First-order phase transition and unexpected rigid rotation mode in hybrid improper ferroelectric (La, Al) co-substituted Ca3Ti2O7 ceramics

Ca3Ti2O7 with Ruddlesden-Popper structure exhibits the largest polarization among the known hybrid improper ferroelectrics. However, the high Curie temperature impedes the thorough study of phase transition through dielectric characterization. According to the previous theoretical design rule, the Curie temperature can be suppressed by increasing the tolerance factor. So, in the present work, high-quality Ca3-xLaxTi2-xAlxO7 (x = 0.0, 0.1, 0.2, 0.3) ceramics with increased tolerance factors were successfully prepared. The amplitude of oxygen octahedral tilt mode indeed decreases with increasing tolerance factors, leading to a degeneration of ferroelectric polarization. However, the unexpected rigid rotation mode causes the composition-invariable coercive fields. The Curie temperatures decrease linearly with increasing x and tolerance factors. The variable-temperature dielectric constant confirms first-order improper ferroelectric transitions in Ca3Ti2O7-based ceramics. The results of variable temperature X-ray diffraction reveal the coexistence of two-phases below Curie temperature. The present work confidently confirms the first-order improper ferroelectric transition in Ca3Ti2O7-based ceramics by combining results of variable-temperature dielectric response and in-situ X-ray powder diffraction. Keywords: Hybrid improper ferroelectricity, Ca3Ti2O7, Rigid rotation mode, First-order phase transition, Dielectric response