Quasielastic diffuse neutron scattering from yttria-stabilized zirconia at elevated temperatures

SolidState l ~ i ~ 2 ~ 3 B (1988) 4 ~ 9 2

QUAS!F.LASTIC DIFFUSE NEUTRON S O . T 3 X R I N G F R O M YTIRIA-STABKffZED ZIRCONIA A T ELEVATED TEMPERATURES s. HULL, T.W.D. FARLEy, M,A. ItACKETT =, W. HAYES, a . OSBORN * N M , ANDERS~'U, K. C L A t l S ~ I M.T. H u ' r c m ~ G s an~

W.G, ffI'IRLING

"

At room t~mperatn~ p u ~ zi~onla (ZIO2) has a m~ocilnic crystal stn*ture. On heating it traa~ f o ~ firstly to a tetragonal p h a ~ at ~ 1273 K ~ d finaUy to a fluotlt e mbiz struct u ~ which is stable bet ~ n 2643 K a n d the melting temperature of 2988 IC The high temperature fluorite plm~ of ZrO2 ~ n bes~abilizedat ambient tempe~turebytheaddition of various ~bic oxides having di-val~t (CaO, M40, etc.) or trl-valent (Y20~, LabOr, etc.) cations in a wide rouge o f ~noentrations. yttria.stabifized zir~ n i a (YSZ) finds many d i v ~ technologist al~ "v~t a ~ X~h~i~ L~., 7112Tavis~k ~ , ~ a don' WCIH 9LT' gI~ 9 p r e ~ t address: R u t ~ r d Appleton ~ m t ~ , CMItua, OXll ~ X , tSK. - - P r i n t ~ d r ~ ~~ t of ~ y , i ~ U m v e ~ of Km~, Keete, Stuffs,,ST5 5BG,UK.

fuel cells, i . oxygen ~ . ~ o ~ and as a high t ~ p e r a ~ ~ a m i ~ Its use as a n artificialgem stone hasled to the ready availabilily of large s~ngle crystals [ l ] . The eler i ~ l neut rallt y o f YSZ is maintained b y ~cancies in the o x y g ~ sublat t i ~ , a n d t his defccfive stmctu~ g i ~ rise to its ~ y g e n ion conductivity. Diffraction studie~ of stabilized zirconla ruing ele~ trons [2,3], X-~ys [4] a n d neutrons [5] have e ~ t ablished the ~ t l m ~ a ~ r a g e d structure of the unit r a n d s h o w t h a t ~ l a x a t i o n ~ a m u n d ~ r vacancy, H ~ r "Mtb such high v ~ n c y concentmtions s t ~ n g ~ e i z t i o n s b e t w e ~ the v a ~ c i ~ ~ r , ~ d t h e ~ have b ~ p ~ b c d b y diffuse ele~ t ~ [ ~,31, X-my [ 4] a n d neutron 16-8 ] s~ttetlng, In this paper ~ ~port the ext~sion of ~ m e of the ~rlizr diffu~ n e u t ~ n ~ t t e r i n g mcasu~ments to elevated temperatures in o~der to gain furth~ in for. r a t i o n on the d y n m l e bebaviouy of the defects, a n d

to relate tats to the ~ n t r a l i o n a n d tempera~a~ dependence of tho ionic ~nductivlty. The principal r~ults oCthe cartier~rk 16-8] ~ brieflyreviewed in s~lio~ 3.

Exper~ml

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The y male% y=O~/ZlO~ single clTstals o f nominal ~ m p o s k l o n y = 9 . 1 2 . 1 5 . 1 8 . 21 a n d 24 ~ gro~ b y J.F. W~ekus o f the Ce~s Corporation ushlg the skull melting )~hnique. They w e ~ aligned with eillaer the [ 110] or [001 ] erystallot~taphic ~ i s v e ~ lieal. For hlgh t e m p e ~ t u ~ m e . ~ e n t s they ~ cut altra~al~lly into r162 typically o f 15 m m d i ~ e t e r ~ d 25 ~ length, ~ d then encapsulated under v a ~ inside molybdenum or t~gsten ~ . T h e sample ~ s then mounted inside spechlly d ~ signed high-tempem~rr f u m a ~ s , constl~cted of materials with 1 ~ n e u r o n absorption. T h e expefim ~ l s w e ~ performed using triple axis spectromr t e ~ in the D I D O and PLUTO ~ a c t o ~ , H ~ e U , a n d at Risla ~ t d the Insitut Lau~Langevln (ILL). G ~ n oble. The l a t t ~ two thst~ments ~ s i t . t e d on beams from cold reoriented n e u r o n ~ s to give good energy ~solution. T w o types o f esperim~t ~ perfo~r In lbe f ~ t , the elastic intensity at zero e n e l ~ l r a ~ f ~ ~ , was s ~ n n e d to deteminr S( Q, w = 0 ) as a function of s~tterthg vector ~ i n the ( l l O ) or (100) plane. If the ~atterthg is elastic, or has quasielastir broadealng well ~rilin the imtr~m~tal ~ e r g y ~ l u t i o n as is th~ silualion at 293 K, the intensity contoun ,All be orS(Q) =fS(Q, ~)doJ. A t eeaain values of ~ energy s ~ n s ~ per f o x e d to m ~ 1(~r At all t ~ p ~ m r e s the intensity profde I(Qa.~o) ~ a l d be ~11 fired using tl~ routine P K F I T [9 ] b y a combi~t thn of a n elastic resolution limited Gauss i ~ function plus a ~ v o l u t i o n of a Lo~ntzisn funclion with the Gaussian ~ e r g y ~solulion f u n ~ lion, together with a l i n e r back~'ound level. The elastie component ~ t a l n e d a ~mribulio~ f ~ m the i n . h e l m ~ t teling f ~ the ~ p l e ~ntalner. The diffu~ scattel~ng i~tensity ~ s b~ugb~ onto a n absolute scale by a ~mparisnn ~ t h the ~attering ohse~ed from selected acoustic phonons w h o ~ cross section ~ u l d be ~leulate&

3, De ft'el suuclvte at 293 K The defecl structure of crystalsof an six dopant levelshas beea studied at 293 K, using the DIDO 3axis s p e ~ e t e r , by dast~e ~iffvsr n e ~ t ~ ~ t t e ~ inS, The principal ~ l t s have been repoaed in ~fs, [ 6-8]. Dala obtained at elected t ~ p ~ r a l u ~ h a ~ helped to id~t~fy thre~ regions of r e . p r o w l s p ~ w h ~ the principal ~ t t e r i n g arls~ from d i f f ~ m origins. The idenlifl~lion is rinsed on calculations of the expected scattering from a single oxygen ~ e y ~ d its distortion fidd [7,10], a n d from a mnSr o f slzesofaggregalesofva~neypaL~nt~don~lion sites, the v a ~ e i e s in each pair being sepa~t 9 by

(ll2112112)ao[g].Thel~hniqueisin~nsiliveto the location o f the dopant ions, owing m the similar neutron ~ t t e l i n g lenghts o f the Z P + ~ d Y ; § nuclei. A typical ~ t o ~ map of S(Q) for a cr~tal of yffi 12, at 293 K i s s h ~ n in fig. 1. Similar ~ p s h~ve bcenoblained forboth ( l I 0 ) ~ d (001)planesfor a n six ~ n t r a ~ i o ~ , ~. T w o ~gions of the crystal which give r i ~ to diffuse stuttering may be i d ~ l i fled. One ~mprises a relat[vely v a ~ n o y = f ~ telrag= onally-dist or ted fluorite structe~ [ I 11 which gives r i ~ to rather sharp peaks at the (l 12 ) a n d (l 14) points. These d ~ a ~ in int~sity as y ine~e.ses showing thai the letragonal ~gion is thsignifi~nt above y ~ ] 8. T h e ~ p ~ k s ~ found to I~ s o m ~

rig. 1. D J ~ d a ~ l e ~ u m ~ ~a~oun tn~e 0 I0) ~la.e.~m a s~.~ ~ 1 or 12 mel~ y~O~Z~O~at 293 K,

what degcnd~z ~ fae t h ~ a ] h l s ~ l ~ o f the ~ . pie, particularly i f i t is held at i n t e ~ e d i a t e t ~ p e r a u ~ above ~ 1273 K. Eoer~/~sly~iss h ~ ~ t Ibey broaden fa~tcr, with a pr~Ic~minantly Lorcntzian i a t ~ s l r / profde, with / r i c k i n g zero~tu~ t ~ t~e ~ m omer r ~ o n ~ The

tor o f the v e ~ c y pair [ 6]. Si~.le ~ n c i e s ~ v v r i ~ toanumbetofb~ddiffu~peak~ofwhichthclw ak, wlth the f o i s t ~ i s ~ t r e d at ~ ( 1.4,1.4, L4) [ lO], This weak d / f f ~ ~aglc v a ~ n c y ~ t t e r i ~ g ~ be ob~edovera]ar~ ~geos ~d ~do~te Jn ~ r ~ n r ~ s cf r ~ s~ ~ Qs e ~ r

aggregalcs o f v ~ c y pm~ ~ r . In the agSlegate the v a ~ c y ~ai~ ~ e separated by {L 0~-5, 0.5} latt i ~ ve~1o~, ~ d g i ~ n ~c i o d Lffuse" ~ l e l I i l e "pe~
teHng arises from ~ n d c s and *their s f , g ~ g a ~ is ~ f ' ~ C ~ i b y the ~]ativr i~depeadcn~ o f ZIncn o ~ lir~d ~ t t g r l n goa y, This~ g ~ from 141o 25 b ~ s /

~ n t e ~ d at p~inm • (0A. 0.4, _+0 8) reJalive to t~e ~ g ~ fl~o~e ~ p r ~ l a ~ point, such a~ at

v e ~ y at ~ e poinl (L6, I.~, 1.~)~ ~nd i r o ~ 28 Io 38 bar~e~va~cy e~ (0.6, 0.6, 3.~). T~c w~e~hin

~ o b s g ~ e d be~usec~'~heenvr

o f the ~Sgreg~te~ ~s 9 few l s v 2 ~ sp~cin~- R ~

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~ 2 r~et~3~ s ~1coesl~.-12~h~uf,b (heq~S~r dif~se ~ued~g r r ~ YSZ,Theexpedmenl~ld~t~~refiued~y~~ b i n a lionof a ~sa[uti~n] i ~ i ~ Gau~ienpe~ ( - - - ), a b ~ ~ t zianc ~ e ~ t con~ved wi~ ~e G~u~iafite~lut ~onf ~ q=( [.~ 1.4.1.~).T=]8~ K~(c) 0,7, LT,t.O),T= [825K;and ~ m [ 8mol~ y~O~Z~ b (d) o~ (].7,1.7, t.0), T ~ 7 0 0

~cases only slightly with ~,indi~ting that ~ the d ~ pant level i n ~ the aggrcgat~ ~ in number 'out net in size, 4. Dynamic b e b a v i ~ at high tempexatme New m e ~ u ~ m e n l s o f S ( ~ , ~ = 0 ) and 8(Q~ m) have been per~o~ed on a y ~ 12 clvslaJ using the IN12 spcct ~ e t ~ at ILL at t~Bemtures u p to 1825 K~ a n d on a y = 18 ~ using the PL[ITO 3-~is spect~met~ at t e l n g e r a t u ~ u p 10 2780 K, c l ~ to the meltin 8 point of 2950 K for y = 18. A t ambient tem~x~tu~, all ~ n s t a m - Q ~ n s o f energy ~ v e Gaussian intensity profil~ with the inst, umenta] eno orgy width of 0.057 THZ I=WHM ~ s h o ~ for I N l 2 data in fig. 2a. Tais indi~tes that the defect s t m ~ t ~ s h a ~ lifetimes greater t h ~ ~ lO-~~ s.

Thcinte~sityeontourofS(Q,~=O)f~y=12at 1825 K o ~ a ~glon o f the (1 [ 0 ) plane is s h o ~ in fif~3. A t t h l s t e m p ~ a t u ~ c r ~ b m a d ~ i n g o ~ as seen from the i n ~ s i l y profile s h ~ in figs. 2b a n d 2c, It is seen that there ~ n ~ both Gaussian and Lo~nt2ian ~ m p o n ~ t s to the s~ttel~ng. The integrated int~si'~y and FWHM of the Lor~tzian

aos.. . . . .

.

m m p o n ~ t ~ g l v ~ as n ~ b e ~ in figs+ 3a a n d 3b ~pectively at the values of Q w h e n S ( ~ , ~ ) ~ s s ~ n ~ d , Fig. 2b s h o w that at the difin~ "superlatfi~" point the scattering is don~nate~ by the Gaussian ~ml~nCnl, indicting that ~hc strongly ~ l a t e d ~glons of v ~ n c y pairs ~ m a i n ~latively mabl~ The intensity deCTea~ewlth t ~ l : ~ f a l u ~ increase ~ n be a ~ u n t e d for b y a Debyc-waner factor, indicting that the density of the agsiega~s remains ~ughly ~nsuutt. T h ~ is h ~ e v e r a ~ a l l broadenecl LoRnU~an ~mpoaent. The L e f t ' a n ~ m p o n ~ l t becom~ m o ~ inten~ than the C r a u ~ i ~ component in the ~ t t e t J n 8 ~ f the(1.7,1.7,1.0)polnt, asseeninilf~2c.OurmOdel attlibut ~ ~ t t e r l n g in this I ~ i o n to a r i ~ p~domi ~ n t l y from single v a ~ n c i ~ . ~he iut~sity of the I ~ n t . ~ a n ~ m p o n ~ t a n d its cncrg~-w~dth at this point i n ~ s e s with tempetatur while the Gaussian ~mponcnt d~re~ in intensity, ~ f ~ i n g the

dynamicnat~ofth~dcfeclsandsugges~h3gtheir diffusi~ t h o u g h the latti~. Fig, 2d s h o ~ thc b ~ a d ~ profile at 2700 g f ~ the y = 18 ~ p l e as m ~ s u ~ d with s o - - h a t ~ r ~ r ~ l u t i o n of ~ 0.22 THz.

492

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$. C c n c l m l ~ s C o h ~ t difPas* n ~ t ~ ~ t t c r ~ g i~cli~tr that two tylgs o f m a ~ # r ~ # o n exist i n Y,O~ aabilizcdz i n n i a + A mlatlw]ydr fr162f ~ rcglon with a tetmson~ c~stoaion d e c ~ rapidly J~ s ~ as the dop~m level i n c a s e s , As the t m l ~ r a l u ~ is mi~cl it has an i ~ f i n g l y short llfetimt,t Rker due to ~qua~atlons b~twc~D cubic aad tet~gonal w m me~mor i s L ~ dhCfuslono f r a t t l e s . I~ tllc $ ~ n d ~ o n . va~xc~ pa+~ a m ~ i m ~ into agis'cgax~

is sup~0rted in part by the LrK S.E.R.C., the Danish Minlst1~ o f Easier. H ~ I I EMR Contractg and a C.]E.C.T'.~nningGrant No. S'TJ.033~I-C. It is also supporlcd by the Und~lyingR e h a s h P ~ z r a m m e o f 1he U.ICA,E.A.

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with t ha magc e oPsi~es i nt ~ s ec swhich . m Both ~ p~gions ~ l am t i ~ iarc ylstable relatively mat

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small in spatial exlent, wlth m ~ n ~ l a t i o a over only a few l a t t i ~ spacLngs.As y inc~a~s, tbr humbcr o f a~greg~tcsi n ~ c a ~ s rather than ~ e i r size. The

13]B.Hud~n~d p.T.~ . 3 ~lid $ t a ~ e m 19(197~] 3s~. 14] LB-C~ NLM~ ~ $"F a ~ Jr'. ~fia ~a~ Iaa" its M4(1981)61+tnd ~ f ~ s t ~ n . [5] D.St~qeandB.E.F.Fender,L pb~s,C7 (L974}L [~] R. Cs~om,N,I~.A ~ m a , IL C h ~ , ts l~a*km~W, a a ~ , M.T. Hu~h~n~~ d ~.~. M a ~ 1 0 . M ~ . S~; F ~ m 7 (1986)55. I71N'H" ~dr ~ Oaus~' MA" nsc~tt' W"na~g M'T' HutchinD,LE.~M~r and ~ ~ b ~ , i~:T ~ stmau~ ~lafiom ia faroion and m~ed ~od~m~. cds, r,w. eoulCn,N~. A n d e s . g O a u ~ S. S ~ m p aad OT. Screnm ( RiseNail.Lab..Rosklldc.19g~) p. 279. IS] N,H,Andev~n,K,ORu~n.M,A.H~kctLW,Ha~,M.T. Hu~hings,~.E. MaCdonaldaad K. Osb~. physi~ I36 O0S~ ~LS, [9] ~.T. Hu~ings, R.D.Lowdcan4~. Tin~,ia:~ P~. Sl (Ins~~ys.+~ndon, L986)p.151. [ L0] M.A.Hack~t,Ph.D.~esis (O~fOMUmve~ly. 1997). {111~,Fab:r~r,,M.H Muell~ra,d aa.Coop~r, phys l~v. ~I~ (I9VS)4gg4. [ 121C'R'A ~tl~ S~ ~ t c I~ 12 (t 984} 67-

m a x l m u mi~ Ihr do~ant dr162 o f t ~ condu~ tivi~y at y ~ 8--10% at a given temperature is a t t f i b utcd to the trappingo f vacancies i n the agg~galesi n a m a n n ~ s l m l h r to lh~t suggcstextby Catlow [ 12 ] and c4he~. 5r attn~.me~ tr singlevacancies ~ d ~ n ] l ~ e ~ I C S i ~ 6 g s as the l r 1 6 2 inc ~ a ~ s ~ d indi~tcs the d~e.~sin~ lifr162 o f the c o l l a t e d d~f~ls, Thls suggests an in~easing diff ~ i o R o f v a ~ n c [ ~ n ~ r the pcr/phcry o f lalgcr aggrcgalcs, Ack~ledgemeat W e wish IO lhank Dr. J+D. ComJas for the Io~n of the 21 and 24 mole% yzOsl~O~ ~ p l ~ . This work