Dynamics of antipolar distortions

Materials possessing antipolar cation motions are currently receiving a lot of attention because they are fundamentally intriguing while being technologically promising. Most studies devoted to these complex materials have focused on their static properties or on their zone-center phonons. As a result, some important dynamics of antipolar cation distortions, such as the temperature behavior of their phonon frequencies, have been much less investigated, despite the possibility to exhibit unusual features. Here, we report the results and analysis of atomistic simulations revealing and explaining such dynamics for BiFeO3 bulks being subject to hydrostatic pressure. It is first predicted that cooling such material yields the following phase transition sequence: the cubic paraelectric Pm $$\bar 3$$ m state at high temperature, followed by an intermediate phase possessing long-range-ordered in-phase oxygen octahedral tiltings, and then the Pnma state that is known to possess antipolar cation motions in addition to in-phase and antiphase oxygen octahedral tiltings. Antipolar cation modes are found to all have high phonon frequencies that are independent of temperature in the paraelectric phase. On the other hand and in addition to antipolar cation modes increasing in number, some phonons possessing antipolar cation character are rather soft in the intermediate and Pnma states. Analysis of our data combined with the development of a simple model reveals that such features originate from a dynamical mixing between pure antipolar cation phonons and fluctuations of oxygen octahedral tiltings, as a result of a specific trilinear energetic coupling. The developed model can also be easily applied to predict dynamics of antipolar cation motions for other possible structural paths bringing Pm $$\bar 3$$ m to Pnma states. Temperature dependent structural dynamics in pressurized BiFeO3 are simulated and analyzed for ferroelectric-related phases. Kinnary Patel and coworkers from the University of Arkansas in US and Southern Federal University in Russia performed atomistic simulations on the dynamics of antipolar distortions, which are cation distortions against ferroelectric polarization, with respect to temperature in BiFeO3 under hydrostatic pressure. The material undergoes a paraelectric phase, an intermediate phase with long-range-ordered FeO6 octahedral tilting and a motion-mixed phase with both antipolar distortion and FeO6 octahedral tilting upon cooling. In paraelectric phase, the antipolar distortions always have high frequencies independent of temperature while they become soft with low frequencies in the intermediate and the motion-mixed phases. These results can be applied to predict dynamics of antipolar motions on the phase evolutions for other materials with similar crystal structures.