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

Abstract

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 (α-quartz). This compound possesses a tetragonal polar structure with space group P42 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 μC/cm2, 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.

Improper ferroelectric polarization in a perovskite driven by intersite charge transfer and ordering

“…Based on crystal field stabilization arguments, Mn 2+ in the square-planar environment tends to restore the expected orbital degeneracy preferred by the symmetrically filled e g and t 2g orbitals by making its crystal field environment more regular by shortening second-nearestneighbor Mn-O bond distances. Such a shortening is evident in our low temperature refinements for Mn A , which have a nextnearest-neighbor distance of 2.4Å, very close to that reported in other systems with Mn A 2+ [31,32], and is well below the shortest second-nearest-neighbor distance of 2.65Å observed for any previously reported Mn A 3+ system. We hence assign a formal charge ordering of 1:2, Mn 2+ :Mn 3+ on the A -site.…”

Ambient-temperature high-pressure-induced ferroelectric phase transition in CaMnTi2O6

“…CaMnTi 2 O 6 was synthesized using pure ilmenite-type MnTiO 3 and CaTiO 3 sealed inside a platinum capsule at 7 GPa and 1700…”

“…• C in a multianvil press for 30 min following the work by Aimi et al [3]. The multianvil experiment was performed at GFZ Potsdam with a 18/11 assembly, which was calibrated at room temperature against the phase transitions in Bi metal [4,5].…”

“…Unfortunately, while most type-I multiferroics have a large spontaneous polarization, they do not show bulk ferroelectricity due to their large leakage and coercive field [2]. Recently, Aimi et al [3] have discovered that CaMnTi 2 O 6 is a multiferroic with a moderate spontaneous polarization of ∼24 μC/cm 2 and the first example of an oxide containing Mn 2+ allowing a polarization reversal at ambient temperature, thus leading to a novel class of type-I multiferroics.…”

“…However, in contrast to CaFeTi 2 O 6 , the square-planar Mn 2+ and the octahedrally coordinated Ti 4+ are shifted along the c axis in CaMnTi 2 O 6 . This breaks the center of inversion, lowering the symmetry from space group P 4 2 Aimi et al [3] have shown that with increasing temperature the off-centering of the square-planar coordinated Mn 2+ and of the Ti 4+ decreases. Concomitantly, the second-harmonicgeneration (SHG) signal decreases monotonically with temperature up to the Curie temperature T c = 630 K, where it becomes zero due to the ferroelectric (space group P 4 2 mc) to paraelectric (space group P 4 2 /nmc) phase transition.…”