Hybrid improper ferroelectricity and possible ferroelectric switching paths in Sr3Hf2O7.

Hybrid improper ferroelectricity, which is induced by the coupling of oxygen octahedral rotation and tilt in oxide materials, has great potential to create single-phase multiferroic materials with intrinsic electric field-controlled magnetism. Based on previous experiments with bi-layered Ruddlesden-Popper Sr-based materials, hybrid improper ferroelectricity in Sr3Hf2O7 was investigated by first-principle calculations in the present work. The results show that the ferroelectric A21am phase has the lowest energy among all possible phases, indicating that it is the ground state. The spontaneous polarization calculated by the Berry phase approach and Born effective charge model is approximately 4.6 μC/cm2, in agreement with those reported for other Sr-based compounds. Compared to Ca3Ti2O7, the smaller polarization of Sr3Hf2O7 is attributed to the negative contribution of the HfO2 layer to global polarization. The possible ferroelectric switching path was determined using the nudged elastic band method. The energy barriers of the two four-step paths across the non-polar Pbnm and Pnab phases are almost the same, and they are the lowest energies among all possible paths. Competition between these paths can help us understand complex phase transitions in analogous compounds. In summary, hybrid improper ferroelectricity was predicted in Sr3Hf2O7 for the first time, and the possible ferroelectric switching paths were determined. [ABSTRACT FROM AUTHOR]