High-Pressure Synthesis of A-Site Ordered Double Perovskite CaMnTi2O6 and Ferroelectricity Driven by Coupling of A-Site Ordering and the Second-Order Jahn-Teller Effect

We successfully synthesized a novel ferroelectric A-site-ordered double perovskite CaMnTi2O6 under high-pressure and investigated its structure, ferroelectric, magnetic and dielectric properties, and high-temperature phase transition behavior. Optical second harmonic generation signal, by frequency doubling 1064 nm radiation to 532 nm, was observed and its efficiency is about 9 times as much as that of SiO2 (alpha-quartz). This compound possesses a tetragonal polar structure with space group P4(2)mc. P-E hysteresis measurement demonstrated that CaMnTi2O6 is also ferroelectric. A spontaneous polarization calculated by use of point charge model and the observed remnant polarization are 24 and 3.5 mu C/cm(2), respectively. CaMnTi2O6 undergoes a ferroelectric-paraelectric order-disorder-type phase transition at 630 K. The structural analysis implies that both the ordering of shift of Mn2+ from the square-planar and the off-center displacement of Ti4+ in TiO6 octahedra are responsible for ferroelectricity. CaMnTi2O6 belongs to a new class of ferroelectrics in which A-site ordering and second-order Jahn-Teller distortion are cooperatively coupled. The finding gave us a new concept for the design of ferroelectric materials.