MnCaTa(2)O(7)—A Magnetically Ordered Polar Phase Prepared via Cation Exchange

[Image: see text] Reaction between the pseudo-Ruddlesden-Popper phase Li(2)CaTa(2)O(7) and MnCl(2) at 375 °C yields MnCaTa(2)O(7), a paramagnetic polar phase (space group P2(1)nm), which adopts an a(–)b(–)c(+)/b(–)a(–)c(+) distorted, layered perovskite structure. Magnetization and neutron diffraction data show that MnCaTa(2)O(7) adopts an antiferromagnetically ordered state below T(N) = 56 K and exhibits large lattice parameter anomalies and a transient increase in its polar distortion mode at T(A) = 50 K. However, in contrast to the related phase MnSrTa(2)O(7), MnCaTa(2)O(7) shows no strong signature of weak ferromagnetism and thus shows no signs of magnetoelectric coupling. The differences in physical behavior between the two MnATa(2)O(7) phases appear to be related to their differing Mn cation-order and differing TaO(6) tilting schemes and demonstrate that even subtle changes to these orderings can have large effects on the distortion-mode couplings, which drive complex behavior of this class of “hybrid improper” ferroelectric material.