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Characterization of strontium titanate ceramics by infrared reflectivity spectroscopy and electron paramagnetic resonance
Mat. R e s . B u l l . , Vol. 22, p p . 1629-1633, 1987. P r i n t e d in t h e USA. 0025--5408/87 $3.00 + .00 C o p y r i g h t (c0 1987 Pergamon J o u r n a l s Ltd.
C H A R A C T E R I Z A T I O N OF S T R O N T I U M T I T A N A T E C E R A M I C S BY I N F R A R E D R E F L E C T I V l T Y S P E C T R O S C O P Y AND E L E C T R O N P A R A M A G N E T I C
Centre
RESONANCE
F. Gervais, B. Cal~s and P. Odier de R e c h e r e h e s sur la Physique des Hautes T e m p e r a t u r e s C.N.R.S., 4507] Orl~ans, France.
( R e c e i v e d J u n e 22, 1987; Communicated b y E . F .
Bertaut)
ABSTRACT Undoped strontium titanate ceramics prepared by organic e o m p l e x a t i o n route exhibits a sharp p a r a m a g n e t i c line near g = 2 the intensity of which varies r a p i d l y with the Sr/Ti ratio. This p a r a m a g n e t i c center v a n i s h e s in the p r e s e n c e of free c a r r i e r s i n t r o d u c e d either by d o p i n g w i t h Nb 9+ or reduction under CO/CO 2 or Ar/H 2 mixtures at high temperature. High carrier c o n c e n t r a t i o n s may be deduced from the a n a l y s i s of infrared r e f l e c t i o n s p e c t r a that display l o n g i t u d i n a l optical p h o n o n - p l a s m o n c o u p l i n g behavior. This natural p a r a m a g n e t i c center is i n t e r p r e t e d as a hole t r a p p e d on a t i t a n i u m site. Its study allows accurate control of one of the main factors which g o v e r n the e l e c t r i c a l c o n d u c t i v i t y of the ceramics, i.e. the Sr/Ti ratio w h i c h is o t h e r w i s e d i f f i c u l t to control. MATERIALS
INDEX
: perovskites,
oxides,
strontium,
titanium
i. I n t r o d u c t i o n Strontium titanate is a p r o t o t y p e of ceramics with the p e r o v s k i t e structure. It is s u f f i c i e n t l y simple to allow m o d e l i n g of e l e c t r o n i c p r o p e r t i e s of p e r o v s k i t e ceramics and is itself c a n d i d a t e for technological applications, possibly as a capacitor owing to its incipient f e r r o e l e c t r i c properties. The e l e c t r i c a l p r o p e r t i e s are known to be very s e n s i t i v e to the lattice defects c r e a t e d in p a r t i c u ] a r by any deviation from unity of the ratio of strontium and titanium concentrations. For example, under c e r t a i n conditions, a change of this ratio by only 0.1% may change the e l e c t r i c a l c o n d u c t i v i t y by a factor as large as i0 (i). W i t h i n this context, a c o r r e l a t i o n b e t w e e n this ratio and the intensity of the EPR line of an "intrinsic" p a r a m a g n e t i c center is r e p o r t e d in this paper. The existence of this c o r r e l a t i o n opens the way to a m e t h o d of control of the Sr/Ti ratio - what is o t h e r w i s e difficult to estimate accurately and thus allows a better c h a r a c t e r i z a t i o n of s t r o n t i u m titanate ceramics.
1629
1630
F.
GERVAIS,
et al.
Vol.
22,
No.
2. Sample preparation The synthesis of ultra-fine-grain and homogeneous powders of strontium titanate is performed via an organic complexation route. After formation of an organic complex, obtained by reaction of citric acid with ethylene glycol and titanium IV isopropoxyde, to which anhydrous strontium carbonate has been added, a resin is obtained by polymerization at 150°C in a platinum crucible. Then the powder is calcinated in air at 600°C. After sintering at 1400°C, the density reaches 99.8% of the theoretical density. In those conditions, the Sr/Ti ratio of the final product can be controlled to better than 1%, and any additional dopant as well (2).
3. Results Undoped ceramics of strontium titanate display a narrow EPR line at g = 1.985 (figure i). The linewidth is only 0.5 G at 200 K, and increases roughly linearly with temperature between 200 and 400 K (figure 2). The study of the same paramagnetic center in a single crystal shows that the line is isotropic, which explains that it is equally well observed in ceramics samples. The relaxation time is very long and extremely low microwave powers (typically 1 watt) should be used to prevent the saturation of the line. However, this paramagnetic center is observed only when the sample is free from charge carriers. Charge carriers can be introduced either by doping with Nb^O_ or heating under b reducing atmosphere (CO/CO 2 or Ar/H 2 mixtures) w~ich creates oxygen deficiencies.
Sr/Ti
= 1.01
..~g =1.985
B
50 G
FIG i. Electron paramagnetic resonance spectrum titanate ceramics in the X band.
of
an
undoped
strontium
12
Vol. 22, No. 12
CERAMICS
FIG 2. Temperature dependence of the width of the main isotropie E.P.R. line shown in the spectrum of figure [.
1631
/
A
~2 I i-w z u
/
l-
no_ W
/
J i I
0
I-LIJ
o
J z uJ
J
100 300 TEMPERATURE
l°°°l response, ~,ln~ ,v,
lo
decoupled plasrnon
z_
]
-
< ~E
" 7 / % '. . . . I
1
(K)
5OO
LL
~
~,
--
f
I ~-~
,
0 l
80K >> D
I-(.3 W .J LI_ W r'~
Q5
, SrTi03(0.5°/oNb) t ~z"~ ceramics ~t / 100
300 FREQUENCY
500
700
900
(cm -1)
FIG 3. I n f r a r e d r e f l e c t i o n s p e c t r u m (dots) of a ceramic sample of s t r o n t i u m titanate doped w i t h 0.5 % Nb and best fit (full line) of the f a c t o r i z e d form od the d i e l e c t r i c function to the data. The upper part shows the r e s p o n s e of t r a n s v e r s e (imaginary d i e l e c t r i c function) and l o n g i t u d i n a l optical (inverse d i e l e c t r i c function) modes.
1632
F.
GERVAIS,
e t al.
Vol.
22,
No.
Highly-doped or highly-reduced samples exhibit infrared reflection spectra that are characteristic of LO p h o n o n - p l a s m o n coupling. A typical spectrum and the results of its analysis with the factorized form of the dielectric function is shown in figure 3. The details of the extraction of the p l a s m o n frequency from the experimental d_~ta are explained elsewhere (3). K n o w i n g the plasma frequency of 300 cm which is found in this example, and the effective mass m of the free carriers (3,4), one can deduce the carrier c o n c e n t r a t i o n v i ~ COp
Z
~
= (ne /m e~)
1/2
-3 to be 8 1018 electrons em in the example of Fig. 3. There are two other spectral features that are seen in figure i. First the s e x t u p l e t is u n a m b i g u o u s l y a s s i g n e d to m a n g a n e s e impurities. Comparison of the h y p e r f i n e splitting m e a s u r e d in figure 1 and the results already p u b l i s h e d in (4) and (5) indicates the presgnce of small amount of m a n g a n e s e impurities in the o x y d a t i o n state Mn ~+. The other line seen at g = 2 is assigned to V'~ center by Kutty et al (5). ~r The main EPR line discussed at the beginning of this paragraph, o b s e r v e d only when the sample is free from charge carriers (oxidized samples), is found to vary with the Sr/Ti ratio. When the Sr/Ti ratio is I.O1, the intensity of the EPR line is increased by 2/3 with respect to its intensity when the sample contains equal proportions of strontium and titanium. If the ratio Sr/Ti is lowered to 0.99, then the line intensity is r e d u c e d by 2/3. found
3. D i s c u s s i o n
and
conclusion
The n a r r o w n e s s of the resonance, its p o s i t i o n very near g = 2, its isotropy, the fact that the line vanishes in presence of free carriers and its dependence upon the Sr/Ti ratio - two ions that are diamagnetic - all this tends to indicate that the p a r a m a g n e t i c center likely is an electron or a hole trapped somewhere in the structure. In the cubic perovskite structure, both cationic sites possess the oetahedral symmetry whereas the symmetry of anionic sites is lower. The fact that the observed p a r a m a g n e t i c center is isotropie indicates that the site likely p o s s e s s e s the highest symmetry in the structure, i.e. is a cationic site. Besides, since the intensity of the signal increases with titanium deficiency, we arrive at the c o n c l u s i o n that we observe a hole trapped on a titanium site. The c o m b i n a t i o n of both m e t h o d o l o g i e ~ - EPR and infrared reflectivity spectroscopy combined with an a p p r o p r i a t e analysis of the spectra - allows accurate d e t e r m i n a t i o n of some important c h a r a c t e r i s t i c s of strontium titanate ceramics either near their insulating pole or in the h i g h l y - d o p e d s e m i e o n d u c t i n g regime. References
i.
N.H. Chan, R.K. 1762 (1981).
2.
Y. Dansui,
3.
F. Gervais, J.L. Servoin, A. Baratoff, J. Bednorz, and G. Binnig, Annual M e e t i n g of the G e r m a n Physical Society, MUnster (1982) - and to be published.
Sharma,
B. Cal~s,
and D.M.
Smyth,
and F. Gervais,
J. Electrochem.
J. Physique,
C1 47,
Soc.
871
128,
(1986).
12
Vol. 22, No. 12
4.
CERAMICS
1633
T.R.N. Kutty, L. Gomathi Devi, and P. Murugara, Mat. Res. Bull. 2]__, 1093
(1986).
5.
K.W. Blazey, J.M. Cabrera, 903 (1983).
6.
H.P.E. Frederikse, W.R. Thurber, and W.R. Hosler, Phys. Rev. ]34, A442 (]964).
and K.A. MUller,
Solid St. Commun. 45,
C H A R A C T E R I Z A T I O N OF S T R O N T I U M T I T A N A T E C E R A M I C S BY I N F R A R E D R E F L E C T I V l T Y S P E C T R O S C O P Y AND E L E C T R O N P A R A M A G N E T I C
Centre
RESONANCE
F. Gervais, B. Cal~s and P. Odier de R e c h e r e h e s sur la Physique des Hautes T e m p e r a t u r e s C.N.R.S., 4507] Orl~ans, France.
( R e c e i v e d J u n e 22, 1987; Communicated b y E . F .
Bertaut)
ABSTRACT Undoped strontium titanate ceramics prepared by organic e o m p l e x a t i o n route exhibits a sharp p a r a m a g n e t i c line near g = 2 the intensity of which varies r a p i d l y with the Sr/Ti ratio. This p a r a m a g n e t i c center v a n i s h e s in the p r e s e n c e of free c a r r i e r s i n t r o d u c e d either by d o p i n g w i t h Nb 9+ or reduction under CO/CO 2 or Ar/H 2 mixtures at high temperature. High carrier c o n c e n t r a t i o n s may be deduced from the a n a l y s i s of infrared r e f l e c t i o n s p e c t r a that display l o n g i t u d i n a l optical p h o n o n - p l a s m o n c o u p l i n g behavior. This natural p a r a m a g n e t i c center is i n t e r p r e t e d as a hole t r a p p e d on a t i t a n i u m site. Its study allows accurate control of one of the main factors which g o v e r n the e l e c t r i c a l c o n d u c t i v i t y of the ceramics, i.e. the Sr/Ti ratio w h i c h is o t h e r w i s e d i f f i c u l t to control. MATERIALS
INDEX
: perovskites,
oxides,
strontium,
titanium
i. I n t r o d u c t i o n Strontium titanate is a p r o t o t y p e of ceramics with the p e r o v s k i t e structure. It is s u f f i c i e n t l y simple to allow m o d e l i n g of e l e c t r o n i c p r o p e r t i e s of p e r o v s k i t e ceramics and is itself c a n d i d a t e for technological applications, possibly as a capacitor owing to its incipient f e r r o e l e c t r i c properties. The e l e c t r i c a l p r o p e r t i e s are known to be very s e n s i t i v e to the lattice defects c r e a t e d in p a r t i c u ] a r by any deviation from unity of the ratio of strontium and titanium concentrations. For example, under c e r t a i n conditions, a change of this ratio by only 0.1% may change the e l e c t r i c a l c o n d u c t i v i t y by a factor as large as i0 (i). W i t h i n this context, a c o r r e l a t i o n b e t w e e n this ratio and the intensity of the EPR line of an "intrinsic" p a r a m a g n e t i c center is r e p o r t e d in this paper. The existence of this c o r r e l a t i o n opens the way to a m e t h o d of control of the Sr/Ti ratio - what is o t h e r w i s e difficult to estimate accurately and thus allows a better c h a r a c t e r i z a t i o n of s t r o n t i u m titanate ceramics.
1629
1630
F.
GERVAIS,
et al.
Vol.
22,
No.
2. Sample preparation The synthesis of ultra-fine-grain and homogeneous powders of strontium titanate is performed via an organic complexation route. After formation of an organic complex, obtained by reaction of citric acid with ethylene glycol and titanium IV isopropoxyde, to which anhydrous strontium carbonate has been added, a resin is obtained by polymerization at 150°C in a platinum crucible. Then the powder is calcinated in air at 600°C. After sintering at 1400°C, the density reaches 99.8% of the theoretical density. In those conditions, the Sr/Ti ratio of the final product can be controlled to better than 1%, and any additional dopant as well (2).
3. Results Undoped ceramics of strontium titanate display a narrow EPR line at g = 1.985 (figure i). The linewidth is only 0.5 G at 200 K, and increases roughly linearly with temperature between 200 and 400 K (figure 2). The study of the same paramagnetic center in a single crystal shows that the line is isotropic, which explains that it is equally well observed in ceramics samples. The relaxation time is very long and extremely low microwave powers (typically 1 watt) should be used to prevent the saturation of the line. However, this paramagnetic center is observed only when the sample is free from charge carriers. Charge carriers can be introduced either by doping with Nb^O_ or heating under b reducing atmosphere (CO/CO 2 or Ar/H 2 mixtures) w~ich creates oxygen deficiencies.
Sr/Ti
= 1.01
..~g =1.985
B
50 G
FIG i. Electron paramagnetic resonance spectrum titanate ceramics in the X band.
of
an
undoped
strontium
12
Vol. 22, No. 12
CERAMICS
FIG 2. Temperature dependence of the width of the main isotropie E.P.R. line shown in the spectrum of figure [.
1631
/
A
~2 I i-w z u
/
l-
no_ W
/
J i I
0
I-LIJ
o
J z uJ
J
100 300 TEMPERATURE
l°°°l response, ~,ln~ ,v,
lo
decoupled plasrnon
z_
]
-
< ~E
" 7 / % '. . . . I
1
(K)
5OO
LL
~
~,
--
f
I ~-~
,
0 l
80K >> D
I-(.3 W .J LI_ W r'~
Q5
, SrTi03(0.5°/oNb) t ~z"~ ceramics ~t / 100
300 FREQUENCY
500
700
900
(cm -1)
FIG 3. I n f r a r e d r e f l e c t i o n s p e c t r u m (dots) of a ceramic sample of s t r o n t i u m titanate doped w i t h 0.5 % Nb and best fit (full line) of the f a c t o r i z e d form od the d i e l e c t r i c function to the data. The upper part shows the r e s p o n s e of t r a n s v e r s e (imaginary d i e l e c t r i c function) and l o n g i t u d i n a l optical (inverse d i e l e c t r i c function) modes.
1632
F.
GERVAIS,
e t al.
Vol.
22,
No.
Highly-doped or highly-reduced samples exhibit infrared reflection spectra that are characteristic of LO p h o n o n - p l a s m o n coupling. A typical spectrum and the results of its analysis with the factorized form of the dielectric function is shown in figure 3. The details of the extraction of the p l a s m o n frequency from the experimental d_~ta are explained elsewhere (3). K n o w i n g the plasma frequency of 300 cm which is found in this example, and the effective mass m of the free carriers (3,4), one can deduce the carrier c o n c e n t r a t i o n v i ~ COp
Z
~
= (ne /m e~)
1/2
-3 to be 8 1018 electrons em in the example of Fig. 3. There are two other spectral features that are seen in figure i. First the s e x t u p l e t is u n a m b i g u o u s l y a s s i g n e d to m a n g a n e s e impurities. Comparison of the h y p e r f i n e splitting m e a s u r e d in figure 1 and the results already p u b l i s h e d in (4) and (5) indicates the presgnce of small amount of m a n g a n e s e impurities in the o x y d a t i o n state Mn ~+. The other line seen at g = 2 is assigned to V'~ center by Kutty et al (5). ~r The main EPR line discussed at the beginning of this paragraph, o b s e r v e d only when the sample is free from charge carriers (oxidized samples), is found to vary with the Sr/Ti ratio. When the Sr/Ti ratio is I.O1, the intensity of the EPR line is increased by 2/3 with respect to its intensity when the sample contains equal proportions of strontium and titanium. If the ratio Sr/Ti is lowered to 0.99, then the line intensity is r e d u c e d by 2/3. found
3. D i s c u s s i o n
and
conclusion
The n a r r o w n e s s of the resonance, its p o s i t i o n very near g = 2, its isotropy, the fact that the line vanishes in presence of free carriers and its dependence upon the Sr/Ti ratio - two ions that are diamagnetic - all this tends to indicate that the p a r a m a g n e t i c center likely is an electron or a hole trapped somewhere in the structure. In the cubic perovskite structure, both cationic sites possess the oetahedral symmetry whereas the symmetry of anionic sites is lower. The fact that the observed p a r a m a g n e t i c center is isotropie indicates that the site likely p o s s e s s e s the highest symmetry in the structure, i.e. is a cationic site. Besides, since the intensity of the signal increases with titanium deficiency, we arrive at the c o n c l u s i o n that we observe a hole trapped on a titanium site. The c o m b i n a t i o n of both m e t h o d o l o g i e ~ - EPR and infrared reflectivity spectroscopy combined with an a p p r o p r i a t e analysis of the spectra - allows accurate d e t e r m i n a t i o n of some important c h a r a c t e r i s t i c s of strontium titanate ceramics either near their insulating pole or in the h i g h l y - d o p e d s e m i e o n d u c t i n g regime. References
i.
N.H. Chan, R.K. 1762 (1981).
2.
Y. Dansui,
3.
F. Gervais, J.L. Servoin, A. Baratoff, J. Bednorz, and G. Binnig, Annual M e e t i n g of the G e r m a n Physical Society, MUnster (1982) - and to be published.
Sharma,
B. Cal~s,
and D.M.
Smyth,
and F. Gervais,
J. Electrochem.
J. Physique,
C1 47,
Soc.
871
128,
(1986).
12
Vol. 22, No. 12
4.
CERAMICS
1633
T.R.N. Kutty, L. Gomathi Devi, and P. Murugara, Mat. Res. Bull. 2]__, 1093
(1986).
5.
K.W. Blazey, J.M. Cabrera, 903 (1983).
6.
H.P.E. Frederikse, W.R. Thurber, and W.R. Hosler, Phys. Rev. ]34, A442 (]964).
and K.A. MUller,
Solid St. Commun. 45,