A family of ferroelectric bismuth compounds

A large number of compounds of the general form (Bi2O2)2+ (Mem−1RmO3m+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), Me0.5Bi4.5Ti4O15 (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.