Thermal and water vapor pressure dependence of electrical conductivity in HTaWO6·xH2O (0<-x<-1)

~l~d SlateIoni~ 2~30 (1988)~2-~6 N~-Hoffaa& Amst~am

THERMAL AND WATER VAPOR pRESSURE DE~ENDENOg OF ELECTRICAL CONDUCTIVITY IN HTaWO~.sH~O (0'g x ~< I) C.M. MARL A. A/~GHILERL M. CATTI

and G. CHIODELLI

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In eke f ~ m e ~s a research devoted ~o line~mdv o~ new proton ~ n d u c t o ~ that should oi~eraIe i a ~hc 200-4~)0"C range ~ ~ l i d ete~rol~es i~ fuel rolls, the elcctri~t ~nduetlx'ily o f rue pytoehlore HTaWO~.xH~O ( 0 ~ x ~ g l J ~vas invcstlgated [1]. P~liminaryresales suggesteO two differentcoaduct i ~ mechanismsfor h?dregen ions: the f i ~ t at high temperature (1~- 130'C) in which both a dasslcal ion-hoppingmodel and a p h ~ o n ~ i s t e d t unnelling model can be ~ a m e . d , and the second at low ternperalu~ ( T ~ 130r ), not ~r welt defined,i n ~hich the pre~nce o f wat~ moleculesin the crystal s t ~ tureofthematcriataplmarsto~h~the~leetiScal ~ n d u e t i v i t ysubstantiallg i n order *o characterize lhoroughlyIh~ r bebaviourof such py~chlore and to invesllgatetile ~ l e o f ~ t e r m o l ~ l ~ i n the conduction mechanism, detailed eleetfi~t r~islance measurements we~ ~rried auk by complex impedan~ method, on volyet~staliine samples o f the anhydrous and hydrated phase~

2.LCempound~rel/arati~n

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Th~ HTaWO~.HtO p h a ~ ~as prepared ~ M m g to[I]byionexr N HCI from KTaWOe-H~O. 2.2. Apparatus The experimentalset-up r to ~rc~ out the el~trt~l m e a s ~ e n t ~ at ~ wyn-ddlacd water-vapor pressu~ is described in gvf. [ 2 ]. Changingthe ratio bct~ecu the dry and the water va~or saturated gas (air) t~e ~ o u n t o f ga~ous water i n tile gas s t e a m was ~ n t ~ l l e d and its c ~ c ~ t ~ t i o n mensured by an hygromelcr ~ r k i n g i n -n01+90~C temperature rage ~ d in 0-100% F,H ~nge; the a ~ c u r ~ y is • 20,6and • 3% in 1~1e~'-~0% RH and 8010~AI RH ~ugcs, respectivel 7 The cell was placed in an air t h e ~ o s t a t reglllated at _+1oC. 2,3. Electrical conductivit~ measurements Eletrlcal conductivitym e a s ~ e n t s were carried o~t ou f l 13 m m 0alters, formed by ~mpaetin~ at

0 167-2738/88/$ 03.50 9 ElsevierScience Publishe~ B.V, (Noah-Holland Physics PublishingDivision

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2.$1.Electrical~nduct!vityofHTaWO~ The anbyd~us phase wa~ obtalned d~:eetly in ihe cell by heatingthe pellets o f FITaWO~-H~O at 7C' C for four hou~ under the vacuumo f I 0 Pa. The water loss ~ s checked by gravimetricanalysis~nder Ihe ~ m e experlmcmalconditions.The impedancediagramsobtained i n the ~ g e 20-250 oC show, at lower tempe~tu~s, a portion o f s e m i c i ~ l ~that i n ~ a ~ so much to become at first a finished~ i c i ~ l e ~ d then to develop i n a double one ( see fig. I )) as the tempc~tum i n c a s e s , The higher f~queneles ~ m i c i l d ~~ e m the ~mptex impedance o f the bulk: this was e o n f i ~ r by measu~ments on diffe~nt sized penets Which s h ~ d that the diameter o f the e ~ l e dep~ds on the thicknesso f the sp~imen and by the valueofthe ~pscitanees (pF) [ 3], calculated at the r e . h e n c e frequ~cy. Being the center o f the semicircles supp~sscd ~lp to Cew degrees, the fr~ queney ~sonanee valuc~ were obtained by the dingram o f imaginary pan o f the modulus v e n u s f~quency [2]. Tee low f~queney semici~les,having capacitance valueso f some nF, we~ assigned to the grain boundary [21. The semitic ~n~tuetivityo f the bulk as deduced from the results o f the ~ m c t e x a d m i t t a n ~ m ~ -

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o lO~r ( KI) Fig ~ Bulk~nducfivltyvc~s 1/ r of8 r a ~ s u r f e r s is retorted, in a log o v e n u s I/Tdiagram, i n t l g 2; a change o f the sieve, ascribed to a not well definedtransltionat t h e p ~ e u L i s o b ~ e d a t about 100~ ( Tv ) The activationenergies w ~ eslelJlaled to be 0.38 and 0.66 eV for t ~ p c r a t u ~ higherand

IowerthanTx,~spectively.lnfig. 3thebehaviour

o f the grain boundary e l ~ t r i l l ~ n d u c t i v i ty is r ~ porled as a f u n c t l o n o fthe l e m p e r a l u ~ i nlhe 150250~ ~he aeti~r~on energy was calculated to be 0.43 eV.

Z3.2.Electri~l~nducti*,ityofltTalVO,.xlf~O With the aim to inv~tigalethe i n n a ~ e e o f the wat~ mol~ules p ~ t i n the l a t t i ~ o f pyrochlo~ on its electrical ~sistivity, el~trical conductivity measuremenlsas a f~netion o f t e m p e ~ l u ~ (25-

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Fig ~. Grain~ u n ~ ~ndactivi~ ~

1/ T of ~aWO,.

the , , a f i a t i ~ o f the e l e m n ~ lconductivityas a 51n~ tion o f llme, at a well defined ~ t e r vapor p ~ s ~ va]u~ a quite ]ong ~ime (abouI ] 80 mi~} was n ~

es~ry to altaln equl]ibri~ conditions bctw~ the ~lid and the gas pha~. The impedan~diagr'cms obtaiaed, in equilibrium ~nditions,at 25 ' C s h ~ a portion o f semlci~letha~ i n ~ s e s so much 1o become a finished~micirele and then to develop i n a double ~ r ~ the watervapor p~ssum i n ~ a s ~ . At t e m p ~ m t u ~higherthan 25~ a singledlstor~d ~ i c i m l e , the ~dias o f whlch deceases as the ~ter-vapor p m ~ u m i n e r e a ~ , is alwayso b s ~ c d (see fig, 4). D e ~ n ~ l u t i r t ~~r ~ . trapolating1hose cu~cs, the bulk (Rb) ~ d the gealn bonndary ( P~c~) ~sisteu~s were oblained. The vab ues o f R~ and R,b s e ~ to be ~ n s t a n t as the ~ l e r vapor p ~ s s ~ ~aches a certain ~ l u e that depends on 1 ~ p e r a t u ~ . In fig. 5 and fig. 6 the log a verses IogP v a l u ~ for Re and R ~ ~spec~ively, a ~ r ~ I~orted. One ~ n o b s ~ r tha~ Rb depends on ternperalu~ and on waler vapor pressure accordingto:

~p~(r~,

( 1)

while R ~ is independento f t e m p ~ a l u ~ as: a~P

(2)

I?~ .

The n value o f eq. ( I } d e ~ s e s with i n ~ s m g tempe~ture, The R~ cons~ent valuesobtainedby extraI~latingthelog a v e n u s P ~ e s w~e tentetivc]y plotted ~us 1] Tand the activalionener~ was ~ l -

culaled 1o be 0.22 CV.

3. ~ i s ~ i a n Allbongh the ~ n d u ~ i n n m ~ h a n i ~ s in the pytoehlore i n ~ t l g a t e d have nol yet b e ~ ~ t i~.ly defined, the p m ~ n t work might be a ~ h e r e ~ t r i b u t i ~ to their ~ p l a ~ t i o n . As for the anhydrousphase, both propo~d mo~e]s (ion-hopping and p h o u o n ~ i s t e dW ~ e l l n g ) for the r ~ n d u c t i o n mcchanism14,5 ] am suhs~ntiallysupported by the presenl re~lts. In fuel, some prellmlnary~lculatlonso f the potential en~gy for theprotonmotioninthcst~ctureoflhispyroch[o~ were p e r f o ~ c d [6] and yielded a computed value o f the a e l i ~ l i o nenerg~ (in the a~umptiono f a thermally activated hopping mechanism} which ~ m par~ favourablywith the experimental~ae o f 0,66 eV (for temperaturesI ~ e r than Tx). Several p~blems a ~ slln ~ ~ Ihe ~ hand' for lhe hydrated p~sei At first, some analogies m n ~

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Fi~.~. BulkconOucti~ityor HT~WO~~dm~edphaseas a fun~i~ofwatervapor~ i ~ p ~ a m at fourdifferent~mver~t~res be found with the simUac pyrochlo~ HSbTeOs.xH20 {2]: the ~lcctrlcal conductivity,in air, shows a m i n i m u mat about IO0~C for both pha~s, The initiali n c r ~ s e o f the elect rJml ~sistivitF with temperature ~ s ascribed to a progressiv~ loss o f water. H ~ v e r a dctaUcd analysis o f the depCn~ dance o f the electrical conductivityon wmer vapor p r ~ u ~ indicates a diffe~ut bchaviouro f the two p y ~ c h l o ~ . HSbTcWO6.xH20 shows an cxpo~ntiai d e p e n d e n t , while in HTaWO,.xH~O a v a r i ~ ,with a p o ~ r o f p (see cq. ( I ) ) ; yet the a c f i ~ t i o n

encrgi~ are ~mpamble: 0.44 and 0.22 eV ~spectlvely.Our ~ u l t s , showinga constant slope in the log o ~ u s log P diagrams, migh~.be interpreted i n

an analogy with the nonstoichioaletri~oxide sys-

-4

terns. In the present case, the slope shouldbe ~lat ed to a parti~lar vibrationalor tunnelingstate o f the water m o l ~ l ~ s among disordered positions [7] instead o f a well defined ionizationstate o f defects in the case ofoxme~ This statementis ~nsistent with eq, ( 1), beingn independento f the wat~ ~ n t e n t in the ]anice and dependent on temperam~ only On the basis o f th~o ~nside~tions, the followir.B ~nductlon m ~ h a n i s m i n three steps can be sug-

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Oat i n the latti~, the lower activation energy obs ~ in the hydrated phase is a e r a t e d for.

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