P., Mercier,R., and Chieux,P., Solid State lonics 25, (1987), 263. I201 Borjesson,L., and Howells,W.S., Solid State lonics 40141, (1990), 702. I211Borjesson,L., Mat.Res. Soc. Symp Proc, 210, (1991), 559. I221 Borjesson,L., Torell,L.M., and Howells,W.S., Phil. Mag. 859, 105, (1989). I231Borjesson, L. and McGreevy. R.L., Phil. Mag. (in press) I241Fontana,A., Rocca,F., Fontana,M.P., Phys.Rev. Lett. 58, (1987), 503. 1251Fontana,A., Rocca,F., Fontana,M.P., Rosi,B., and Dianoux, A.J., Phys. Rev. 841, (1990), 3778. -. . -. I261Borjesson, L., McGreevy, R.L. and Wicks, J., (to be published)