Abstract
In ABO3-type perovskite oxides with Pnma symmetry, rotation (QR+, a0a0c+) and tilt (QT, a–a–c0) of BO6 octahedra are the two primary order parameters. These order parameters establish an inherent trilinear coupling with anti-ferroelectric A-site displacement (QAFE) to form the low-symmetry phase. The symmetry is further lowered in double perovskite oxides (DPOs) due to A/A′ cation ordering. It in turn makes these systems polar via hybrid improper ferroelectric mechanism, primarily driven by QR+ and QT. Naturally, it has been believed that functionalities such as polarization can also be switched by tuning these primary order parameters. However, mystery around finding switching mechanism still remains. Our study based on density functional theory calculations combined with finite-temperature molecular dynamics simulations shows that the polarization switching is a two-step process, driven by out-of-phase rotation (QR–, a0a0c– when QT = 0 or, a–a–b– when QT ≠ 0). A series of polar DPOs such as KLnFeOsO6 [Ln = Sm, Gd, Dy, Tm (lanthanides) and Y (rare earth)], all belonging to P21 symmetry, are considered in this investigation. The polarization switching P (𝑃⃗ ) occurs at a very high temperature of ∼1150 K through a phase transition, from a polar (P21) phase with P(+) to P(-) via a non-polar P4/n phase. The switching itself is metastable in nature. The switching (both polarization and spin state) is only observed for a very short period of time (∼23 ps) that poses limitation on using such a mechanism in memory device realization. We demonstrate a concurrent heating–cooling procedure to overcome such shortcoming. Simulations conducted at 600 K further imply that long lasting switching can be achieved, at least for 1.2 ns for 600 K, and ideally for an infinite time, if the material is heated just above the Tc followed by rapid cooling to a temperature below Tc.
