Structural transitions in hybrid improper ferroelectric Ca3Ti2O7 tuned by site-selective isovalent substitutions: A

Abstract

Ca3Ti2O7 is an experimentally confirmed hybrid improper ferroelectric material, in which the electric polarization is induced by a combination of the coherent TiO6 octahedral rotation and tilting. In this work, we investigate the tuning of ferroelectricity of Ca3Ti2O7 using iso-valent substitutions on Ca-sites. Due to the size mismatch, larger/smaller alkaline earths prefer A'/A sites respectively, allowing the possibility for site-selective substitutions.Without extra carriers, such site-selected iso-valent substitutions can significantly tune the TiO6 octahedral rotation and tilting, and thus change the structure and polarization. Using the first-principles calculations, our study reveals that three substituted cases (Sr, Mg, Sr+Mg) show divergent physical behaviors. In particular, (CaTiO3)2SrO becomes non-polar, which can reasonably explain the suppression of polarization upon Sr substitution observed in experiment. In contrast, the polarization in (MgTiO3)2CaO is almost doubled upon substitutions, while the estimated coercivity for ferroelectric switching does not change. The (MgTiO3)2SrO remains polar but its structural space group changes, with moderate increased polarization and possible different ferroelectric switching paths. Our study reveals the subtle ferroelectricity in the A3Ti2O7 family and suggests one more practical route to tune hybrid improper ferroelectricity, in addition to the strain effect.

Charged domain boundaries stabilized by translational symmetry breaking in the hybrid improper ferroelectric Ca3–xSrxTi2O7

“…The core-valence electron interaction was described using the projector augmented wave method 36 , and the exchange-correlation part was treated within the Perdew-Burke-Ernzerhof generalized gradient approximation 37 . The on-site Coulomb interaction (U eff ) of the Ti ions was set to 3 eV 38 . The cutoff energy for the plane-wave basis was 550 eV.…”

Suppressing the ferroelectric switching barrier in hybrid improper ferroelectrics

“…(We refer to phases where dipole moments of each slab are zero as 'nonpolar'.) Many of these phases are observed to emerge in various A 3 B 2 O 7 oxides under biaxial strain or equivalent doping, and are also shown to be important as intermediate states in the coherent switching of polarization 5,16,[20][21][22]33 . This is in addition to single-tilt systems observed, for example, at finite temperature 7 .…”

First-Principles Investigation into Hybrid Improper Ferroelectricity in Ruddlesden–Popper Perovskite Chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se)

“…reported that HIF was active in the layered perovskite oxides with the Ruddlesden–Popper (RP) structure and the chemical formula of (AO)­(ABO 3 ) n , n = 2, and recently predicted in n = 2 RP perovskite oxides (RPPOs) and chalcogenides (RPPCs), i . e ., Ca 3 Ti 2 O 7 , Sr 3 Sn 2 O 7 , Sr 3 Zr 2 O 7 , Ca 3 Zr 2 S 7 , Ca 3 Zr 2 Se 7 , Ca 3 Hf 2 S 7 , Ca 3 Hf 2 Se 7 , Sr 3 Zr 2 S 7 , and Sr 3 Hf 2 S 7 . ,− On the one hand, it is worth addressing that the HIF in RPPOs, i . e ., Ca 3 Ti 2 O 7 , Sr 3 Sn 2 O 7 , Sr 3 Zr 2 O 7 , and Ca 3 (Ti, Mn) 2 O 7 , was already experimentally confirmed. ,,, So far, all predicted HIF in RPPCs is still in the stage of theoretical predictions and in urgent need of experimental verification.…”