MnCaTa 2 O 7 -A Magnetically Ordered Polar Phase Prepared via Cation Exchange.

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 P 2 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 Mn A Ta 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.