Contribution of Pb to Ferroelectricity in Perovskite-type Oxides
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Contribution of Pb to Ferroelectricity in Perovskite-type Oxides Hiromu Miyazawa1, Fumiyuki Ishii2, Masaya Ishida1, Eiji Natori1, Tatsuya Shimoda1 and Tamio Oguchi2 1 Technology Platform Research Center, SEIKO EPSON Corporation 281 Fujimi, Fujimi-machi, Nagano-ken 399-0293, Japan 2 Graduate School of Advanced Sciences of Matter, Hiroshima University 1-3-1 Kagamiyama, Higashi-Hiroshima-shi, Hiroshima 739-8530, Japan ABSTRACT We have studied the nature of the covalent bond between Pb 6s, 6p and O 2p orbitals in perovskites based on first-principles calculations. We conclude that the Pb 6p - O 2p covalent bond, not a bond involving Pb 6s, is crucial for the large ferroelectric effect in PbTiO3. The Pb 6s states behave like a shallow core level. To examine the effect of a Pb atom at the A site in the perovskite-type structure, we compared several calculated properties of PbTiO3 and BaTiO3 - the electric polarization, piezoelectric stress tensor and Young's modulus. The piezoelectric stress tensor e33 was calculated through Berry phase theory. BaTiO3 was found to have a larger e33 value than PbTiO3. In the case of BaTiO3, the response of Ti-O(2) to c-axis distortion is larger than in PbTiO3. We found that PbTiO3 is softer than BaTiO3 because of the cooperation of both Ti-O(2) and Pb-O(1) covalent bonds. We also found that the ferroelectric state is much softer than the paraelectric state. We propose that in the structural phase transition the system with the lower symmetry avoids ion repulsion through the additional degree of freedom available from atom displacement. INTRODUCTION Perovskite-type oxides represented by the chemical composition ABO3 are widely used as piezoelectric and ferroelectric devices. Recently, use of these materials as thin-films has opened new frontiers for materials science. This is in part because the opportunities presented by the presence of new effects in thin films, such as huge inhomogeneous stresses, allow us to easily obtain nonequilibrium and stable states such as superlattices. In these exciting circumstances, in order to find much more efficient materials we should carefully study the basic physics, i.e., the electronic structure of the system and investigate the mechanism that operates in piezoelectric and ferroelectric materials from this point of view. Cohen, Singh, and Vanderbilt established the basic notion of the electronic structure of ferroelectric materials based on first-principles calculations[1,2,3,4,5]. Their work emphasized that the covalent bond between the B-site transition metal and its nearest oxygen ions is the origin of the double-well potential corresponding to the ferroelectric state[1]. It has also been proposed that the experimental Curie temperature, which indicates the strength of ferroelectricity, can be simply explained by covalency (the intensity of covalent bond) between B-site transition metal and its nearest oxygen ions[6]. Until now, it has been supposed that the role of the A-site atom in the strong ferroelectricity in PbTiO3 is due to additiona
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