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

Neutron powder diffraction data indicate that Li2La­(TaTi)­O7 adopts a polar, a–a–c+/a–a–c+ distorted, n = 2 Ruddlesden–Popper structure described in space group A21am (#36), consistent with a hybrid-improper stabilization of the polar structure via a trilinear coupling between the X3–, X2+, and Γ5– distortion modes. In contrast, Na2La­(TaTi)­O7 adopts a polar, a–a–c–/a–a–(-c–) distorted, n = 2 Ruddlesden–Popper structure described in space group Pna21 (#33) with the polar Γ3– distortion mode apparently stabilized by a second-order Jahn–Teller distortion of the d0 Ta5+/Ti4+ cations. The change in class of polar distortion of the A2La­(TaTi)­O7 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.