Mat. Res. Bull., Vol. 17, pp , Printed in the USA /82/ /0 Copyright (c) 1982 Pergamon Press Ltd.

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1 Mat. Res. Bull., Vl. 17, pp , Printed in the USA /82/ /0 Cpyright (c) 1982 Pergamn Press Ltd. THE INFLUENCE OF FOREIGN IONS ON THE CRYSTAL LATTICE OF BARIUM TITANATE G. H. 7nker Department f Electrical Engineering Technical University Twente, Enschede and E. E. Havinga Philips Research Labratries Eindhven, The Netherlands (Received December 22, 1981; Cmmunicated by A. Rabenau) ABSTRACT Frm investigatins f phase diagrams f ternary xides the lattice sites f freign ins and cmpensating vacancies are established. Large trivalent ins ccupy barium sites and are cmpletely cmpensated by titanium vacancies. Small pentavalent ins ccupy titanium sites and are mainly cmpensated by titanium vacancies. During these investigatins a new cmpund was fund, Ba La 4 Ti ismrphus with Ba 5 Nb 4 O15. Intrductin In the literature n P.T.C. thermistrs based n semicnducting BaTiO 3 it is generally assumed that the varius dpants (Nb, Sb, La) are fr ne part cmpensated by electrns and fr the rest by vacancies n the barium sites f the pervskite lattice I)2). The chice f these vacancies is analgus t the case f SrTiO~ and CaTiO] where it is knwn that bth Sr and Ca can be replaced t larqe extent-by La r Ti by Nb accmpanied by the creatin f Sr r Ca vacancies 3)4). In an lder paper, hwever, it was shwn that Ta 5+ and Nb 5+ as substltutes fr T± 4+ are mst prbably cmpensated by a mlxture f Ba and Ti vacancies 5). Furthermre it is knwn that in PbTiO3, dped with La, bth Pb and Ti vacancies can exist with a rati dependina n the PbO partial pressure durin 6)7) ~ ~ ~ g the preparatln. This means that fr every cmpund the vacancy mdel needs a separate and careful examinatin. During ur investigatins f the phase diagrams BaO-TiO2-Nb~O 5 and BaO- TiO~-La_O 3 we fund indicatins pinting t the frmatin f T1 vacancies ratler ~han Ba vacancies. Therefre we made a detailed study f the parts f these diagrams arund the pervskite phase. Althugh different methds have been used, the relatins fund in these systems are mre r less cnsistent. 345

2 346 G.H. JONKER, et al. Vl. 17, N. 3 Especially interesting is the fact that in bth diagrams ismrphus phases are fund n extraplating the line f pervskite slid slutins, namely Ba 5 Nb and a new cmpund Ba La 4 Ti 4 O15. Experimental methds After several rientatinal experiments we prepared series f samples with cnstant dpe cncentratins (Nb, Sb, La) and varying BaO-TiO 2 rati arund the value expected fr the pure pervskite phase. A usual ceramic technique was applied fr the preparatin, starting frm mixtures f xides r carbnates f the cmdsinq metals, milling, prefiring in air at 900 C during 20 hurs, secnd milling, cmpressing t pellets, and then, as a last step, firing these pellets in air at high temperature (1250 r 1350 C) and slwly cling tgether with the furnace. All samples prepared in this way were white r slightly clured (yellw r green shade), s that we assumed that all metal ins had btained their highest valency states (Ba 2+, Ti 4+, Nb 5+, Sb 5+, La 3+) and n free electrns were present. A first indicatin f the psitin f the pure phase was the sharp transitin in sintering behaviur. Excess BaO caused a high prsity, excess TiO 2 a high density. The latter samples cnsisted f rather large crystallites, > I0 ~m. The samples were further investigated by X-ray pwder diagrams and by micrscpic examinatin f plished sectins. The latter methd prved t be the accurate ne, as samples prepared with excess TiO O at 1350 C shwed a nncrystallized glassy phase and runded crystal bundarles, as is als knwn in the pure BaO-TiO 2 systems. By these methds we culd determine the psitin f the pervskite phase with an accuracy in the BaO-TiO 2 rati f abut I%. The system BaO-TiO2-Nb205 The investigatins in this system at 1200 C frmed part f a general study f the phase equilibria between the many cmpunds. The results f this study are less accurate in the exact psitin f the series f slid slutins. They are shwn in figure i. The mst striking pint is that the cmpunds BaTiO 3 and Ba 5 Nb 4 01~ frm series f slid slutins frm bth sides, but with a brad miscibil[ y gap. The width f the slid slutin area indicates the inaccuracy f these experiments. Equilibria with the surrunding phases will nt be discussed in this paper. The results btained with samples prepared at 1350 C are shwn in figure 2. In the first place the cmpsitins with the purest pervskite phase are indicated. These frm a straight line pinting frm Ba TiO 3 in a directin t the cmpsitin ibao-inbo (nt a cmpund). Frm a series f cmpsitins alng this line we fund ~6~ endpint f the slid slutin frmatin at abut 12.5% Nb. This endpint is nt sharp, because a very fine segregatin ccurred, due t the decreasing slubility during the slw cling prcess. The results btained at this temperature are in agreement with thse mentined in 7). These tw investigatins shw that Nb 5+ can be substituted fr Ti 4+ t a high percentage, ~20% at 1200 Cand%25% at 1350 C. Hwever, there is an bvius difference in the cmpensatin mechanism shwing the frmatin f mainly Ti-vacancies at 1200 C and f equal cncentratlns f Ti and Ba vacancies at 1350 C. In figure 5the variatin f the cubic cell parameters is shwn.

3 Vl, 17, N. 3 BARIUM TITANATE 347 The system BaO-TiO2-Sb205 In this system nly ne series f cmpsitins was investigated, with 6% Sb, fired at 1350 C. These cmpsitins crrespnd t thse with 6% Nb in the BaO-TiO2-Nb205 diagram. The purest pervskite phase was fund at the same rati. This means that Sb is taken up in the lattice as sb5 + and with the same vacancy cmpensatin as in the case f Nb. A remarkable pint is that in this case the prus samples with excess BaO were yellw, but that the dense samples with excess TiO 2 were greenish. N BN 848~0"/~ "2), ', / I / /, 20/ ~ 'i, / / t tt 81N 2 i,, s B,N I / ~/ v ',, B~N4 _~ BQ~ k%_15 ~ ~k "', t /. N /,,,,./ & ', / PS, 9 X'~ -/ -- * 6 ~ ~5 / / / O 55 t /at ~- Fig. I. Fig. 2. Figure i: Part f the ternary phase diagram BaO-TiO2-NbO^ 5 at 1200 C shwing the equilibria between Ba Nb405 ss and BaTiO_ ss with ~e surrunding 5 1 cmpunds. The binary BaO-NbO 25 system is taken frm 13). In the binary system BaO-TiO 2 the phase BT 3 ~as pssibly t be replaced by B6T17 (see 8). Figure 2: Enlarged part f the phase diagram BaO-TiO--NbO22.5 at 1350 C. xxx series f cmpsitins investigated, with 000 fr the purest pervskite phases. VTi and VBa dente the directin f the pervskite phase fr either Ti vacancy r Ba vacancy cmpensatin. The system BaO-TiO2-La203 This system was investigated at 1350 C nly, again by first preparing a series f samples with cnstant dpe cncentratin (8% La) in rder t find the psitin f the line f pervskite slid slutins. On extensin this line pints t the cmpsitin La 4 Ti 30.~. The endpint f the slid slutins line is near 13% La, but nt sharply de~fned, because f a precipitatin f very fine particles, prbably during the cling prcess. These results are indicated n figures 3 and Here, the cnclusins can be drawn, that the La ins replace Ba ins, and that the excess charge is cmpletely cmpensated by Ti vacancies. Anther result is that n line frm BaTiO 3 t La 4 Ti 3012 a new cmpund was detected, Ba La 4 Ti 4015, which shwed a pwder X-ray diagram very simular t that f the cmpund ~a 5 Nb. O15. Finally a series f samples cntaining 8% La was fired at 1350 C in a gas mixture cnsisting f 95% CO 2 and 5% H 2.

4 348 G.H. JONKER, et al. Vl. 17, N. 3 B L /~L2TL2T = ~ //' ~/,.~,~_ '~, T = TiO 2-2 L = LEO1, 5 /, ~- 50 / BLaT4~/ /~, :L2T3" / / 40 at % L ///// ~ ~ ~ i / 0 ',/ L4T9 /--at % L / / /, iu. -' / / BT 10 / / SS./ BT 3 BT 4 / ~'/B 2T 6~ 0 I I t I [ r'~o T B~ (/45.C at % B if 5O ~k /, ~ L4T3 / /' L 2T / I L2T3, / b Fig. 3. Fig i 6O t%t --8%L 65 ~T Figure 3: Part f the ternary phase diagram BaO-TiO~-LaO 1 5 at 1350 C with BaTiO--I--~ s-~, Ba La A TiAO~= and LaA Ti~O~ n the V. li~e. The binary LaO, ~- e ±a ~ a iz Ti I.D TiO 2 system after 4) and 12). Figure 4: Enlarged part f figure 3 shwing the series f cmpsitins investigated, and the pure pervskite cmpsitins i i i O 25 Figure 5: Variatin f the pervskite cell parameters as functin f the varius substitutins. Results fr Ta after 5) and fr Sb after 9). The dark blue, electrically cnducting samples shwed their highest phase purity at cmpsitin C f figure 4, situated n the line frm BaTiO 3 t the reduced cmpund La Ti This means that under these highly reducing circumstances the excess charge f the La 3 ins is cmpletely cmpensated by.3+ Tl ins. This result at lw xygen pressure crrespnds t thse f Daniels and H&rdtl - and thse f Hennings 2). The variatin f the cubic cell parameters is shwn in figure 5. At present we have n explanatin fr the fact that linear extraplatin t pure BaTiO_ in this case differs frm the J linear extraplatin f thenb, Ta and Sb lines. Discussin The investigatins cnfirm in a simple way the knwn r expected sites f the substituted ins in BaTiO 3. The large trivalent La ins ccupy Ba sites and the small pentavalent Nb and Sb ins ccupy Ti sites. The analgy between the Nb and the Sb system leads thus t a descriptin f the Sb substitutin that differs frm that f Ebersp~cher 9).

5 Vl. 17, N. 3 BARIUM TITANATE 349 In the case f La substitutin the excess charge is cmpletely cmpensated by Ti vacancies. In the case f Nband prbably als in the case f Sb substitutin, hwever, a cmplete cmpensatin by Ti vacancies ccurs nly at lwer temperatures. At higher temperatures a mixed cmpensatin by Ti and Ba vacancies is fund. This easy variatin f the rati f Ba and Ti vacancies pints t a rather small value f the difference between the enthalpy f frmatin f a single Ti vacancy and that f tw Ba vacancies. At first sight ne wuld expect the same vacancy cmpensatin mechanism bth fr La and fr Nb dped BaTiO 3 as the enthalpy difference mentined abve wuld be independent f the type f dpe. Hwever, in view f the high dpe cncentratins3~n ur samples the frmatin f assciated defects is very prbable. Fr a La in n a Ba site the shrtest distance t a pssible ~i vacancy (½a /3) is smaller than that t a pssible Ba vacancy (a). Fr a Nb + in n a Ti site the ppsite hlds. Fr simple electrstatic reasns, therefre, ne expects a strnger preference fr Ti vacancies in the case f La dpe than in the case f Nb dpe. This is in accrdance with the bserved behaviur. Daniels and H~rdtl I) assumed that in the system BaTiO3-La cmpensatin wuld take place by vacancies n the Ba sites, and calculated fr that case frm their experiments a value f 7.7 ev fr the enthalpy f frmatin f a Ba vacancy. In view f the present results n this system the cmpensatin takes place by Ti vacancies. A cmparable calculatin f the enthalpy f frmatin f ne Ti vacancy with the help f their experimental data results in 12.4 ev. Crystallgraphy The trignal structure f Ba 5Nb 4 O15 is ismrphus with the structure f Ba 5 Ta which has been described in detail in I0) and 11). In principle this structure can be described as a sequence ABC AB f 5 BaO 3 layers perpendicular t the trignal axis. This resembles the structure f BaTiO 3 viewed in the directin f the cube diagnal, which shws a repetitin f an AB C sequence. Between the BaO 3 layers the Nb 5+ r Ti 4+ ins are situated in crner-sharing xygen ctahedra. In the Ba 5 Nb structure the cubic sequence is interrupted after each five layers, as there, lcally, a hexagnal sequence AB AB ccurs, cntaining nn-ccupied duble face-sharing xygen ctahedra. Therefre, this structure can be cnsidered as a typical rdered structure with respect t the vacancies. A cmparable rder will be present in the ismrphus structure f Ba La 4 Ti. O15. Frm simple electrstatic cnsideratins ne expects that the Ba ins wil~ be cncentrated in the center f the cubic sequence and the La ins in the hexagnal sequence arund the vacancy. Hwever, the X-ray diagrams give n infrmatin abut rder f Ba and La ins as the scattering pwer f bth is practically equal. It is interesting that Ba 5 Nb 4 O15 frms a lng series f slid slutins with BaTiO 3, whereas Ba La 4 Ti401& frms n slid slutins r nly in a very restricted way. This again can be the result f a strnger stability f Ti vacancies between La neighburs than f Nb vacancies between Nb neighburs. Table i gives the cell parameters f a number f the trignal structures, cmpared with the cell dimensins f BaTiO 3. Recently, a new cmpund La 4 Ti 3 O12 is mentined in literature 12). The structure f this cmpund is described as a 12 layer hexagnal cell, als related t the pervskite structure, with 3/4 f the available xygen ctahedra ccupied by Ti. In ur study f the binary system La203-TiO 2 we have nt yet identified this cmpund.

6 350 G.H. JONKER, et al. Vl. 17, N. 3 Table 1 Cell parameters f trignal cmpunds a (~) c (~) c/a Ba 5Ta 4 O Ba 5Nb Ba 5Nb3, 2 Til, BaLa 4 Ti Cubic BaTiO 3 ax/2 5 am/3 M a = A References i. J. Daniels and K.H. H~rdtl, Philips Res. Repts. 31, 489 (1976). 2. D. Hennings, Philips Res. Repts. 3_~i, 516 (1976). 3. T.Y. Tien and F.A. Hummel, TRans. Brit. Ceram. Sc. 66, 233 (1967). 4. G.H. Jnker, persnal cmmunicatin, quted by J.B. Mac Chesney and H.A. Sauer, J. Amer. Ceram. Sc. 45, 416 (1962). 5. E.C. Subbara and G. Shirane, J. Amer. Ceram. Sc. 42, 279 (1959). 6. D. Hennings and G. Rsenstein, Mat. Res. Bull. 7, 1505 (1972). 7. K. Keizer, Thesis Technical University Twente, The Netherlands (1976). 8. H.M. O'Bryan Jr. and J. Thmsn Jr., J. Amer. Ceram. Sc. 57, 522 (1974). 9. O. Ebersp~cher, Naturwissenschaften 49, 155 (1962). i0. F. Galass and L. Katz, Acta Cryst. 14, 647 (1960). ii. J. Shannn and L. Katz, Acta Cryst. B26, 102 (1970). 12. N.F. Federv, O.V. Melnikva, V.A. Saltikva, Zh. Nerg. Khim. 4, 1166 (1979) and Ceramic Abstracts Amer. Ceram. Sc. 59, 184 (1980). 13. R.S. Rth and J.L. Waring, J. Research N.B.S. 65, 337 (1961).

Belgian Building Research Institute. Lightweight self-compacting concrete with Argex aggregates

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