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The influence of foreign ions on the crystal lattice of barium titanate
Mat. Res. B u l l . , Vol. 17, pp. 345-350, 1982. Printed in the USA. 0025-5408/82/030345-06503.00/0 Copyright (c) 1982 Pergamon Press Ltd.
THE INFLUENCE
OF F O R E I G N IONS O N T H E C R Y S T A L OF BARIUM TITANATE
LATTICE
G. H. 7onker Department of Electrical Engineering Technical University Twente, Enschede and E. E. Havinga Philips Research Laboratories Eindhoven, The Netherlands
(Received December 22, 1981; Communicated by A. Rabenau) ABSTRACT F r o m i n v e s t i g a t i o n s of phase d i a g r a m s of t e r n a r y oxides the lattice sites of foreign ions and c o m p e n s a t i n g v a c a n c i e s are established. Large t r i v a l e n t ions o c c u p y b a r i u m sites and are c o m p l e t e l y c o m p e n s a t e d by t i t a n i u m vacancies. Small p e n t a v a l e n t ions o c c u p y t i t a n i u m sites and are m a i n l y c o m p e n s a t e d by t i t a n i u m vacancies. D u r i n g these i n v e s t i g a t i o n s a new c o m p o u n d was found, Ba La 4 Ti 4 015 i s o m o r p h o u s w i t h Ba 5 Nb 4 O15.
Introduction In the l i t e r a t u r e on P.T.C. t h e r m i s t o r s based on s e m i c o n d u c t i n g BaTiO 3 it is g e n e r a l l y a s s u m e d that the v a r i o u s dopants (Nb, Sb, La) are for one p a r t c o m p e n s a t e d by e l e c t r o n s and for the rest by v a c a n c i e s on the b a r i u m sites of the p e r o v s k i t e lattice I)2) . The choice of these v a c a n c i e s is a n a l o g o u s to the case of SrTiO~ and CaTiO] where it is known that both Sr and Ca can be r e p l a c e d to larQe e x t e n t - b y La or Ti by Nb a c c o m p a n i e d by the c r e a t i o n of Sr or Ca v a c a n c i e s 3)4) . In an older paper, however, it was shown that Ta 5+ and Nb 5+ as s u b s t l t u t e s for T± 4+ are m o s t p r o b a b l y c o m p e n s a t e d by a m l x t u r e of Ba and Ti v a c a n c i e s 5) . F u r t h e r m o r e it is known that in PbTiO3, doped w i t h La, b o t h Pb and Ti v a c a n c i e s can exist with a ratio d e p e n d i n a on the PbO p a r t i a l p r e s s u r e d u r i n • 6)7) ~ ~ ~ g the p r e p a r a t l o n . This means that for every compound the v a c a n c y model needs a separate and careful examination. D u r i n g our i n v e s t i g a t i o n s of the phase d i a g r a m s B a O - T i O 2 - N b ~ O 5 and BaOT i O ~ - L a _ O 3 we found i n d i c a t i o n s p o i n t i n g to the f o r m a t i o n of T1 v a c a n c i e s ratler ~han Ba vacancies. T h e r e f o r e we made a d e t a i l e d study of the p a r t s of these d i a g r a m s around the p e r o v s k i t e phase. A l t h o u g h d i f f e r e n t m e t h o d s have been used, the r e l a t i o n s found in these systems are more or less consistent.
345
346
G.H.
JONKER, et al.
Vol. 17, No. 3
Especially interesting is the fact that in both diagrams isomorphous phases are found on extrapolating the line of perovskite solid solutions, namely Ba 5 Nb 4 015 and a new compound Ba La 4 Ti 4 O15. Experimental methods After several orientational experiments we prepared series of samples with constant dope concentrations (Nb, Sb, La) and varying BaO-TiO 2 ratio around the value expected for the pure perovskite phase. A usual ceramic technique was applied for the preparation, starting from mixtures of oxides or carbonates of the comDosinq metals, milling, prefiring in air at 900 °C during 20 hours, second milling, compressing to pellets, and then, as a last step, firing these pellets in air at high temperature (1250 or 1350 °C) and slowly cooling together with the furnace. All samples prepared in this way were white or slightly coloured (yellow or green shade), so that we assumed that all metal ions had obtained their highest valency states (Ba 2+, Ti 4+, Nb 5+, Sb 5+, La 3+) and no free electrons were present. A first indication of the position of the pure phase was the sharp transition in sintering behaviour. Excess BaO caused a high porosity, excess TiO 2 a high density. The latter samples consisted of rather large crystallites, > I0 ~m. The samples were further investigated by X-ray powder diagrams and by microscopic examination of polished sections. The latter method proved to be o the accurate one, as samples prepared with excess TiO O at 1350 C showed a noncrystallized glassy phase and rounded crystal boundarles, as is also known in the pure BaO-TiO 2 systems. By these methods we could determine the position of the perovskite phase with an accuracy in the BaO-TiO 2 ratio of about I%. The system BaO-TiO2-Nb205 The investigations in this system at 1200 °C formed part of a general study of the phase equilibria between the many compounds. The results of this study are less accurate in the exact position of the series of solid solutions. They are shown in figure i. The most striking point is that the compounds BaTiO 3 and Ba 5 Nb 4 01~ form series of solid solutions from both sides, but with a broad miscibil[£y gap. The width of the solid solution area indicates the inaccuracy of these experiments. Equilibria with the surrounding phases will not be discussed in this paper. The results obtained with samples prepared at 1350 °C are shown in figure 2. In the first place the compositions with the purest perovskite phase are indicated. These form a straight line pointing from Ba TiO 3 in a direction to the composition iBaO-iNbO (not a compound). From a series of compositions along this line we found ~6~ endpoint of the solid solution formation at about 12.5% Nb. This endpoint is not sharp, because a very fine segregation occurred, due to the decreasing solubility during the slow cooling process. The results obtained at this temperature are in agreement with those mentioned in 7). These two investigations show that Nb 5+ can be substituted for Ti 4+ to a high percentage, ~ 2 0 % at 1 2 0 0 ° C a n d % 2 5 % at 1350°C. However, there is an obvious difference in the compensation mechanism showing the formation of o mainly Ti-vacancies at 1200 C and of equal concentratlons of Ti and Ba vacancies at 1350°C. In figure 5 t h e variation of the cubic cell parameters is shown.
Vol,
17, No.
3
BARIUM T I T A N A T E
347
The system BaO-TiO2-Sb205 In this system only one series of compositions was investigated, with 6% Sb, fired at 1350 °C. These compositions correspond to those with 6% Nb in the BaO-TiO2-Nb205 diagram. The purest perovskite phase was found at the same ratio. This means that Sb is taken up in the lattice as sb5 + and with the same vacancy compensation as in the case of Nb. A remarkable point is that in this case the porous samples with excess BaO were yellow, but that the dense samples with excess TiO 2 were greenish.
N
81N 2
BN848~0"/~\
i,,\ B,NI
"2),
B~N4
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ot o/. N ,,
&
/
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k%_15
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ot °toT Fig.
35
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\
',
PS,9X'~ \
40
45
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Fig. 2.
Figure i: Part of the ternary phase diagram BaO-TiO2-NbO^ 5 at 1200 °C showing the equilibria between Ba N b 4 0 5 ss and BaTiO_ ss with ~ e surrounding 5 1 compounds. The binary BaO-NbO 2 5 system is taken from 13). In the binary system BaO-TiO 2 the phase BT 3 ~as possibly to be replaced by B6T17 (see 8). Figure 2: Enlarged part of the phase diagram BaO-TiO--NbO22 .5 at 1350°C. xxx series of compositions investigated, with 000 for the purest perovskite phases. VTi and VBa denote the direction of the perovskite phase for either Ti vacancy or Ba vacancy compensation. The system BaO-TiO2-La203 This system was investigated at 1350 °C only, again by first preparing a series of samples with constant dope concentration (8% La) in order to find the position of the line of perovskite solid solutions. On extension this line points to the composition La 4 Ti 30.~. The endpoint of the solid solutions line is near 13% La, but not sharply de~fned, because of a precipitation of very fine particles, probably during the cooling process. These results are indicated on figures 3 and 4. 3+ 2+ Here, the conclusions can be drawn, that the La ions replace Ba ions, and that the excess charge is completely compensated by Ti vacancies. Another result is that on line from BaTiO 3 to La 4 Ti 3 0 1 2 a new compound was detected, Ba La 4 Ti 4015, which showed a powder X-ray diagram very simular to that of the compound ~ a 5 Nb. O15. o Finally a series of samples containing 8% La was fired at 1350 C in a gas mixture consisting of 95% CO 2 and 5% H 2.
348
G.H.
JONKER, et al.
Vol. 17, No. 3
L
~k
/~L2TL2T
~oo
=
/ \,~L4T3
//' ~/,.~,~_ '~o,
T = TiO 2 L = LEO1,5
/
-2 , ~- 50 BLaT4~/ / ~ , :L2T3" / 40
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BT
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~'/B 2T6~0
\
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50
t
40
30
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20
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BT3 BT4
I
.C
\
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10
if or'~OT
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(/45
515
5O
ot%T
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Fig.
3.
Fig.
i 6O
~T 65
4.
Figure 3: Part of the t e r n a r y phase d i a g r a m B a O - T i O ~ - L a O 1 5 at 1350 °C w i t h BaTiO--I--~s-~, Ba La A TiAO~= and LaA T i ~ O ~ on the V . li~e. The b i n a r y LaO, ~e ±a ~ a iz Ti I.D T i O 2 s y s t e m after 4) and 12). F i g u r e 4: E n l a r g e d p a r t of figure 3 s h o w i n g the series of c o m p o s i t i o n s investigated, and the pure p e r o v s k i t e compositions. The d a r k blue, e l e c t r i c a l l y c o n d u c t i n g samples showed their h i g h e s t phase p u r i t y at c o m p o s i t i o n C of figure 4, situated on the line from B a T i O 3 to the r e d u c e d c o m p o u n d La Ti 3+ 03 . This m e a n s that under these h i g h l y r e d u c i n g c i r c u m s t a n c e s the excess charge of the La 3 ÷ ions is c o m p l e t e l y c o m p e n s a t e d by .3+ Tl ions. This r e s u l t at low o x y g e n p r e s s u r e c o r r e s p o n d s to those of D a n i e l s and H&rdtl and those of H e n n i n g s 2) . The v a r i a t i o n of the cubic cell p a r a m e t e r s is shown in figure 5. At p r e s e n t we have no e x p l a n a t i o n for the fact that linear e x t r a p o l a t i o n to pure BaTiO_ in this case differs from the J linear e x t r a p o l a t i o n of t h e N b , Ta and Sb lines.
4.04 -
4.03 -
402-
4.01
-
4.00 -
Discussion
3.99 0
5
i 10
15
i 25i 2O
F i g u r e 5: V a r i a t i o n of the p e r o v s k i t e cell p a r a m e t e r s as f u n c t i o n of the v a r i o u s substitutions. R e s u l t s for Ta after 5) and for Sb after 9).
The i n v e s t i g a t i o n s c o n f i r m in a simple way the known or e x p e c t e d sites of the s u b s t i t u t e d ions in B a T i O 3. The large t r i v a l e n t La ions o c c u p y Ba sites and the small p e n t a v a l e n t Nb and Sb ions o c c u p y Ti sites. The a n a l o g y b e t w e e n the Nb and the Sb system leads thus to a d e s c r i p t i o n of the Sb s u b s t i t u t i o n that d i f f e r s from that of E b e r s p ~ c h e r 9) .
Vol. 17, No. 3
BARIUM TITANATE
349
In the case of La s u b s t i t u t i o n the excess charge is c o m p l e t e l y c o m p e n s a t e d by Ti vacancies. In the case of N b a n d p r o b a b l y also in the case of Sb substitution, however, a c o m p l e t e c o m p e n s a t i o n by Ti v a c a n c i e s occurs only at lower temperatures. At h i g h e r t e m p e r a t u r e s a m i x e d c o m p e n s a t i o n by Ti and Ba v a c a n c i e s is found. This easy v a r i a t i o n of the ratio of Ba and Ti v a c a n c i e s points to a rather small value of the d i f f e r e n c e b e t w e e n the e n t h a l p y of f o r m a t i o n of a single Ti v a c a n c y and that of two Ba vacancies. At first sight one w o u l d expect the same v a c a n c y c o m p e n s a t i o n m e c h a n i s m both for La and for Nb doped B a T i O 3 as the e n t h a l p y d i f f e r e n c e m e n t i o n e d above would be i n d e p e n d e n t of the type of dope. However, in v i e w of the high dope concent r a t i o n s 3 ~ n our samples the f o r m a t i o n of a s s o c i a t e d defects is v e r y probable. For a La ion on a Ba site the s h o r t e s t distance to a p o s s i b l e ~i v a c a n c y (½a /3) is smaller than that to a p o s s i b l e Ba v a c a n c y (a). For a Nb + ion on a Ti site the o p p o s i t e holds. For simple e l e c t r o s t a t i c reasons, therefore, one e x p e c t s a stronger p r e f e r e n c e for Ti v a c a n c i e s in the case of La dope than in the case of Nb dope. This is in a c c o r d a n c e w i t h the o b s e r v e d behaviour. D a n i e l s and H~rdtl I) a s s u m e d that in the system B a T i O 3 - L a c o m p e n s a t i o n would take place by v a c a n c i e s on the Ba sites, and c a l c u l a t e d for that case from their e x p e r i m e n t s a value of 7.7 eV for the e n t h a l p y of f o r m a t i o n of a Ba vacancy. In v i e w of the p r e s e n t r e s u l t s on this system the c o m p e n s a t i o n takes place by Ti vacancies. A c o m p a r a b l e c a l c u l a t i o n of the e n t h a l p y of f o r m a t i o n of one Ti v a c a n c y w i t h the help of their e x p e r i m e n t a l data results in 12.4 eV. Crystallography The trigonal structure of Ba 5 N b 4 O15 is isomorphous w i t h the structure of Ba 5 Ta 4 015 w h i c h has been d e s c r i b e d in detail in I0) and 11). In p r i n c i p l e this structure can be d e s c r i b e d as a sequence A B C A B of 5 BaO 3 layers p e r p e n d i c u l a r to the trigonal axis. This r e s e m b l e s the structure of B a T i O 3 v i e w e d in the d i r e c t i o n of the cube diagonal, which shows a r e p e t i t i o n of an A B C sequence. B e t w e e n the BaO 3 layers the Nb 5+ or Ti 4+ ions are situated in c o r n e r - s h a r i n g o x y g e n octahedra. In the Ba 5 Nb 4 015 structure the cubic sequence is i n t e r r u p t e d after each five layers, as there, locally, a h e x a g o n a l sequence A B A B occurs, c o n t a i n i n g n o n - o c c u p i e d double f a c e - s h a r i n g oxygen octahedra. Therefore, this structure can be c o n s i d e r e d as a typical o r d e r e d s t r u c t u r e w i t h r e s p e c t to the vacancies. A c o m p a r a b l e order will be p r e s e n t in the i s o m o r p h o u s structure of Ba La 4 Ti. O15. F r o m simple e l e c t r o s t a t i c c o n s i d e r a t i o n s one e x p e c t s that the Ba ions wil~ be c o n c e n t r a t e d in the center of the cubic sequence and the La ions in the h e x a g o n a l sequence around the vacancy. However, the X-ray d i a g r a m s give no i n f o r m a t i o n about order of Ba and La ions as the s c a t t e r i n g power of b o t h is p r a c t i c a l l y equal. It is i n t e r e s t i n g that Ba 5 Nb 4 O15 forms a long series of solid solutions with B a T i O 3, w h e r e a s Ba La 4 T i 4 0 1 & forms no solid solutions or only in a very r e s t r i c t e d way. This again can b e the r e s u l t of a stronger s t a b i l i t y of Ti v a c a n c i e s b e t w e e n La n e i g h b o u r s than of Nb v a c a n c i e s b e t w e e n Nb neighbours. Table i gives the cell p a r a m e t e r s of a number of the trigonal structures, c o m p a r e d w i t h the cell d i m e n s i o n s of B a T i O 3. Recently, a new compound La 4 Ti 3 O12 is m e n t i o n e d in l i t e r a t u r e 12) . The s t r u c t u r e of this c o m p o u n d is d e s c r i b e d as a 12 layer h e x a g o n a l cell, also r e l a t e d to the p e r o v s k i t e structure, w i t h 3/4 of the a v a i l a b l e o x y g e n o c t a h e d r a o c c u p i e d by Ti. In our study of the b i n a r y system L a 2 0 3 - T i O 2 we have not yet i d e n t i f i e d this compound.
350
G.H.
JONKER, et al.
Vol. 17, No. 3
Table 1 Cell parameters of trigonal compounds a (~)
c (~)
c/a
Ba 5 T a 4 O15
5.79
11.75
2.02
Ba 5 N b 4 015
5.80
11.80
2.03
Ba 5Nb3, 2 Til, 0 0 1 5
5.77
11.55
2.002
B a L a 4 Ti 4 0 1 5
5.57
11.22
2.02
Cubic BaTiO 3
aX/2
a
M
o
= 4.007 A
5 aM/3
5.66
11.59
2.04
References i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13.
J. Daniels and K.H. H~rdtl, Philips Res. Repts. 31, 489 (1976). D. Hennings, Philips Res. Repts. 3_~i, 516 (1976). T.Y. Tien and F.A. Hummel, TRans. Brit. Ceram. Soc. 66, 233 (1967). G.H. Jonker, personal communication, quoted by J.B. Mac Chesney and H.A. Sauer, J. Amer. Ceram. Soc. 45, 416 (1962). E.C. Subbarao and G. Shirane, J. Amer. Ceram. Soc. 42, 279 (1959). D. Hennings and G. Rosenstein, Mat. Res. Bull. 7, 1505 (1972). K. Keizer, Thesis Technical University Twente, The Netherlands (1976). H.M. O'Bryan Jr. and J. Thomson Jr., J. Amer. Ceram. Soc. 57, 522 (1974). O. Ebersp~cher, Naturwissenschaften 49, 155 (1962). F. Galasso and L. Katz, Acta Cryst. 14, 647 (1960). J. Shannon and L. Katz, Acta Cryst. B26, 102 (1970). N.F. Federov, O.V. Melnikova, V.A. Saltikova, Zh. Neorg. Khim. 4, 1166 (1979) and Ceramic Abstracts Amer. Ceram. Soc. 59, 184 (1980). R.S. Roth and J.L. Waring, J. Research N.B.S. 65, 337 (1961).
THE INFLUENCE
OF F O R E I G N IONS O N T H E C R Y S T A L OF BARIUM TITANATE
LATTICE
G. H. 7onker Department of Electrical Engineering Technical University Twente, Enschede and E. E. Havinga Philips Research Laboratories Eindhoven, The Netherlands
(Received December 22, 1981; Communicated by A. Rabenau) ABSTRACT F r o m i n v e s t i g a t i o n s of phase d i a g r a m s of t e r n a r y oxides the lattice sites of foreign ions and c o m p e n s a t i n g v a c a n c i e s are established. Large t r i v a l e n t ions o c c u p y b a r i u m sites and are c o m p l e t e l y c o m p e n s a t e d by t i t a n i u m vacancies. Small p e n t a v a l e n t ions o c c u p y t i t a n i u m sites and are m a i n l y c o m p e n s a t e d by t i t a n i u m vacancies. D u r i n g these i n v e s t i g a t i o n s a new c o m p o u n d was found, Ba La 4 Ti 4 015 i s o m o r p h o u s w i t h Ba 5 Nb 4 O15.
Introduction In the l i t e r a t u r e on P.T.C. t h e r m i s t o r s based on s e m i c o n d u c t i n g BaTiO 3 it is g e n e r a l l y a s s u m e d that the v a r i o u s dopants (Nb, Sb, La) are for one p a r t c o m p e n s a t e d by e l e c t r o n s and for the rest by v a c a n c i e s on the b a r i u m sites of the p e r o v s k i t e lattice I)2) . The choice of these v a c a n c i e s is a n a l o g o u s to the case of SrTiO~ and CaTiO] where it is known that both Sr and Ca can be r e p l a c e d to larQe e x t e n t - b y La or Ti by Nb a c c o m p a n i e d by the c r e a t i o n of Sr or Ca v a c a n c i e s 3)4) . In an older paper, however, it was shown that Ta 5+ and Nb 5+ as s u b s t l t u t e s for T± 4+ are m o s t p r o b a b l y c o m p e n s a t e d by a m l x t u r e of Ba and Ti v a c a n c i e s 5) . F u r t h e r m o r e it is known that in PbTiO3, doped w i t h La, b o t h Pb and Ti v a c a n c i e s can exist with a ratio d e p e n d i n a on the PbO p a r t i a l p r e s s u r e d u r i n • 6)7) ~ ~ ~ g the p r e p a r a t l o n . This means that for every compound the v a c a n c y model needs a separate and careful examination. D u r i n g our i n v e s t i g a t i o n s of the phase d i a g r a m s B a O - T i O 2 - N b ~ O 5 and BaOT i O ~ - L a _ O 3 we found i n d i c a t i o n s p o i n t i n g to the f o r m a t i o n of T1 v a c a n c i e s ratler ~han Ba vacancies. T h e r e f o r e we made a d e t a i l e d study of the p a r t s of these d i a g r a m s around the p e r o v s k i t e phase. A l t h o u g h d i f f e r e n t m e t h o d s have been used, the r e l a t i o n s found in these systems are more or less consistent.
345
346
G.H.
JONKER, et al.
Vol. 17, No. 3
Especially interesting is the fact that in both diagrams isomorphous phases are found on extrapolating the line of perovskite solid solutions, namely Ba 5 Nb 4 015 and a new compound Ba La 4 Ti 4 O15. Experimental methods After several orientational experiments we prepared series of samples with constant dope concentrations (Nb, Sb, La) and varying BaO-TiO 2 ratio around the value expected for the pure perovskite phase. A usual ceramic technique was applied for the preparation, starting from mixtures of oxides or carbonates of the comDosinq metals, milling, prefiring in air at 900 °C during 20 hours, second milling, compressing to pellets, and then, as a last step, firing these pellets in air at high temperature (1250 or 1350 °C) and slowly cooling together with the furnace. All samples prepared in this way were white or slightly coloured (yellow or green shade), so that we assumed that all metal ions had obtained their highest valency states (Ba 2+, Ti 4+, Nb 5+, Sb 5+, La 3+) and no free electrons were present. A first indication of the position of the pure phase was the sharp transition in sintering behaviour. Excess BaO caused a high porosity, excess TiO 2 a high density. The latter samples consisted of rather large crystallites, > I0 ~m. The samples were further investigated by X-ray powder diagrams and by microscopic examination of polished sections. The latter method proved to be o the accurate one, as samples prepared with excess TiO O at 1350 C showed a noncrystallized glassy phase and rounded crystal boundarles, as is also known in the pure BaO-TiO 2 systems. By these methods we could determine the position of the perovskite phase with an accuracy in the BaO-TiO 2 ratio of about I%. The system BaO-TiO2-Nb205 The investigations in this system at 1200 °C formed part of a general study of the phase equilibria between the many compounds. The results of this study are less accurate in the exact position of the series of solid solutions. They are shown in figure i. The most striking point is that the compounds BaTiO 3 and Ba 5 Nb 4 01~ form series of solid solutions from both sides, but with a broad miscibil[£y gap. The width of the solid solution area indicates the inaccuracy of these experiments. Equilibria with the surrounding phases will not be discussed in this paper. The results obtained with samples prepared at 1350 °C are shown in figure 2. In the first place the compositions with the purest perovskite phase are indicated. These form a straight line pointing from Ba TiO 3 in a direction to the composition iBaO-iNbO (not a compound). From a series of compositions along this line we found ~6~ endpoint of the solid solution formation at about 12.5% Nb. This endpoint is not sharp, because a very fine segregation occurred, due to the decreasing solubility during the slow cooling process. The results obtained at this temperature are in agreement with those mentioned in 7). These two investigations show that Nb 5+ can be substituted for Ti 4+ to a high percentage, ~ 2 0 % at 1 2 0 0 ° C a n d % 2 5 % at 1350°C. However, there is an obvious difference in the compensation mechanism showing the formation of o mainly Ti-vacancies at 1200 C and of equal concentratlons of Ti and Ba vacancies at 1350°C. In figure 5 t h e variation of the cubic cell parameters is shown.
Vol,
17, No.
3
BARIUM T I T A N A T E
347
The system BaO-TiO2-Sb205 In this system only one series of compositions was investigated, with 6% Sb, fired at 1350 °C. These compositions correspond to those with 6% Nb in the BaO-TiO2-Nb205 diagram. The purest perovskite phase was found at the same ratio. This means that Sb is taken up in the lattice as sb5 + and with the same vacancy compensation as in the case of Nb. A remarkable point is that in this case the porous samples with excess BaO were yellow, but that the dense samples with excess TiO 2 were greenish.
N
81N 2
BN848~0"/~\
i,,\ B,NI
"2),
B~N4
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/
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ot o/. N ,,
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35
I.
\
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PS,9X'~ \
40
45
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\
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Fig. 2.
Figure i: Part of the ternary phase diagram BaO-TiO2-NbO^ 5 at 1200 °C showing the equilibria between Ba N b 4 0 5 ss and BaTiO_ ss with ~ e surrounding 5 1 compounds. The binary BaO-NbO 2 5 system is taken from 13). In the binary system BaO-TiO 2 the phase BT 3 ~as possibly to be replaced by B6T17 (see 8). Figure 2: Enlarged part of the phase diagram BaO-TiO--NbO22 .5 at 1350°C. xxx series of compositions investigated, with 000 for the purest perovskite phases. VTi and VBa denote the direction of the perovskite phase for either Ti vacancy or Ba vacancy compensation. The system BaO-TiO2-La203 This system was investigated at 1350 °C only, again by first preparing a series of samples with constant dope concentration (8% La) in order to find the position of the line of perovskite solid solutions. On extension this line points to the composition La 4 Ti 30.~. The endpoint of the solid solutions line is near 13% La, but not sharply de~fned, because of a precipitation of very fine particles, probably during the cooling process. These results are indicated on figures 3 and 4. 3+ 2+ Here, the conclusions can be drawn, that the La ions replace Ba ions, and that the excess charge is completely compensated by Ti vacancies. Another result is that on line from BaTiO 3 to La 4 Ti 3 0 1 2 a new compound was detected, Ba La 4 Ti 4015, which showed a powder X-ray diagram very simular to that of the compound ~ a 5 Nb. O15. o Finally a series of samples containing 8% La was fired at 1350 C in a gas mixture consisting of 95% CO 2 and 5% H 2.
348
G.H.
JONKER, et al.
Vol. 17, No. 3
L
~k
/~L2TL2T
~oo
=
/ \,~L4T3
//' ~/,.~,~_ '~o,
T = TiO 2 L = LEO1,5
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t
40
30
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20
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I
.C
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if or'~OT
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Fig.
3.
Fig.
i 6O
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4.
Figure 3: Part of the t e r n a r y phase d i a g r a m B a O - T i O ~ - L a O 1 5 at 1350 °C w i t h BaTiO--I--~s-~, Ba La A TiAO~= and LaA T i ~ O ~ on the V . li~e. The b i n a r y LaO, ~e ±a ~ a iz Ti I.D T i O 2 s y s t e m after 4) and 12). F i g u r e 4: E n l a r g e d p a r t of figure 3 s h o w i n g the series of c o m p o s i t i o n s investigated, and the pure p e r o v s k i t e compositions. The d a r k blue, e l e c t r i c a l l y c o n d u c t i n g samples showed their h i g h e s t phase p u r i t y at c o m p o s i t i o n C of figure 4, situated on the line from B a T i O 3 to the r e d u c e d c o m p o u n d La Ti 3+ 03 . This m e a n s that under these h i g h l y r e d u c i n g c i r c u m s t a n c e s the excess charge of the La 3 ÷ ions is c o m p l e t e l y c o m p e n s a t e d by .3+ Tl ions. This r e s u l t at low o x y g e n p r e s s u r e c o r r e s p o n d s to those of D a n i e l s and H&rdtl and those of H e n n i n g s 2) . The v a r i a t i o n of the cubic cell p a r a m e t e r s is shown in figure 5. At p r e s e n t we have no e x p l a n a t i o n for the fact that linear e x t r a p o l a t i o n to pure BaTiO_ in this case differs from the J linear e x t r a p o l a t i o n of t h e N b , Ta and Sb lines.
4.04 -
4.03 -
402-
4.01
-
4.00 -
Discussion
3.99 0
5
i 10
15
i 25i 2O
F i g u r e 5: V a r i a t i o n of the p e r o v s k i t e cell p a r a m e t e r s as f u n c t i o n of the v a r i o u s substitutions. R e s u l t s for Ta after 5) and for Sb after 9).
The i n v e s t i g a t i o n s c o n f i r m in a simple way the known or e x p e c t e d sites of the s u b s t i t u t e d ions in B a T i O 3. The large t r i v a l e n t La ions o c c u p y Ba sites and the small p e n t a v a l e n t Nb and Sb ions o c c u p y Ti sites. The a n a l o g y b e t w e e n the Nb and the Sb system leads thus to a d e s c r i p t i o n of the Sb s u b s t i t u t i o n that d i f f e r s from that of E b e r s p ~ c h e r 9) .
Vol. 17, No. 3
BARIUM TITANATE
349
In the case of La s u b s t i t u t i o n the excess charge is c o m p l e t e l y c o m p e n s a t e d by Ti vacancies. In the case of N b a n d p r o b a b l y also in the case of Sb substitution, however, a c o m p l e t e c o m p e n s a t i o n by Ti v a c a n c i e s occurs only at lower temperatures. At h i g h e r t e m p e r a t u r e s a m i x e d c o m p e n s a t i o n by Ti and Ba v a c a n c i e s is found. This easy v a r i a t i o n of the ratio of Ba and Ti v a c a n c i e s points to a rather small value of the d i f f e r e n c e b e t w e e n the e n t h a l p y of f o r m a t i o n of a single Ti v a c a n c y and that of two Ba vacancies. At first sight one w o u l d expect the same v a c a n c y c o m p e n s a t i o n m e c h a n i s m both for La and for Nb doped B a T i O 3 as the e n t h a l p y d i f f e r e n c e m e n t i o n e d above would be i n d e p e n d e n t of the type of dope. However, in v i e w of the high dope concent r a t i o n s 3 ~ n our samples the f o r m a t i o n of a s s o c i a t e d defects is v e r y probable. For a La ion on a Ba site the s h o r t e s t distance to a p o s s i b l e ~i v a c a n c y (½a /3) is smaller than that to a p o s s i b l e Ba v a c a n c y (a). For a Nb + ion on a Ti site the o p p o s i t e holds. For simple e l e c t r o s t a t i c reasons, therefore, one e x p e c t s a stronger p r e f e r e n c e for Ti v a c a n c i e s in the case of La dope than in the case of Nb dope. This is in a c c o r d a n c e w i t h the o b s e r v e d behaviour. D a n i e l s and H~rdtl I) a s s u m e d that in the system B a T i O 3 - L a c o m p e n s a t i o n would take place by v a c a n c i e s on the Ba sites, and c a l c u l a t e d for that case from their e x p e r i m e n t s a value of 7.7 eV for the e n t h a l p y of f o r m a t i o n of a Ba vacancy. In v i e w of the p r e s e n t r e s u l t s on this system the c o m p e n s a t i o n takes place by Ti vacancies. A c o m p a r a b l e c a l c u l a t i o n of the e n t h a l p y of f o r m a t i o n of one Ti v a c a n c y w i t h the help of their e x p e r i m e n t a l data results in 12.4 eV. Crystallography The trigonal structure of Ba 5 N b 4 O15 is isomorphous w i t h the structure of Ba 5 Ta 4 015 w h i c h has been d e s c r i b e d in detail in I0) and 11). In p r i n c i p l e this structure can be d e s c r i b e d as a sequence A B C A B of 5 BaO 3 layers p e r p e n d i c u l a r to the trigonal axis. This r e s e m b l e s the structure of B a T i O 3 v i e w e d in the d i r e c t i o n of the cube diagonal, which shows a r e p e t i t i o n of an A B C sequence. B e t w e e n the BaO 3 layers the Nb 5+ or Ti 4+ ions are situated in c o r n e r - s h a r i n g o x y g e n octahedra. In the Ba 5 Nb 4 015 structure the cubic sequence is i n t e r r u p t e d after each five layers, as there, locally, a h e x a g o n a l sequence A B A B occurs, c o n t a i n i n g n o n - o c c u p i e d double f a c e - s h a r i n g oxygen octahedra. Therefore, this structure can be c o n s i d e r e d as a typical o r d e r e d s t r u c t u r e w i t h r e s p e c t to the vacancies. A c o m p a r a b l e order will be p r e s e n t in the i s o m o r p h o u s structure of Ba La 4 Ti. O15. F r o m simple e l e c t r o s t a t i c c o n s i d e r a t i o n s one e x p e c t s that the Ba ions wil~ be c o n c e n t r a t e d in the center of the cubic sequence and the La ions in the h e x a g o n a l sequence around the vacancy. However, the X-ray d i a g r a m s give no i n f o r m a t i o n about order of Ba and La ions as the s c a t t e r i n g power of b o t h is p r a c t i c a l l y equal. It is i n t e r e s t i n g that Ba 5 Nb 4 O15 forms a long series of solid solutions with B a T i O 3, w h e r e a s Ba La 4 T i 4 0 1 & forms no solid solutions or only in a very r e s t r i c t e d way. This again can b e the r e s u l t of a stronger s t a b i l i t y of Ti v a c a n c i e s b e t w e e n La n e i g h b o u r s than of Nb v a c a n c i e s b e t w e e n Nb neighbours. Table i gives the cell p a r a m e t e r s of a number of the trigonal structures, c o m p a r e d w i t h the cell d i m e n s i o n s of B a T i O 3. Recently, a new compound La 4 Ti 3 O12 is m e n t i o n e d in l i t e r a t u r e 12) . The s t r u c t u r e of this c o m p o u n d is d e s c r i b e d as a 12 layer h e x a g o n a l cell, also r e l a t e d to the p e r o v s k i t e structure, w i t h 3/4 of the a v a i l a b l e o x y g e n o c t a h e d r a o c c u p i e d by Ti. In our study of the b i n a r y system L a 2 0 3 - T i O 2 we have not yet i d e n t i f i e d this compound.
350
G.H.
JONKER, et al.
Vol. 17, No. 3
Table 1 Cell parameters of trigonal compounds a (~)
c (~)
c/a
Ba 5 T a 4 O15
5.79
11.75
2.02
Ba 5 N b 4 015
5.80
11.80
2.03
Ba 5Nb3, 2 Til, 0 0 1 5
5.77
11.55
2.002
B a L a 4 Ti 4 0 1 5
5.57
11.22
2.02
Cubic BaTiO 3
aX/2
a
M
o
= 4.007 A
5 aM/3
5.66
11.59
2.04
References i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13.
J. Daniels and K.H. H~rdtl, Philips Res. Repts. 31, 489 (1976). D. Hennings, Philips Res. Repts. 3_~i, 516 (1976). T.Y. Tien and F.A. Hummel, TRans. Brit. Ceram. Soc. 66, 233 (1967). G.H. Jonker, personal communication, quoted by J.B. Mac Chesney and H.A. Sauer, J. Amer. Ceram. Soc. 45, 416 (1962). E.C. Subbarao and G. Shirane, J. Amer. Ceram. Soc. 42, 279 (1959). D. Hennings and G. Rosenstein, Mat. Res. Bull. 7, 1505 (1972). K. Keizer, Thesis Technical University Twente, The Netherlands (1976). H.M. O'Bryan Jr. and J. Thomson Jr., J. Amer. Ceram. Soc. 57, 522 (1974). O. Ebersp~cher, Naturwissenschaften 49, 155 (1962). F. Galasso and L. Katz, Acta Cryst. 14, 647 (1960). J. Shannon and L. Katz, Acta Cryst. B26, 102 (1970). N.F. Federov, O.V. Melnikova, V.A. Saltikova, Zh. Neorg. Khim. 4, 1166 (1979) and Ceramic Abstracts Amer. Ceram. Soc. 59, 184 (1980). R.S. Roth and J.L. Waring, J. Research N.B.S. 65, 337 (1961).