Structural phase transitions of robust insulating Bi_(1-x)La_xFe_(1-y)Ti_yO_3 multiferroics

摘要

In contrast to leaky BiFeO_3, of which two structural phase transitions of cycloidal modulated antiferromagnetic-paramagnetic and ferroelectric-paraelectric are observed below 850 ℃, chemical co-substitution to form Bi_(1-x)La_xFe_(1-y)Ti_yO_3 ternary solid solution makes these multiferroic ceramics become robust insulating low dielectric loss and exhibit rich structural phase transitions. Differential thermal analysis and temperature-dependent X-ray diffraction measurements probe four first-order structural phase transitions, e.g., T_H = 305℃, T_N = 365℃, T_C = 810℃, and T_S = 830℃ observed in the Bi_(0.98)La_(0.02)Fe_(0.99)Ti_(0.01)O_3 system, which are reasonably attributed to Brazovskii-type cycloidal modulated antiferromagnetic-helimagnetic (at T_H) and helimagnetic-paramagnetic (at T_N) magnetic phase transitions, ferroelectric-paraelectric (at T_C) and rhombohedral-cubic (at T_S) structural phase transitions, respectively. Magnetic phase transition temperatures change a little but ferroelectric and lattice structural phase transition temperatures decrease gradually with increasing co-substitution up to composition-induced rhombohedral-(pseudo-)cubic structural phase boundary. The first-order nature of magnetic phase transition and emergence of helimagnetic phase were attributed to Ti~(3+) d~1 magnetic disorder distribution in the Fe~(3+) d~5-O-Fe~(3+) d~5 chains, while the first-order nature becomes weak with increasing co-substitution, owing to decreased ferroelectric rhombohedral lattice distortion. An intermediate rhombohedral paraelectric phase is discovered intervening between ferroelectric rhombohedral and paraelectric cubic phase, of which the temperature range defined by difference between T_C and T_S increases with increasing co-substitution. It was found that T_C and T_S are able to be predicted quantitatively by reduced mass of unit cell. These findings enrich our understanding of ferroic phase transitions and advance designing novel high temperature multiferroic compounds.