Asymmetry in the avalanche dynamics of the interferroelectric phase transformation in barium titanate

The jerky character of displacive phase transformations in ferroelectrics generates measurable ultrasonic signals that provide information about the transformation dynamics. This work investigates the acoustic emission from the interferroelectric phase transition between the tetragonal and orthorhombic states in the model ferroelectric BaTiO3. Similar to the paraelectric–ferroelectric transition, the interferroelectric transformation exhibits scale-invariant avalanche dynamics reflected in power laws of characteristic quantities such as the amplitude and energy of the ultrasonic pulses. In contrast to the paraelectric–ferroelectric case, the interferroelectric transformation exhibits a pronounced directional asymmetry: the energy exponent is 1.38±0.06 for heating and 1.59±0.08 for cooling. Within experimental uncertainties, the results agree with the basic mean-field prediction of 1.33 and with the field-integrated value of 1.67. The exponent asymmetry and the 260% higher event count during cooling suggest distinct dynamical pathways of the forward and reverse transformations, a picture further supported by systematic differences in the waiting-time statistics between successive avalanches. We attribute this asymmetry to different starting conditions for nucleation into the respective product phase. The strong correlation between avalanche amplitude and energy and the validity of the Omori law for aftershocks do not depend on the transformation direction.