Mapping the structural transitions controlled by the trilinear coupling in Ca3-xSrxTi2O7

Abstract

We present the results of the high-temperature neutron and x-ray diffraction experiments on the Ca 3-xSrxTi2O7 (x = 0.5, 0.8, 0.85, 0.9) compounds. The ferro-to paraelectric transition in these hybrid improper ferroelectric materials arises from the so-called trilinear coupling. Depending on the Strontium content, various structures and phase transitions, different from theoretical predictions, emerge. The in-situ x-ray powder diffraction indicates a direct ferro-to paraelectric transition between the orthorhombic A2 1am and tetragonal undistorted I4/mmm phase for x ≤ 0.6. We identified a reduction in the trilinear coupling robustness by increasing the Sr-doping level to lead to the emergence of the intermediate tetragonal P4 2/mnm phase and the gradual suppression of the orthorhombic phase. The observed character of the structure transitions and the Ca 3-xSrxTi2O7 phase diagram are discussed in the framework of theoretical models of other related hybrid improper ferroelectric systems.

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

“…This continuous sequence provides an explanation for the second-order nature of the phase transitions, which is consistent with the OP amplitude refined against the diffraction data, the continuous evolution of lattice parameters, and the absence of any phase coexistence. This is a much richer phase diagram than that previously proposed [37], which assumed a first-order phase transition from A2 1 am directly to P4 2 /mnm at this composition. Thus, in summary, x = 0 shows first-order while x = 0.85 shows second-order phase transitions.…”

Complex Structural Phase Transitions of the Hybrid Improper Polar Dion–Jacobson Oxides RbNdM2O7 and CsNdM2O7 (M = Nb, Ta)

“…A surprising feature of the Ruddlesden−Popper T c α t relation is that it spans compounds that undergo structural transitions to a variety of high-temperature phases above T c : Sr 3 Zr 2 O 7 and (Sr,Ca)Sn 2 O 7 (A2 1 am → Pnab), 20,22 Ca 3 Mn 2 O 7 (A2 1 am → Acaa), 28,29 and (Ca,Sr) 3 Ti 2 O 7 (A2 1 am → I4/ mmm). 30 The apparent insensitivity of the T c α t relation to the identity of the high-temperature phase at the ferroelectric phase transition suggests that T c scales with some feature of the low-temperature polar phase alone. The structural tolerance factor, t, parameterizes the driving force, on simple packing grounds, for the adoption of distorted structures, and as such can be considered as a measure of the stability of distorted structures compared to an undistorted aristotype phase.…”

“…A surprising feature of the Ruddlesden–Popper T c α t relation is that it spans compounds that undergo structural transitions to a variety of high-temperature phases above T c : Sr 3 Zr 2 O 7 and (Sr,Ca)­Sn 2 O 7 ( A 2 1 am → Pnab ), , Ca 3 Mn 2 O 7 ( A 2 1 am → Acaa ), , and (Ca,Sr) 3 Ti 2 O 7 ( A 2 1 am → I 4 /mmm ) . The apparent insensitivity of the T c α t relation to the identity of the high-temperature phase at the ferroelectric phase transition suggests that T c scales with some feature of the low-temperature polar phase alone.…”

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

“…This continuous sequence provides an explanation for the second-order nature of the phase transitions, which is consistent with the OP amplitude refined against the diffraction data, the continuous evolution of lattice parameters, and the absence of any phase coexistence. This is a much richer phase diagram than that previously proposed [33], which assumed a first-order phase transition from A2 1 am directly to P 4 2 /mnm at this composition. Thus in summary, x = 0 shows first-order while x = 0.85 shows second-order phase transitions.…”