BaTiO_3およびPbTiO_3の自発分極の結晶異方性と単位胞体積依存性の第一原理解析(<特集>半導体エレクトロニクス)

概要

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Using first-principles calculations we have analyzed the dependence of spontaneous polarization on lattice parameters and tetragonality ratio in natural and artificial structures of tetragonal BaTiO_3 (BTO) and PbTiO_3 (PTO), in order to establish the conditions needed for obtaining increased spontaneous polarization. Conventional knowledge of the polarization in PTO being higher than in BTO due to its larger tetragonality ratio and increased chemical activity of the A-site cation has been confirmed. In addition, a peculiar dependence of the spontaneous polarization on unit cell volume has been revealed. Specifically, at the equal tetragonality ratio, a given ferroelectric material (be it BTO or PTO), with a larger unit cell volume will have a larger spontaneous polarization. This may be related to the augmenting or reduction of short-range repulsion forces that tend to stabilize a centrosymmetrical structure. Therefore, a large tetragonality ratio is not absolutely necessary for obtaining increased spontaneous polarization, if the unit cell volume is large. The present results might partly explain why one should expect that BiFeO_3, with a rather modest tetragonality ratio in thin film form, should have very large spontaneous polarization. That is, the chemical bonding and the unit cell volume are similar to PTO, whose polarization is close to 100μC/cm^2.
2006-02-15