STRUCTURAL STUDY OF AgNbO3 AND NaNbO3 AT 292 K
Jan Fábry1, Zdenek Zikmund1, Antoni Kania2, Václav Petricek1
1Institute of
Physics of the Czech Academy of Sciences, Na Slovance 2, 180 40
Praha 8, Czech Republic
(e-mail: fabry@fzu.cz)
2Institute of Physics, Silesian University, ul. Uniwersytecka
4,40-007 Katowice, Poland
Keywords:
perovskites, ferroelectricity, twinning
The present structure determination of AgNbO3 was performed [1] on a twinned single crystal with predominant presence (~93(1) %) of one twin component. The structure turned out to be isostructural with NaNbO3 [2,3] which was redetermined though the crystal contained considerable proportion of a second domain (~27(3) %). (It should be noted that two almost homometric models can be assumed.) Both compounds are tilted perovskites, 4-fold superstructures along the c-axis with quite regular oxygen octahedra. At room temperature in AgNbO3 a week ferroelectricity [4] was observed while NaNbO3 is antiferroelectric [5]. The structural deviations in AgNbO3 from the centrosymmetry were not detected in this experiment, and should be very small. Perhaps the observed ferroelectricity may be related to the reported deficiency of Ag in AgNbO3 which should cause point defects. An analogy to the ferroelectric Q-phase of NaNbO3 can be drawn. The structure of the Q-phase corresponds to the contents of one half of the unit of the room temperature phase of NaNbO3, i. e. it is a 2-fold superstructure along the c axis. The Q-phase can be called forth by addition of a few % of K, e.g. [6] which should act as point defects as well. The Q-phase can even coexist with a 4-fold superstructure of NaNbO3. The experiments of HREM [7] indicated in AgNbO3 planar faults along the c-axis. This argument results in a hypothesis that ferroelectricity in AgNbO3 may stem from a phase which is analogous to the Q-phase in NaNbO3.
Acknowledgments:
This work was supported by the grant 203/96/0111 of the GA ER.