Silver-ammonium beta alumina

Solid State Ionics 5 (1981) 245-248 North-Holland PublishingCompany

SILVER-AMMONIUM BETA ALUMINA

B C Tofield Materials Development Division AERE Harwell Oxfordshire OXII ORA UK The exchange of ammonium by silver ions in beta alumina is described and the thermal evolution of silver-ammonium beta alumina is outlined. Treatment of silver beta alumina in molten ammonium nitrate yields two separate beta-alumina phases within the same crystal rather than a mixed single-phase material as is generally assumed to occur in beta alumina ion exchanges. The extent of exchange at 210°C is indicated by an opaque exchange front and the c-axis lattice constants on either side are close but not identical to those of the ammonium and silver compounds. On heating, the two compositions interdiffuse and, ultimately, stoichiometric silver beta alumina is formed within the crystal. As well as providing a single-stage preparation of a stoichiometric metal beta alumina, such a mixed crystal should provide a fascinating vehicle for the study of the interaction of related but insoluble phases produced by controlled solid state reaction. The conclusions drawn from the silver-to-ammonium exchange are confirmed by study of the exchange of partially-decomposed ammonium beta alumina in molten silver nitrate. INTRODUCTION The preparation of hydrogen beta alumina (1,2) has broadened our understanding of beta alumina chemistry (3) by demonstrating the possible loss of interstitial oxygen under suitable circumstances. The formation and structure of stoichiometric hydrogen beta alumina, has also allowed the preparation (4,5) of more-nearlystoichiometric metal beta aluminas by reverse ion exchange. The most advantageous preparation of hydrogen beta alumina is by the action of hydrogen gas on polycrystalline silver beta alumina (1,2), but hydrogen beta alumina may also be obtained by the thermal decomposition of ammonium beta alumina (6), and more-nearlystolchiometric metal beta aluminas have been prepared via this route (7). Hydrogen beta alumina obtained via the thermal decomposition of ammonium beta alumina appears disordered relative to material made from silver beta alumina (8) but may be prepared in single-crystal form suitable for structural and spectroscopic studies (8). Attempts to prepare single crystal material by the action of hydrogen gas on silver beta alumina yield very poor quality material, rendered completely opaque by the exfoliated silver, which is unsuitable either for accurate structural or spectroscopic study (Figure i) although IR of mulled fully-exchanged single-crystal material confirms the presence of an H-O(5) stretch (9).

Figure 1

The preparation and decomposition of ammonium beta alumina from the sodium compound is reviewed in an accompanying paper (8). Because of the very small weight change expected, about 1%, this measurement cannot be used as an accurate probe of the degree of

The action of hydrogen gas on silver beta alumina crystals. On the lefthand side, a crystal treated for a few hours at 450°C. A silvery precipitate is seen within the outer portion of the crystal. In the centre a more opaque deposit is produced in a larger crystal by treatment for 24 hours at 550°C. The clear centre portion is essentially unchanged silver beta alumina. Extended treatment for two weeks at 550°C allows complete conversion to hydrogen beta alumina with total loss of opacity (right-hand side) and very degraded crystal quality.

exchange and the measurement of the c-axis lattice constant across the crystal proved

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B. C Tofield / Silver-ammoniurn beta alumina

246

ultimately a very satisfactory method of assessing the progress of the exchange (8,10). For a 2-3 man thick (in the basal plane) crystal, an initial exchange of ~50% is often observed. Neither at this point, nor as exchange proceeds, has any evidence for the presence of more than one phase been forthcoming. The crystals remain optically clear and the c-axis lattice constant varies across the crystal in a manner consistent with the degree of exchange expected. The exchange of silver by ammonium ions should produce a much larger weight change, about 16% for a stoichiometric excess of 25%, and would seem to offer a route to monitor gravimetrically, and hence very readily, the progress towards fully-exchanged ammonium beta alumina. This paper describes the results obtained in attempting an exchange of this type and the thermal evolution of the two-phase material formed. Some preliminary data have been given previously (ii) in a review of hydrogen-containing beta aluminas.

Lattice constant measurements revealed c-axes close to but not identical to those of the end members (3): Outer portion Inner portion 25% exchange 50% exchange

22.826 ~ 22.814 ~

22.511 22.511

These results have been confirmed by the exchange of partially-decomposed ammonium beta alumina in molten silver nitrate at 3OO°C. (NH4) 1 oHo 25AIII017.125, prepared by annealing ammonium beta alumina at 350°C in hydrogen (8), yielded, probably, (NH4)I-xAgxHo.25AIIIOI7.125 where x was close to unity (Figure 3). Although considerable silver deposition throughout the crystal precluded the possible optical observation of separate phases, two lattice constants of 22.82 ~ (minority phase) and 22.52 (majority phase) were measured, in very good agreement with the results above. The exchange of ammonium beta alumina by silver has not been studied in any more detail.

THE FOP~MATION OF SILVER-AMMONIUM BETA ALUMINA The progress of the exchange is indicated by an opaque white band around the crystal. For crystals of approximately 2 ir~n thickness in the basal plane, four days exposure gave 25% exchange as measured gravimetrically, A further twelve days exposure gave almost 50% exchange. The progress of the exchange front through the crystals agreed well with the thermogravimetric result if it was assumed that it separated essentially unchanged silver and ammonium beta aluminas. ,~ - - ~ WAV[NUMSen

Figure 3

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The IR spectrum of (NH4)I_xAgxHo.25AIIIOI7.125 (x-i). The ammonium bands are weakly present and the H-O stretching vibration is increased in energy [o 3540 em -I compared to 3505 cm- for the (NH4)I.oHo.25AIIIOI7.125 starting material.

THE THERMAL EVOLUTION OF SILVER-AMMONIUM BETA ALUMINA

Figure 2

Crystals of silver beta alumina after exchange for 4 days (left-hand side) and 12 days (right-hand side) in ar~nonium nitrate at 210°C. The progress of the exchange fronts across the crystals indicates approximately 20% and 50% exchange in the two cases, in good agreement with gravimetric results.

Crystal slices of material exchanged to approximately 50% have been heated to 3OO°C, 4OO°C, 5OO°C, 6OO°C and 7OO°C in air for periods of up to 24 hours. At 3OO°C, the exchange front has begun to diffuse into the bulk of the crystal (Figure 4). Two lattice constants are now observed in both the inner (22.828 ~, 22.483 ~) and outer portions (22.830 ~ and 22.481 ~) indicating continuing inter-phase insolubility. Indeed, the silverrich lattice constant is now very close to that for silver beta alumina itself (22.488 ~ (3)).

B.C. Tofield / Silver-ammonium beta alumina

247

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Figure 4

Left-hand side: 50% exchanged silverammonium beta alumina heated to 300°C. The exchange front resulting from the 210°C exchange is still observed but has partially diffused through the crystal. Right-hand side: Further heating to 400°C gives an optically clear crystal which, however, still exhibits two c-axis lattice constants.

Figure 5

Heating to 400°C renders the crystals transparent (Figure 4). Once more, however& two lattice constants are observed (22.78 N and 22.50 ~) but with greatly increased linewidths indicating scattering dimensions of approximately iO0 ~. The mixed system is possibly approaching single-phase behaviour and the very small 'particle' size no longer results in optical scattering. The detailed behaviour just above 400°C has not been investigated, but at 500°C, silver is deposited within the crystal (Figure 5) and a single lattice constant is observed for the first time (c = 22.577 ~). The product is probably stoichiometric hydrogen-silver beta a]umina, Hl-xAgxAlllOl7 (x~0.5) produced by loss of ammonia to give a non-stoichiometric hydrogen-silver material, followed by removal of the interstitial oxygen as water, and reduction of a portion of the silver content to silver metal. The product at 600°C is very similar (Figure 5) although c has decreased (22.546 ~), but at 700°C irreversible decomposition of the hydrogen-containing material (8) seems to occur yielding a further degradation in crystal transparency (Figure 5). A well-crystallised beta alumina phase remains, however, (c = 22.540 ~), probably with a composition close to stoichiometric silver beta alumina, AgAIIIOI7 (c = 22.529 ~ (3)). This would be included within a partially decomposed beta alumina framework and similar behaviour is observed in the decomposition of ammonium beta alumina con-

F

I

From left to right, approximately 50% exchanged silver ammonium beta alumina heated to 500°C, 600°C and 700°C. Silver deposition is observed within the crystal at 500°C and 6OO°C and a thinner section is shown here from the 600°C treatment. At 700°C a further loss of transparency occurs as a result of the irreversible decomposition of hydrogen beta alumina.

200

T uElO0 ci c~

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Figure 6

200 crn-I

The far-IR spectrum at 2K (E~ c) of stoichiometric silver beta alumina formed by heating silver-ammonium beta alumina to 700°C in air. The band near 21.5 cm -I arises from motion within the mirror plane of Ag + in a BR-mO site (Refs 3,4). A band at 43 cm -I observed in nonstoichiometric silver beta (Ref 12), which arises from motion of Ag + at an aBR site~ is not seen. The band near 78 cm -i probably arises primarily (Ref 12) from 0(5) motions in the mirror plane.

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B.C. Tofield / Silver-ammonium beta alumina

taining residual sodium (8). The identification of the product as stoichiometric silver beta alumina is supported by far IR-spectroscopy which yields a spectrum (Figure 6) very similar to that obtained for stoichiometric silver beta alumina obtained via ammonium beta alumina (12). It is also of interest that the partial thermal decomposition of ammonium beta alumina yields material with two c-axis lattice constants (8).

REFERENCES l

B C Tofield, A J Jacobson, W A England, P J Clarke and M W Thomas, J Solid State Chem, 30, 1 (1979)

2

J M Newsam, B C Tofield, W A England and A J Jacobson in 'Fast Ion Transport in Solids', Eds P Vashishta, J N Mundy and G K Shenoy (Elsevier-North Holland), p405 (1979)

3

J M Newsam and B C Tofield,

4

J M Newsam and B C Tofield, J Phys C, _14, 1545 (1981)

5

W Hayes, L Holden and g C Tofield, State Ionics, i, 373 (1980)

6

Ph Colomban, J P Boilot, A Kahn and (; Lucazeau, Nouveau Journal de Chimie, J 21 (1978)

7

J P Boilot, Ph Colomban, G Collin and R C o m ~ s , Ref 2, p 243

8

g C Tofield, J M Newsam and A Hooper, this conference

9

W Hayes, L Holden and B C Tofield, J Phys C, 13, 4217 (1980)

CONCLUSIONS A new aspect of beta-alumina chemistry, the preparation of immisible beta aluminas within the same crystal, has been demonstrated. The thermal evolution of silver-ammonium beta alumina has been surveyed and shown to lead to the direct formation of stoichiometric silver beta alumina. It would be of great interest to determine the extent of immisible behaviour in other beta aluminas and, for particular systems, to establish the phase diagrams in more detail and also the particle size evolution as a function of temperature and time. In the silverammonium system it would be very worthwhile to examine both the structures of the various compositions achieved as the temperature is varied as a function of the initial degree of exchange, and also their ion-exchange behaviour.

Solid

IO

A Hooper, B C Tofield and C F Sampson, Ref 2, p 409

ii

B A Bellamy, A Hooper, A E Hughes, J M Newsam, C F Sampson and B C Tofield, in 'Energy and Ceramics', ed P Vincenzini (Elsevier, Amsterdam, Oxford and New York), p 950 (1980)

12

W Hayes, L Holden and B C Tofield, J Phys C, 14, 511 (1981)

ACKNOWLEDGEMENTS I am indebted to C F Sampson (MPD, Harwell) for assistance with sample preparation, B A Bellamy (MDD, Harwell) for lattice constant determinations, J M Butcher (E&MSD, Harwell) for IR spectroscopy, W Hayes and L Holden (Clarendon Laboratory, Oxford) for far-IR spectroscopy and to the Harwell Photographic Group for the photography of beta alumina crystals.

this conference