Short Notes Kttl

phys. stat. sol. (a) 116, K81 (1989) Subject classification: 61.50 and 72.20; S10.15

School of Materials Science and Technology (a) and Department of Ceramic Engineering (b) , Institute of Technology, Banaras Hindau University, Varanasi' ) Preparation and Characterisation of the System Srl,xLaATil-xC&g3

OM PARKASH (a), CH. DURGA PRASAD (a), and DEVENDRA KUMAR (b) BY

Introduction Strontium titanate is a useful dielectric material which forms the basis of barrier layer capacitors /1, 21. On heating in atmospheres of low partial pressure of oxygen it becomes semiconducting due to loss of oxygen according to the reaction

o0 +. 2 1 o2 + V; + 2e',

where all the symbols have been used in Kriiger-Vink notation of point defects. Oxygen is lost during the firing process. On cooling the ceramic from the firing temperatures, reoxidation of the grain boundary takes place making them insulating. Due to insufficient time available during cooling, this reoxidation process is restricted to grain boundaries while the grains still remain oxygen deficient and semiconducting. Thus we have semiconducting grains enveloped in insulating grain boundaries. This gives rise to a high dielectric constant characteristic of barrier layers.

LaCo03 is also an important semiconducting oxide perovskite which finds a wide variety of applications / 3 / . We have investigated the electrical behaviour and spin- state and valence-state equilibria of cobalt ions in the system Sr1-xLaxTil,xCox03 for x < 0.50 / 4 , 51 . This system represents a valence compensated solid solution between SrTi03 and LaCo03 where simultaneous substitution of La3+ and Co3+ for Sr2+ and Ti4+, respectively, leads to internal charge compensation. Recently we found that in the system Pbl-xLaxTil,xCox03 the solid solution forms over the entire range of composition I S / . The compositions with x 5 0.50 are found to exhibit dielectric relaxator behaviour /? / . In view of this we considered it worthwhile to investigate the possibility of formation of solid solution in the system Sr1-xLaxTil-xCox03 for x < 0.50 and to study the electrical behaviour of these materials. In this note we are reporting the preparation, structure, and preliminary results of electrical measurements on these samples.

Experimental Samples with x = 0.01, 0.05, 0.10, 0.20, 0.30, and 0.40 were prepared by the ceramic method using strontium oxalate, lanthanum oxalate, cobalt oxalate, and titanium dioxide all having purity better than 99.5%. Lanthanum oxalate and strontium oxalate were estimated as lanthanum oxide and strontium carbonate by heating to 1223 K. Appropriate quantities of these starting materials

') Varanasi 221005, India. 6 physica (a) -

K82 physica status solidi (a) 116

were accurately weighed, mixed in agate jars in a ball mil l using agate balls as grinding media. The dried powders were transferred to platinum crucibles and calcined at 1473 K for 4 h and then cooled in the furnace to room temperature. The calcined powders were ground, mixed with 1% solution of PVA as binder, and pressed as cylindrical pellets. These pellets were heated slowly to 873 K and kept at this temperature for half an hour. The temperature was then raised to 1523 K and the samples were sintered at this temperature for 1 2 h and then cooled in the furnace.

X-ray 4iffraction patterns of the crushed pellets were taken using CuKal radiation f rom which CuKa2 radiations were filtered in a Rigaku Rotaflex Diffractometer . For dielectric measurements, both surfaces of the sintered pellets were polished and coated with air dried silver paint. Capacitance and dielectric loss were measured using a Hewlett Packard Impedance Analyser Model No. 4192A LF. The Seebeck coefficient, a, was measured at different steady temperatures between 350 to 1000 K relative to platinum.

X-ray diffraction patterns of the various compositions indicated the absence of characteristic X-ray diffraction lines of the constituent oxides indicating the formation of solid solution in all the compositions investigated. Therefore, solid solution forms over the entire range of composition in the system Lal,xSrxCol-xTix03 similar to Lal,xPbxCol-xTix03. X-ray data of the various compositions could be indexed on the basis of a cubic unit cell similar to pure strontium titanate. The lattice parameter, a , for all the compositions is given in Table 1. It is observed that the lattice parameter decreases with x except for the composition with x = 0.20. This can be understood in terms of ionic radii of La3+(0.122 nm), Sr2+ (0.123 nm), Ti4+ (0.060 nm), Co3+ (0.063 nm or0.052 nm depending on exists in the high-spin state t4 e2 or low-spin state t6 eo respectively). La3' and Sr2' have almost equal size. Cobalt ions are known to co-exist in low-as well as high-spin states in LaCo03 181. The decrease of the lattice parameter with increasing x shows that these materials contain a significant concentration of low-spin cobalt ions.

The Seebeck coefficient was found to be positive over the entire temperature range of measurements indicating that holes are the majority charge carriers. The values of a at 500 K for all the samples are given in Table 1. The values of the dielectric constant, E , at 1 kHz and 300 K are also given in Table 1. Very large values of the dielectric constant are observed in the composition with x = 0.30 and 0.40. These compositions also exhibit relaxator behaviour. The dielectric loss is also high in these compositions which may be attributed to the high conductivity of these samples. Detailed results of measurements of dc conductivity, Seebeck coefficient, dielectric constant, and dielectric loss as a function of temperature will be reported elsewhere in near future.

Results and discussion

whether it 2g g

2g g'

Short Notes K83

T a b l e 1

composition

0.01

0.05 0.10

0.20 0.30 0.40

X a (nm) a ( u V / K ) E

0.3904 110 0.3898 160 1450 0.3875 340 720 0.3880 180 410 0.3870 190 45000 0.3865 100 18700

Financial support from Department of Science and Technology, Government of India, New DeLhi is grateful ly acknowledged. W e are g ra t e fu l to Naval Chemical and Metallurgical Laboratory, Bombay f o r p rov id ing the XRD facil i ty during the course of these investigations.

References /I/ I. BURN and S. NEIRMAN, J. Mater. Sci. l7, 3510 (1984). 121 N. YAMAOKA, Bull. h e r . Ceram. SOC. 65, 1149 (1986)

/ 3 / 0. YAMAMOTO, Y. TAKEDA, R. KANNO, and M. NODA,

/ 4 / D. BAHADUR and OM PARKASH,

/5 / V. KUMAR, R.M. SINGRU, OM PARKASH, and D. BAHADUR,

/ 6 / OM PARKASH, CH. DURGA PRASAD, and DEVENDRA KUMAR,

/ 7 / OM PARKASH, CH. DURGA PRASAD, and DEVENDRA KUMAR,

/ 8 / P.M. RACCAH and J.B. GOODENOUGH,

and re fe rences cited therein.

Solid State Ionics 22, 241 (1987) and re fe rences cited therein.

J. Solid State Chem. 46, 197 (1983).

phys. stat. sol. (b) 128, 223 (1985).

J. Solid State Chem. E, 385 (1987).

phys. stat. sol. (a) 106, 627 (1988).

Phys. Rev. 155_, 932 (1967).

(Received A u g u s t 22, 1988; in revised form September 20, 1989)