A family of ferroelectric bismuth compounds

Abstract

A large number of compounds of the general form (Bi₂O₂)²⁺ (Me_m-1R_mO_3m+1)^2- are synthesized. Me and R represent ions of appropriate size and valency and m equals 2, 4 and 5. A dielectric study revealed ferroelectricityin the following compounds: MeBi₂R₂O₂ (Me Ba,Pb, Sr and R Nb, Ta), MeBi₄Ti₄O₁₅ (Me Ba, Ph, Sr), Me_0.5Bi_4.5Ti₄O₁₅ (Me K, Na) and Me₂Bi₄Ti₅O₁₈ (Me Pb, Sr). Curie temperatures exist over a wide range from about 100°C to perhaps as high as 900-950°C (for Bi₃TiNbO₉). The presence of Bi ions seems related to the high (Tc > 500°C) Curie temperatures of many members. In a given series, the Curie points decreased with the divalent ion in the order: lead-strontium-barium. At the Curie temperature, a symmetry change from tetragonal to orthorhombic takes place, with the largest orthorhombic distortion observed being 1-007 Bi₃TiNbO₉ at room temperature. Dielectric studies on single crystals indicate the ferroelectric axis to be the pseudo-tetragonal c-axis.