New Mechanism for Ferroelectricity in the Perovskite Ca2–xMnxTi2O6 Synthesized by Spark Plasma Sintering

Perovskite oxides hosting ferroelectricity are particularly important materials for modern technologies. The ferroelectric transition in the well-known oxides BaTiO3 and PbTiO3 is realized by softening of a vibration mode in the cubic perovskite structure. For most perovskite oxides, octahedral-site tilting systems are developed to accommodate the bonding mismatch due to a geometric tolerance factor t = (A–O)/[√2(B–O)] < 1. In the absence of cations having lone-pair electrons, e.g., Bi3+ and Pb2+, all simple and complex A-site and B-site ordered perovskite oxides with a t < 1 show a variety of tilting systems, and none of them become ferroelectric. The ferroelectric CaMnTi2O6 oxide is, up to now, the only one that breaks this rule. It exhibits a columnar A-site ordering with a pronounced octahedral-site tilting and yet becomes ferroelectric at Tc ≈ 650 K. Most importantly, the ferroelectricity at T < Tc is caused by an order–disorder transition instead of a displacive transition; this character may be use...