The structural phase transition in hexagonal BaMnO3 occurring at TC = 130 K was studied in ceramic samples using electron and x-ray diffraction, second harmonic generation, as well as by dielectric and lattice dynamic spectroscopy. 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 TC and it hardens according to the Cochran law. Upon cooling below TC, 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.
Unusual ferroelectric and magnetic phases in multiferroic 2H−BaMnO3 ceramics