JAPAN NANONET BULLETIN - 78th Issue - August 31, 2006

Development of hazardous-lead-free novel ferroelectrics

Lead zirconate titanate (PZT) has been used in a wide variety of device applications, such as non-volatile memories and piezoelectric products. However, pollution and the destruction of ecological systems, because of illegally-dumped PZT devices, which release toxic lead, have become serious problems. We are trying to develop a novel lead-free ferroelectric that can act as an alternative to PZT.

We have been studying a bismuth layered ferroelectric (see Figure 1), which is a promising candidate for lead-free ferroelectrics. The perovskite layers are composed of m layers of TiO₆ octahedra and bismuth-oxide layers that are alternately stacked in the crystal structure. It is well known that the family of the bismuth layered compounds can be ferroelectrics with an m between 1 and 5. We have grown a single-crystal of natural superlattice-structured ferroelectrics having alternating of Bi₄Ti₃O₁₂ (m = 3) and BaBi₄Ti₄O₁₅ (m = 4) layers. The crystal has the largest spontaneous polarization of 62 μC/cm² in lead-free perovskite materials, and this value is larger than that of commercialized PZT (see Figure 2). Structural stress occurs because of the stacking of two different perovskite layers with different in-plane lattice parameters. This peculiar stress in the superlattice structure induces ferroelectric displacement of the Bi ions in the bismuth-oxide layers, which is the origin of the large spontaneous polarization. Furthermore, the introduction of layered defects in the superlattice structure, or in other words, a change in the ratio of the m = 3 and m = 4 layers will result in a larger spontaneous polarization in this system.

Currently, we have been investigating the superlattice-structured crystals with a large amount of layered defects. In addition to crystals, grain-oriented ceramics of this material have synthesized, and we are currently trying to determine their piezoelectric properties.