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THE BaCeO3-SrZrO3 SOLID SOLUTION.
CHARACTERIZATION AND ELECTRICAL CHARACTERISTICS
L. Podda, D. Minichelli
To cite this version:
L. Podda, D. Minichelli. THE BaCeO3-SrZrO3 SOLID SOLUTION. CHARACTERIZATION AND
ELECTRICAL CHARACTERISTICS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-813-C1-
817. �10.1051/jphyscol:19861124�. �jpa-00225521�
JOURNAL DE PHYSIQUE
C o l l o g u e C I , s u p p l e m e n t au n ° 2 . Tome 4 7 , f e v r i e r 1 9 8 6 p a g e c i - 8 1 3
THE B a C e 03- S r Z r 03 SOLID SOLUTION. CHARACTERIZATION AND ELECTRICAL CHARACTERISTICS
L . PODDA a n d D . MINICHELLI*
Istituto dl Chlmlca Rppllcata e Industrlale, Unlverslta dl Trieste, Via Valerlo 2, 1-34127 Trieste, Italy
* SRC Snc, Via d e l Burlo 1, 1-34127 Trieste, Italy
Résumé - On a étudié un grand nombre de compositions delà solution solide BaCeO -SrZrO pour déterminer ses paramètres réticulaires et pour la carac- tériser du point de vue électrique.
Abstract - As a p a r t of a study on p e r o v s k i t e - t y p e compounds based m a t e r i a l s s e v e r a l compositions of BaCeO -SrZrCL s o l i d s o l u t i o n nave been considered with t h e aim t o determine t h e c e l l parameters and t h e e l e c t r i c a l behaviour.
I - INTRODUCTION
The interest in the high temperature properties of materials has been increasing in the last te n years. The ceramic oxydes are suitable for high-temperature uses and their technological and structural behaviour is object of complex investigations.
Aiming at evaluating the correlations between technological and structural properti- es of ceramic oxides, it is convenient to investigate those materials which give or- igin to wide solid solutions. In this field the ABO perovskite-type compounds (A =
= Ca, ST, Ba and B = Ti, Zr, Ce) show large solubility fields /1//2//3//4/.
This paper regards a study concerning the SrZrO -BaCeO solid solution and aims to correlate the structural and the electrical properties of some compositions belong- ing to this solid solution.
II - EXPERIMENTAL
CeO , ZrO , BaCO and SrCO of reagent grade purity were used as raw materials. Sam- ples of four different compositions ( Table 1 ) were prepared by weighing the start- ing materials in stoichiometric quantities, followed by mixing and grinding. The re- sulting powders were fired at ?1300°C for 6 hours to decompose the carbonates and pressed into disks at 4000 Kg/cm ; the disks were then fired at 1400°C for 360 hours in an oxidizing atmosphere.
The sintered materials were checked for phase equilibrium by reflected light micro- scopy and X-ray diffraction techniques. Phase equilibrium was considered to have been reached when no further changes were detected in the X-ray diffraction patterns on successively fired samples.
X-ray analysis of air quenched powdered specimens was carried out by a Guinier - de Wolff camera and by a goniometer with proportional counter, using CuK radiation.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861124
c I - 8 1 4 JOURNAL DE PHYSIQUE
Pb (NO 1 was used as internal standard for calibrating the diffraction patterns. fie 3 2
lattice parm.eter ( Table 1) were refked by least squares method.
Table 1
-
Crystal data for BaCeO -SrZrO solid solution. calculated for samples fir- ed at 1400°C for 360 hours. 3 3For electrical conductivity measurements the method used together with consider- Sample
SrZr03 1 2 3 4 BaCe03
ations about their accuracy and~reproducibility were reported in previous papers /5/
/6/. The specific electrical conductivity in alternatly current (1000 Hz) and in di- rect current were determined at different temperatures and for different oxygen par- tial pressures. Table 2 summarizes the values of conductivity obtained in alternate current. The data regarding the values deternincd in direct current have been omit- ted since these values coincided with those in alternate current.
Z 4 4 4 8 4 4
-1 -1
Table 2 - Specific electrical conductivity (o in R an ) at different temperatu- res and oxygen partial pressures for the samples tested. AC
c ( m ) 0.8195 0.8438 0.8439 0.8590 0.8603 0.8804
%EaCsO 3 0 10 2 5 50 7 5 100
V ( m 3 ) 0.2762 0.2999 0.3116 0.6296 0.3183 0.3397 1
Symmetry OF. 1 OR 1 OR 1 OR 2 TET TET
%SrZrO 3 100
90 75 5 0 2 5 0
a(nm) 0.5797 0.5933 0.6018 0.5961 0.6083 0.6212
b ( m ) 0.5815 0.5990 0.6136 1.2299 0.6083 0.6212
111 - RESULTS AND DISCUSSION a) Structure analysis
The existence of a continuous s o l i d solution over the whole range BaCeO -SrZrO whichwas previously suggested /l /,was confirmed. The compound SrZrO a s previously 3 3' established /7/, is orthorhombic and exhibits a d i s t o r t e d perovszfte-type structu- r e with a
'
b and c-
a & , which is the topologic r e l a t i o n linking an ideal cubic pe- rovskite framework t o a more d i s t o r t e d orthorhombic l a t t i c e .BaCeO shows a tetragonal l a t t i c e /8/ which i s d u e t o a more regular atomic arrange- ment m the perovskite-type 3 s t r u c t u r a l model. The behaviour of BaCeO -SrZrO s o l i d solution is complex and shows the presence of pronounced displacive cl?anges gappen- ing i n the structure. Starting from SrZrO , t h e s u b s t i t u t i o n of Sr and Z r atoms with Ba and Ce ones brings i n i t i a l l y t o a sligl?tly preater orthorhombic u n i t c e l l . For the intermediate composition 50% BaCeO
-
50% SrZrO the s t r u c t u r e abruptly changes3 . 3'
i n t o a more complex orthorhombic c e l l wlth b
=
2a; The doubling of 6 axisis requir- ed by t h e sharp presence of few, well detectable r e f l e c t i o n s which disappearfor a l l the compositions d i f f e r i n g from t h i s one. Successively i n the BaCeO r i c h p a r t of s o l i d solution, a simpler tetragonal structure was detected. 3b) E l e c t r i c a l conductivity analysis
The parameters influencing the e l e c t r i c a l conductivity data a r e the current frequen- cy of the measuring system, the oxygen v a r t i a l pressure and the temperature. A dis- cussion regarding these parameters could y i e l d informations about e l e c t r i c a l con- duction mechanism.
The differences found between s p e c i f i c e l e c t r i c a l conductivity measured i n a l t e r n a t e and i n d i r e c t current were negligible. Therefore, it can be s t a t e d t h a t the conduc- t i o n mechanism i s of e l e c t r o n i c type.
Far a l l the investigated compositions and f o r a l l temperatures the s p e c i f i c electric- a l conductivitymsincreasing with increasingof oxygen p a r t i a l pressure and there- f o r e it can be deduced t h a t the predominant charge c a r r i e r s were electron holes (p- -type semiconductors).
E l e c t r i c a l conductivity being a k i n e t i c phenomenon, the temperature dependence of the constant describing it, should be of the ~ r r h e n i u s type, i . e . a l i n e a r law f o r log of conductivity versus the inverse of absolute temperature. This r e l a t i o n was found f o r a l l the compositionstestedand f o r a l l oxygen p a r t i a l pressures. I n Fig. 1 a typical p l o t logo versus 1/T.
logo \
- \ (1) 1
' ' \ I
(3) 3 x
"
-3
-- .\
(4) 5 X I 0-4
--
I I I I I f
1200 1100 1000 9000 T ("C) 800
Fig. 1
-
Log of the s p e c i f i c e l e c t r i c a l conductivity in AC versus 1/T f o r sample 2.C1-el6 JOURNAL DE PHYSIQUE
The values of activation energy of the conduction mechanism determined by least- - square calculationasthermal gaps were reported i n Table 3.
Table 3
-
Thermal gap (eV) f o r the samples tested a t different oxygen p a r t i a l pres- sures.The correlation coefficient r l i e s in the range 0.9972
+
1 .In Figure 2 the thermal gap ( eV ) i s plotted versus the sample composition. I t can be seen that the thermal gap increases with the increase of oxygen p a r t i a l pr, ~ s s u r e except f o r the 50%-50% composition: t h i s phenomenon can be related with the changes of the structure of t h i s composition.
Figure 2
-
Thermal gaps (eV) a t different oxygen p a r t i a l pressures f o r the solid solution examined.IV - CONCLUSIONS
The exsistence of a continuous s o l i d solution BaCeO
-
SrZrO was ~ o n f i r m e d ~ b u t t h e i r3 3
l a t t i c e parameters do not follow the Vegardls law: f o r the intermediiite composition the l a t t i c e i s stroiigly distorted and the doubling of the b axis i s required by the presence of new reflections.
For a l l the samples tested the ionic contribution t o t o t a l conduction i s negligible and it can be s t a t e d t h a t the charge c a r r i e r s a r e the electron holes.
The thermal gap cannot by simply related with the composition or with the l a t t i c e parameters of the solid solution.
REFERENCES
/1/ Barbariol I . , Longo V. and Cocco A., Annali d i Chimica,
59
(1969) 878./2/ Longo V. and Meriani S., La Ceramica,z (1979) 1 .
/3/ Longo V., Ricciardiello F. and Sbaizero O., "Energy and Ceramics" P. Vincenzini Ed.; Elsevier, Amsterdam(l980) 1123.
/4/ Minichelli D., Ricciardiello F. and Sbaizero O . , Mater. Chem.and Phys.
-
10 (1984l 487./5/ Longo V. and R o i t t i S., Ceramurgia (1972) 172.
61 de P r e t i s A . , Longo V., Ricciardiello F. and Sbaizero O., S i l i c a t e s Industriels 1984 (1984) 139.
/7/ LongoV. and Minichelli D . , La Ceramica
Z
(1974) 9./8/ Longo V . , Minichelli D. and Ricciardiello F., J. Mater. S c i . ,
