The influence of the preparation method on the properties of Sr8(Al12O24)(CrO4)2 ferroelectric ceramics

Mat. Res. Bull., Vol. 23, pp. 481-486, 1988. Printed in the USA. 0025-5408/88 $3.00 + .00 Copyright (c) 1988 Pergamon Press plc.

T H E I N F L U E N C E O F T H E P R E P A R A T I O N M E T H O D ON T H E P R O P E R T I E S OF Sr8(AI12024)(CrO4) 2 FERROELECTRIC CERAMICS

N. Setter* and A. Bhalla Materials Research Laboratory The Pennsylvania State University University Park, PA 16802 ( R e c e i v e d O c t o b e r 30, 1987; R e f e r e e d )

ABSTRACT Strontium-aluminate-chromate (SACR) ceramics have been prepared by the citrate method. As a result the sintering temperature was reduced from 1700 ° C (in conventional oxide method) to 1500 ° C. These ceramics reveal a new phase transition. The dielectric and pyroelectric measurements show that SACR is an important candidate material for pyroelectric applications. The two phase transitions observed in the present study can be useful to enhance the figure of merit of a device at room temperature. MATERIALS INDEX: Strontium - Aluminum- Chromium - Oxide.

Introduction Strontium-aluminate-chromate (SACR) is a ferroelectric material with a phase transition close to room temperature (1). It is a member of a family of silica-free aluminate-sodalites with a general formula M8(Al12024)(X)2, where M is a divalent cation like Sr +2, Ca +2 or Ba +2 and X is a tetrahedral anion like (CrO4) -2 or (WO4) -2. Many of the aluminate-sodalites undergo phase-transition from cubic, I43m, to tetragonal or orthorhombic polar phases (2,3,4). SACR is the first of this group to show ferroelectricity, but its structure, properties, and phase transition have not yet been fully investigated. Recently, the preparation of SACR ceramics has been reported (5) and the samples have shown a relatively high spontaneous polarization (Ps = 2 gC/cm 2) with a low dielectric constant (K - 15). These properties make the aluminate-sodalite ceramics the potential materials for pyroelectric applications. However, the sintering temperature of the S A C R ceramiic is high and even at 1700 ° C full sintering could not be achieved. *Permanent address: M . O . D , P.O.B. 2250, Haifa 31021, Israel. 481

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Figure 1. Therrnograms showing the endo-thermal peaks o f S A C R fired at various temperatures: a) 825°C, 10 hrs. b) 1100 o C, 10 hrs. c) 1170 ° C, 12 hrs. d) 1320 ° C, 12 hrs. e) 1420 ° C, 12 hrs. f) 1530 ° C, 5 hrs. g) 1670 ° C, 5 hrs.

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Since the last decade, chemical methods for the preparation of ceramics have been gaining importance over the other methods of ceramics processing due to the fact that ceramics prepared by this route have shown relatively better properties. Among these, the citrate process, first used by Van de Graaf et al. (6), provides highly sinteractive powders while still utilizing carbonates, oxides or readily available nitrates as raw materials. Therefore, the citrate process was chosen for the preparation of SACR in the present investigation with an aim to explore a method for preparing a dense ceramic at a lower possible sintering temperature, making SACR more attractive and at the same time obtaining a highly homogeneous material for further studies of its ferroelectric properties.

The starting chemicals were reagent grade CrO3, Sr(NO3)2 and A l ( N O 3 ) 3 . 9 H 2 0 (Baker Ltd.). The stoichiometric amount corresponding to 0.005 M SACR was dissolved in water. 0.1 M citric acid and 0.3 M ethylene glycol were then added, and after complete dissolution the temperature was increased while stirring to form a resin. Then the resin material was further heated to form a dry powder. The powder obtained was amorphous as defined from the XRD studies. When fired at 670 ° C the organics and the nitric compounds decomposed and the resulted powder showed mainly the sodalite structure with particle size of 160 A ° diameter as determined from the XRD line width. In comparison, mixed-oxidescarbonates attempts to prepare the sodalite phase below 1100 o C have not been successful. The pressed pellets were fired at several temperatures in the range 825 ° C to 1700 ° C. Dense sintered pellets were obtained at temperatures - 1500 ° C which was relatively lower by 200 ° C as compared with the sintering temperature required for the powder prepared by the solid-state reaction technique. The sintered pellets were green in color. The particle growth was studied by XRD (SINTAG PAD V) and SEM (ISI DS 130). The phase-transition was studied by differential scanning calorimetry (Dupont 900 130). For the DSC measurements 20-23 mg of samples were used and a typical heating rate of 10 ° C/rain was employed. For the dielectric studies pellets were polished and gold sputtered on both sides and the capacitance was measured by HP4140 LCR meter. The pyroelectric coefficients were measured on the poled samples by the Byer-Roundy technique. Results

and Discussion

The particle size of the samples fired at various temperatures are listed in Table 1 and the thermal scans of these samples are shown in Fig. 1. There is practically no obvious indication of phase transitions on samples fired below 1100 o C. Only with materials fired at or above 1100 ° C and with particle size > 400 ]~ the phase transitions were observed. Also in all the samples two endothermal peaks corresponding to two phase transitions in SACR were observed. The two transitions (Tcl - 43 ° C, Tc2 - 16 ° C) in the samples fired at 1170 o C were separated apart by ~ 27 ° C. The separation between the two transitions gradually decreased in the samples fired at higher temperatures and finally reduced to the value - 11 ° C for the samples fired at 1700 ° C (Table 1).

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TABLE 1 Particle size of S A C R fired at different temperatures and the temperature width of the intermediate phase. Firing temp. (o C)

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In the case o f powder prepared by the conventional oxide method, two phase transitions were observed for the powder calcined at 1170 ° C whereas only one peak in the DSC scan was observed for the samples sintered at 1700 ° C (Fig. 2).

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Figure 2. Thermograms of SACR prepared by the mixed oxide method: a) 1170 ° C, 12 hrs. b) 1700 ° C, 5 hrs. Two anomalies were observed in the dielectric constant measurements in the samples prepared by the citrate method (Fig. 3), though the Tc values differed from the values determined by the DSC scans. Also the K vs T anomalies were rather weak. In contrast to the samples prepared by the citrate method, the samples prepared by conventional oxide method showed only one anomaly in the K vs T measurements (5).

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Figure 3. The temperature dependence of the dielectric constant of SACR prepared by the citrate method. The pyroelectric measurements on the poled samples prepared by the citrate method showed two anomalies in the p vs T plot (Fig. 4a). The pyroelectric current was reversed upon reversal of the laoling field, indicating polar-polar phase transition in SACR. Since the

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Figure 4. Temperature dependence of the pyroelectric response of SACR. Samples cooled under electric field of 8.5 KV/cm. pyroelectric method is very sensitive to the small changes in the polarization of the samples, it could pick up also the weaker phase transition. However, for the SACR prepared by the mixed-oxides method only one anomaly appeared. The structure of the paraelectric phase of SACR has been reported recently (7). It consists of an almost fully expanded aluminate-sodalite framework and its chromate cagetetrahedra are disordered. The ferroelectric phase is orthorhombic (1) but the full structure is not known by this time. Ca8(Al12024)(WO4)2 (CAW), another aluminate sodalite which undergoes two phase transitions at 341 ° C and at 383 ° C has been studied in more detail (2,3,8,9). In C A W the transition sequence is cubic --> tetragonal (probable) --> orthorhombic (the intermediate phase has not been studied yet). In both the cubic and the orthorhombic

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phases of CAW the framework is partially collapsed but the cage anion tetrahedra are disordered in the cubic phase and ordered in the orthorhombic phase. Following CAW and in accordance with the structure of the cubic phase of SACRa possible phase transition sequence could be cubic (m3m) --> cubic (43m) --> orthorhombic (mm2), which means a collapse of m3m to Z~3mprior to the ferroelectric transition. This is excluded by the present pyroelectric measurement. Also, the lower dielectric constant and large pyroelectfic response and phase transitions close to room temperature make this material attractive for pyroelectric applications. Moreover, the two phase transitions can be placed at the desirable temperatures by choosing the proper sintering conditions for the samples. In conclusion, the preparation of SACR by the citrate method lowered the sintering temperature of SACR by ~ 200 ° C. The powder was chemically homogeneous and thus the measurements revealed new intermediate phase between the higher paraelectric phase and the lower ferroelectric phase. This could be exploited for designing pyroelectric devices with improved figure of merit. The nature of the two phase transitions is presently under investigation. Acknowledgements

We would like to thank W. Yarbrough, W. Heubner, and S. Komarneni for their advice on the subject of chemical methods of ceramics preparation. The work was carded under the ONR-DARPA contract No. N00014-86-K-0767. References

1.

N. Setter, M.E. Mendoza-Alvarez, W. Depmeier, and H. Schmid, Ferroelectric 56, 49 (1984).

2.

W. Depmeier, J. Appl. Cryst. 12, 623 (1979).

3.

W. Depmeier, Z. Kristallogr. 17_4,41 (1986).

4.

N. Setter and W. Depmeier, Ferroelectrics 56, 45 (1984).

5.

N. Setter, Ferroelectrics Lett. 7, 1 (1987).

6.

M. Van de Graaf, T. Van Dijk, M.A. De Jongh, and A.J. Burgraaf, Science of Ceramics9, 75 (1977).

7.

W. Depmeier, H. Schmid, N. Setter, and M.L. Werk, Acta Cryst. C (submitted).

8.

W. Depmeier, Acta Cryst. C40, 226 (1984).

9.

W. Depmeier, Acta Cryst. B (submitted).