Switching between Proper and Hybrid-Improper Polar Structures via Cation Substitution in A2La(TaTi)O7(A = Li, Na)

Neutron powder diffraction data indicate that Li 2 La(TaTi)O 7 adopts a polar, a – a – c + / a – a – c + distorted, n = 2 Ruddlesden–Popper structure described in space group A 2 1 am (#36), consistent with a hybrid-improper stabilization of the polar structure via a trilinear coupling between the X 3 –, X 2 +, and Γ 5 – distortion modes. In contrast, Na 2 La(TaTi)O 7 adopts a polar, a – a – c – / a – a – (- c – ) distorted, n = 2 Ruddlesden–Popper structure described in space group Pna 2 1 (#33) with the polar Γ 3 – distortion mode apparently stabilized by a second-order Jahn–Teller distortion of the d 0 Ta 5+ /Ti 4+ cations. The change in class of polar distortion of the A 2 La(TaTi)O 7 framework, from hybrid improper (trilinearly coupled) to proper (second-order Jahn–Teller stabilized), on A -cation substitution suggests that the two stabilization mechanisms are in competition in these two materials and many other hybrid-improper ferroelectric phases.