From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilization

Abstract

Hybrid improper ferroelectrics (HIFs) have been studied intensively over the past few years to gain an understanding of their temperature-induced phase transitions and ferroelectric switching pathways. Here we report a switching from a first-to a second-order phase transition pathway for HIFs Ca 3−x Sr x Ti 2 O 7 , which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions-such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others-is a crucial step in creating novel functional materials by design.

In situ X-ray diffraction investigation of electric-field-induced switching in a hybrid improper ferroelectric

“…Despite these limitations, our results suggest that the initial switching pathway proceeds predominately via an unwinding of the X þ 2 rotation mode. This is corroborated by analogy with the temperature-based phase transitions found in our previous work on this material, wherein a sequence of phase transitions from A2 1 am to aristotype I4/mmm proceeds initially by a melting of the X þ 2 distortion (Pomiro et al, 2020). We further suggest that a one-step mechanism appears to be dominant under the conditions of the experiment presented above, though the tendency towards this is subtle.…”

“…By incorporating this lowered symmetry into a symmetry-mode model in the Rietveld refinement program TOPAS Academic v6, it is possible to individually vary the magnitudes and directions of the distortions and plot the effects on the intensity of each reflection. Starting with both modes fixed at values refined in a previous publication (defined as switching coordinate 0) and manipulating their amplitudes both individually and simultaneously (Pomiro et al, 2020), we mimic the effect of unwinding the distortion modes (complete unwinding being switching coordinate 0.5), moving through different switching mechanisms to a complete reversal of the distortions (defined as switching coordinate 1). shortest switching coordinate, changing by 10% around a switching coordinate of approximately 0.04.…”

“…The rotation and tilt OPs transform as irreducible representations X þ 2 and X À 3 , respectively, and their combination yields not only a polar ground state but also a very rich domain structure including head-to-head and tail-to-tail charged domain walls (Harris, 2011;Oh et al, 2015). With increasing temperature, the phase transitions of samples of Ca 3Àx Sr x -Ti 2 O 7 (0 x 0.85) have been demonstrated to be strongly influenced by entropy: at x = 0, the phase exhibits a first-order transition between A2 1 am and Acaa; at x = 0.85, a secondorder phase transition between A2 1 am and P4 2 /mnm is observed with smaller distortion magnitudes (Huang et al, 2016;Kratochvilova et al, 2019;Pomiro et al, 2020). The continuous evolution of the OP at x = 0.85 is similar to that expected in a switching mechanism.…”

Pressure-Dependent Phase Transitions in Hybrid Improper Ferroelectric Ruddlesden-Popper Oxides

“…A visualisation of these modes is shown in the SI (Figure S1). By contrast, we have recently shown 23 that Ca 3 Ti 2 O 7 exhibits a first-order phase transition between A2 1 am and Acaa. With doping by Sr, this switches to a continuous second-order transition at Ca 2.15 Sr 0.85 Ti 2 O 7 .…”

Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn2Ti2NbO10 Dion-Jacobson Phases

“…Similarly shallow energy landscapes have been reported for hybrid-improper Ruddlesden–Popper systems and are suggested to influence phase-transition pathways between models with different combinations of tilts. 65 …”