Unusual ferroelectric and magnetic phases in multiferroic 2H−BaMnO₃ ceramics

The structural phase transition in hexagonal BaMnO₃ occurring at T_C=130 K was studied in ceramic samples using electron and x-ray diffraction, second harmonic generation, as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group P63cm) is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary [q=(1/3,1/3,0)]; this mode activates in infrared and Raman spectra below T_C and it hardens according to the Cochran law. Upon cooling below T_C, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing down of ferroelectric domain wall motions.

Temperature dependence of (a) dielectric permittivity and (b) dielectric loss of 2H−BaMnO₃ ceramics measured at various frequencies. The values of tan δ at 6 GHz in the temperature interval between 30 and 90 K are missing due to high losses.

¹Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8, Czech Republic
²Laboratoire de Physique de Systemes complexes, Université de Picardie, 80000 Amiens, France
³Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic
⁴Department of Physics, Northern Illinois University, DeKalb, Illinois, USA