Solid State Ionics 136–137 (2000) 1261–1266www.elsevier.com/ locate / ssi

A mesoscopic heterostructure as the origin of the extreme ionicconductivity in AgI:Al O2 3

1 2 *J.-S. Lee , St. Adams , J. Maier¨ ¨Max-Planck-Institut f ur Festkorperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany

Abstract

A 7H polytype of AgI (characterised by stacking fault arrangement) is detected at the interface of b-AgI and g-Al O ,2 3

which may even form the major (or even the only) AgI-constituent at pronounced Al O -contents. The driving force for the2 31formation of the 7H phase is assumed to be the interaction of Ag with the basic alumina phase similar as in the AgCl and

AgBr composites in which due to the relative stability of the rock salt structure only ideal semi-infinite space charge layersoccur. Considering the 7H structure as a heterostructure of g- and b-phase (b /g/b /g/ . . . ) leads to an explanation that is

1consistent with all the features observed: The extreme Ag -conductivity (that can hardly be explained by semi-infinite spacecharges) and the peculiar phase transition behaviour on one hand, and the qualitative similarities with AgCl:Al O and2 3

AgBr:Al O on the other hand. Since the layer separation is in the sub-Debye length range we expect a mesoscopic effect on2 3

the ionic conductivity as predicted earlier. This is supported by conductivity anomalies in b /g two phase mixtures. Thismesoscopic heterolayer is discussed in the context of nano-ionics (ion conductivity in nanocrystalline materials andnano-composites) and nano-electronics (quantum wells, wires and dots). 2000 Elsevier Science B.V. All rights reserved.

Keywords: Mesoscopic effect; Heterostructure; AgI:Al O ; Ion conduction; Interfacial phase transition2 3

1. Introduction sorption to the basic (nucleophilic) Al O surfaces.2 3

The interaction enthalpy has been assessed to beIn AgCl and AgBr heterogeneously doped with | 0.5 eV. Qualitatively similar space charge effects

Al O particles, the enhanced conductivity can be are observed at grain boundaries and surfaces, in2 3

quantitatively explained by ideal space charge ef- particular if exposed to nucleophilic gas phases.1fects. They occur as a consequence of Ag ad- Similar phenomena occur in the cases of TlCl, Li–

halides, Cu–halides and alkaline earth metal halides[1].

The extremely high absolute values in AgI:Al O*Corresponding author. Tel.: 149-711-689-1720; fax: 149- 2 3

711-689-1722. composites first reported in Ref. [2], however, areE-mail address: weiglein@chemix.mpi-stuttgart.mpg.de (J. beyond such ideal semi-infinite space charge effects;

Maier). this point together with phase transition anomalies1Present address: Center for Microstructure Science of Materi-occurring in this composite led to suggesting theals, Seoul National University.

2 formation of a-AgI-like interphases [3,4]. LaterPresent address: Min.-Kristallographisches Institut der Georg-¨ ¨August-Universitat Gottingen. arguments have been given pointing towards the

0167-2738/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 00 )00583-X

1262 J.-S. Lee et al. / Solid State Ionics 136 –137 (2000) 1261 –1266

formation of thin crystalline or amorphous phases 3. Results[5].

On the other hand, many qualitative facts indi- In Fig. 1 conductivity and DSC curves for acated a behaviour similar to that found for the other AgI:Al O composite with 40 m/o alumina are2 3

silver halides: essentially, the enhanced surface and shown, which reproduce the features of an extreme1grain boundary conductivities of b-AgI which are Ag conductivity and of phase transition anomalies

similarly activated as the values of the composites. already reported in the literature by the group of J.B.Moreover, the gas-sensitivity has to be mentioned in Wagner [2,3,12,13]. A comprehensive report of ourthis context [6]. work on the AgI:Al O composites will be given2 3

In a polymorphic compound such as AgI, inter- elsewhere [14].action energies of the order of 0.5 eV at the interface, The pronounced phase transition hysteresis couldas they occur in heterogeneously doped material (e.g. be observed in both types of measurements: Thewith alumina), are great enough to cause phase transition to the high temperature phase is super-transitions [7,8]. In addition, attractive Coulomb heated by 108C (T ) and the transition on cooling is1

interactions occurring at high defect concentrations undercooled by 408C (T ) with respect to the2

(e.g. core-space charge interactions) can naturallylead to order–disorder transitions [9,10].

It is reported in the following that stacking faultdisorder occurs at AgI /Al O boundaries that ex-2 3

plains all the above phenomena. Moreover, argu-ments are given that these interphases are realisationsof the mesoscopic ion conductivity effect quantita-tively predicted by one of us earlier [11]. In fact thestacking fault arrangement can be conceived as anionic heterostructure with a layer thickness below theDebye length and thus being more or less completelydisordered.

2. Experimental

AgI:Al O composites were prepared by mixing2 3

AgI (Alfa, 99.999%) and g-Al O (‘wet’ undried) of2 3

particle size of 0.06 mm (Meller, 99.992%), heatingthe mixture at 6008C for 12 h and annealing at 1008Cfor 12 h. For the electrical measurements theAgI:Al O powders were uni-axially pressed at 54002 3

2kg /cm into discs of diameter 0.6 cm.DSC (DSC 121, Setaram) and in-situ XRD

(PW302, Philips diffractometer) were performed inthe temperature range of 258C , T , 1808C. Impe-dance spectra were recorded by a Solartron 1260

Fig. 1. Conductivity (bold line), DSC curve (dashed line) andwith silver pasted electrodes in a furnace or using ain-situ XRD for AgI:Al O (40 m/o) composite (insets). Large2 3thermostat (RC 6 CP, Lauda) for 2 208C , T ,thermal hysteresis in phase transition with superheating by 108C

1808C. The heating and cooling rates for conduc- (T ) and undercooling by 408C (T ) is consistently observed. The1 23tivity, DSC and in-situ XRD were consistently extreme conductivity enhancement larger than 10 with respect to

chosen to be 0.28C/min. the pristine AgI (gray line) is in accordance with the literature [2].

J.-S. Lee et al. / Solid State Ionics 136 –137 (2000) 1261 –1266 1263

nominal b–a transition temperature of 1478C of AgI.In-situ XRD analysis directly confirmed the corre-sponding structural phase transitions as the insets inFig. 1 indicate. Impedance measurements revealed amaximum conductivity enhancement by a factor of

3 410 –10 with respect to the pristine b-AgI for thecomposites with 30–50 m/o alumina content inaccordance with Ref. [2].

In spite of various conjectures with respect tostructural anomalies at the internal interfaces ofAgI:Al O composites [3–5,15], no direct structural2 3

evidence for an interfacial phase has been reported,which could account for the anomalies of the com-posites. In this work, however, significant differencesin XRD patterns between the pure b-AgI and thecomposites were detected. In Fig. 2 correspondingXRD patterns for pure b-AgI and for a compositewith 30 m/o alumina are compared.

In addition to the broadening of the peaks, asubstantial decrease in intensity of the (h0l) peaks ofb-AgI (cf. Fig. 2: .) and the emergence of newreflections (cf. Fig. 2: m) in AgI:Al O neighbouring2 3

the reduced (h0l) peaks can be observed. The newFig. 2. XRD patterns for the pristine b-AgI (the reagent powderreflections cannot be identified with other poly-from Alfa) and the AgI:Al O (30 m/o) composite. The (h0l)2 3

peaks of b-AgI (.) are substantially reduced and the new peaks morphs of AgI, metallic silver or alumina.(m) attributed to 7H-AgI are emerging in the composites. The Bragg reflections of b-AgI, which are sub-

Fig. 3. Rietveld refinement for the AgI:Al O (30 m/o) composite employing 7H-AgI as the only AgI phase. The weak contributions of2 3

Al O are treated as background modulations except for the strongest Al O peak, which had to be excluded from the refinement.2 3 2 3

1264 J.-S. Lee et al. / Solid State Ionics 136 –137 (2000) 1261 –1266

stantially affected in AgI:Al O , refer to the stacking These thermal effects are reflected by overlapped2 3

sequence of the close packed planes in the wurtzite broad peaks in the DSC curve (Fig. 5) in thestructure of b-AgI. If the stacking faults occur corresponding temperature range. In-situ XRD re-periodically, layered structures with well-defined veals the coexistence of the high temperature a-stacking sequences result which are characterised by phase and the low temperature b-phase in a broadcorresponding new Bragg reflections. Such structures transition region as indicated in the insets in Fig. 5.with different periodicity are called polytypes of the The gradual transition of b-AgI to a-AgI in thismaterial concerned. In fact, a 7-layer polytype AgI region has been ascribed to the disordered stacking(7H-AgI) with the stacking sequence ABCBCAC region between coherent b-AgI and 7H-AgI [17].that was reported in 1974 by Davis and Johnson In-situ monitoring of the XRD patterns of the[16], explains the spectral features of AgI:Al O mixtures of b-AgI and 7H-AgI rendered it possible2 3

composites. The 7H polytype in AgI (space group to attribute the hysteresis solely to the 7H phase. TheP3m1) is unique in that it has been the only polytype large hysteresis is a characteristic of the martensiticever observed in polycrystalline specimens and also transformation between a-AgI and 7H-AgI, whilethe only one in a trigonal structure with an odd- the transformation between a-AgI and b-AgI occursnumbered periodicity in contrast to 4H, 8H, 12H, reversibly at 1478C, if fully equilibrated.16H, 80H, etc. polytypes observed in single crys- The composition dependence of conductivity intalline AgI.

The Rietveld refinement for the composite with 30˚m/o alumina yields lattice constants of a54.595 A,

˚c526.25 A, and indicates that the silver ions occupyslightly distorted tetrahedral sites (see Fig. 3). Fig. 4displays the 7H structure composed of close-packedlayers of tetrahedral sites. Considering neighbouringstacking layers, the structure could be further splitdown into substructures exhibiting the sequence ofhexagonal close packed b-AgI (ABABAB . . . ) andsubstructures exhibiting the sequence of cubic closepacked g-AgI (ABCABC . . . ).

It turned out that all the AgI in the compositesexists in the 7 layer polytype form if the aluminacontent exceeds 30 m/o. Accordingly, the diffractionpattern of AgI:Al O (30 m/o) composites in Fig. 2,2 3

regardless of the apparent similarity to that of b-AgI,can be essentially attributed to the 7H phase alone asthe Rietveld refinement in Fig. 3 verifies. For higherAl O content exceeding 60 m/o a substantial2 3

broadening of the Bragg reflections indicates theexistence of a large number of random stackingfaults which limits the size of well-ordered 7H phasedomains.

For the composites with alumina contents less than30 m/o, peaks stemming from the pristine b-AgIwere still present. As shown in Fig. 5 for AgI:Al O2 3

(20 m/o), composites with alumina contents lessthan 30 m/o exhibit broad transitions in the con-

Fig. 4. 7H structure represented by close packed layers ofductivity curve ranging from the nominal transition tetrahedral sites occupied by silver ions. Partial g-AgI like (left)(T ) to the superheated temperature (T ) on heating and b-AgI like (right) stacking sequences are indicated ino 1

and to the undercooled temperature (T ) on cooling. coloured regimes.2

J.-S. Lee et al. / Solid State Ionics 136 –137 (2000) 1261 –1266 1265

1Since it is known that a preparation in the Ag rich1solution leads to g-AgI formation, while a Ag

deficit stabilises the b-AgI phase (see e.g. Ref. [18]),a silver transfer from b-AgI to Al O and g-AgI2 3

may be envisaged.According to Refs. [1,11] a phase is completely

charged if its thickness (, ) is comparable to or evensmaller than the Debye-length (l). This is definitelythe case in the 7H heterostructure. Thus, we expectan almost fully disordered periodic arrangement

1 1characterised by Ag -rich parts separated by Ag -deficient parts (see Fig. 6), in which the chargecarrier concentrations are much greater than in anextended phase and are expected to be of the order ofthe nominal silver concentrations. In this way theextreme absolute s-values of the AgI:Al O com-2 3

posites can be well explained. All the compositesexhibit the characteristic activation energy of 0.29 eVjust as other surface- and interface-dominated AgIspecimens: single crystalline AgI cleaved along theab-planes and polycrystalline AgI with smallplatelet-like grains [19]. The activation energy of0.29 eV can be attributed to the transport within theclose-packed layers in the AgI system, which natu-

Fig. 5. Conductivity (bold line), DSC curve (dashed line) and rally applies to the heterolayer, too [19].in-situ XRD for AgI:Al O (20 m/o) composite (insets). The2 3 Even though predicted a decade ago [11], clearconductivity curve exhibits broad transitions ranging from the

evidence for the mesoscopic effect on the ionnominal temperature (T ) to the temperatures of superheated andoconductivity has not been found in typical solid ionundercooled transitions (T on heating and T on cooling). DSC1 2

conductors. The behaviour of extremely thin LiImonitors thermal effects over the corresponding temperaturerange. In-situ XRD (inset) confirmed the coexistence of the hightemperature a-phase and the low temperature b-phase over thetransition region.

the AgI:Al O composites is consistent with the2 3

7H-content [14]. The next section outlines ourhypothesis of a mesoscopic ionic heterostructurewhich explains the above features.

4. Discussions

As set out above, the 7H structure can be approxi-mately conceived as a heterolayer of b-AgI andg-AgI (i.e. b /g/b /g . . . ). Since we have evidence Fig. 6. Schematic conductivity profile of a mesoscopic heterolayer

of b-AgI and g-AgI (b /g/b /g/ . . . ), the layer separation ofof conductivity anomalies in deliberately producedwhich is in the sub-Debye-length (, ,l) range. The dash–dottedtwo phase mixtures of b-AgI and g-AgI [14] weline shows the concentration profile for semi-infinite space charge

expect a silver transfer from one phase to the other layers within a single phase of thickness L . 4l. The dotted lineforming v/ i (vacancy/ interstitial) junctions separat- represents the bulk value. The contact phase is assumed to being the bulk phases as discussed in detail in Ref. [1]. g-Al O on both sides.2 3

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films has been quantified in this way [11], however, mesoscopic ionic conductivity effect is expected tothe unlikely large Debye lengths obtained cast some occur explaining the experimental features in par-doubts on this interpretation [1]. Mesoscopic effects ticular the extreme conductivity and the activationare expected in nanocrystalline ion conductors [20]. energy. Ionic heterostructures with sub-Debye spac-Investigations of nanocrystalline CaF point towards ing are expected to open a field of considerable2

distinct space charge effects; the analysis yet indi- potential in the area of Solid State Ionics.cates — in accordance with the comparatively highimpurity content — semi-infinite space charge layers[21]. Finite space charge effects should definitely Acknowledgementsoccur in swollen ion-exchange membranes such as‘watered’ Nafion (see e.g. [22,20]). The water chan- J.-S. Lee acknowledges the support by Alexandernels with the fluorinated hydrocarbon backbone are von Humboldt foundation in Bonn.extremely thin and protons can be assumed to movein a mesoscale space charge situation while thecounter ions are fixed at the hydrocarbon framework. References

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[11] J. Maier, Solid State Ionics 23 (1987) 59.the regime of cluster chemistry. Details on this can[12] P. Chowdhary, V.B. Tare, J.B. Wagner, J. Electrochem. Soc.be found in Ref. [20].

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[15] A. Schmidt, J.C. Bazan, L.Vico, Solid State Ionics 27 (1988)AgI:Al O composites. In contrast to the stable2 3 1.AgCl and AgBr rock salt structures in which the [16] B.L. Davis, L.R. Johnson, Crystal Lattice Defects 5 (1974)

1235.interaction of Ag with the nucleophilic alumina

[17] J.-S. Lee, J. Maier, Macroscopic mixed phase effects inleads to an ideal diffuse and semi-infinite compensat-AgI:Al O composite electrolytes, in preparation.2 3ing space charge layer, the interaction leads here to

[18] Gmelins Handbuch der anorganischen Chemie, Silber, Teil Ban interfacial phase transition. (Note that such an 2, VCH, Weinheim, 1972, pp. 192–193.interphase is also not observed in the AgI:AgCl [19] J.-S. Lee, S. Adams, J. Maier, Defect chemistry and transportcomposites.) The long-time stability of the 7H phase characteristics in b-AgI, J. Phys. Chem. Solids, accepted.

[20] J. Maier, Point defect thermodynamics and size effects, Solidin the composite needs to be checked further.State Ionics, in press.The stacking fault phase can be approximately

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