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.

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.

13 C-NMR measurements have been performed to investigate the nature of the insulating state of a quasi-one-dimensional conductor, ͑DI-DCNQI͒ 2 Ag, with a quarter-filled band. The 4k F charge-density wave has been evidenced below about 220 K by the line separation of the 13 C-NMR spectra, which shows a clear charge disproportionation instead of the lattice dimerization encountered in most cases. The present case demonstrates the Wigner crystallization of electrons in the charge-transfer salt and a crucial role of the long-range Coulomb interaction. [S0031-9007(98)

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LiNbO(3)-type MnMO(3) (M = Ti, Sn) were synthesized under high pressure and temperature; their structures and magnetic, dielectric, and thermal properties were investigated; and their relationships were discussed. Optical second harmonic generation and synchrotron powder X-ray diffraction measurements revealed that both of the compounds possess a polar LiNbO(3)-type structure at room temperature. Weak ferromagnetism due to canted antiferromagnetic interaction was observed at 25 and 50 K for MnTiO(3) and MnSnO(3), respectively. Anomalies in the dielectric permittivity were observed at the weak ferromagnetic transition temperature for both the compounds, indicating the correlation between magnetic and dielectric properties. These results indicate that LiNbO(3)-type compounds with magnetic cations are new candidates for multiferroic materials.