Tetsuhiro Katsumata

A polar oxide ZnSnO3 with a LiNbO3-type structure.

A polar oxide ZnSnO3 was synthesized by a solid-state reaction under a pressure of 7 GPa and a temperature of 1000 degrees C. The crystal structure was determined by Rietveld analysis of the X-ray powder diffraction data. ZnSnO3 has a rhombohedral LiNbO3-type structure with unit cell parameters, a = 0.52622(1) nm, c = 1.40026(2) nm (space group: R3c). The polar structure is characterized by the large displacement of Zn along the c-axis in the ZnO6 octahedron based on the strong chemical bonding between Zn and three O. ZnSnO3 is a candidate piezoelectric and pyroelectric material as well as nonlinear optical material.

High-pressure synthesis and ferroelectric properties in perovskite-type BiScO3–PbTiO3 solid solution

Stabilization of a perovskite-type solid solution (1−x) PbTiO3–x BiScO3 with x⩾0.45 was demonstrated by high-pressure synthesis, and the phase diagram and the ferroelectric properties of the solid solution were investigated. The crystal symmetry of the perovskite subcell change in turn from tetragonal, to rhombohedral, to pseudocubic, to monoclinic, and then to triclinic as x increases. It was found that the tetragonal, rhombohedral, and pseudocubic phases are ferroelectric, while the monoclinic phase is not. In the morphotropic phase boundary in the vicinity of x=0.37 between tetragonal and rhombohedral phases, the maximum electromechanical coupling factor and the minimum coercive electric field were just like those observed in other Pb-based ferroelectric perovskites. In addition, relaxor behavior in the dielectric constant was observed in the vicinity of x=0.5.

Synthesis, Structural Transformation, Thermal Stability, Valence State, and Magnetic and Electronic Properties of PbNiO3 with Perovskite- and LiNbO3-Type Structures

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.

High-Pressure Synthesis and Correlation between Structure, Magnetic, and Dielectric Properties in LiNbO3-Type MnMO3 (M = Ti, Sn)

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.

First‐Principles Studies on Novel Polar Oxide ZnSnO3; Pressure‐Induced Phase Transition and Electric Properties

Adv. Mater. 2010, 22, 2579–2582 2010 WILEY-VCH Verlag G An increased interest has developed around noncentrosymmetric (NCS) oxides because of their symmetry dependent properties, such as ferroelectricity, piezoelectricity, and second-order non-linear optical behavior. Among these NCS oxides, special attention has been paid to the R3c structure owing to a fascination of fundamental science and technical applications. For instance, LiNbO3 and LiTaO3 are typical representatives of non-linear optical materials, and BiFeO3 is known as a multiferroic material. We recently synthesized novel NCS oxides of ZnSnO3 with a LiNbO3 (LN)-type structure (R3c) under high-pressure (HP) conditions ( 7GPa). The refined crystal structure determined by Rietveld analyses confirmed the non-cetrosymmetry, primarily because of a large displacement of Zn2þ. Later, Son et al. succeeded in synthesizing a LN-type ZnSnO3 thin film with a high ferroelectric polarization of 47 8C cm 2 by pulsed laser deposition (PLD). Since thin film formation enables one to utilize the ferroelectric material in electric circuits, a LN-type ZnSnO3 can potentially replace existing materials in electronic devices. Another aspect of the general interest in LN-type ZnSnO3 materials with high ferroelectric polarization lies in the fact that this compound consists only of main-group cations, Zn2þ and Sn4þ. Until now, Pb-based ferroelectric oxides such as Pb(Zr,Ti)O3 (PZT) have been widely utilized because of their high ferroelectric polarization. Pb, however, causes environmental pollution, so that many attempts have been devoted to find novel lead-free ferroelectric compounds. In addition, most of the recent studies to develop ferroelectric materials have focused on the oxides containing second-order Jahn–Teller (SOJT) distorted cations with d transition metal ions (such as, Nb5þ and Ta5þ) and/or cations with lone pair electrons of ns (such as Bi3þ).[13–22] On the other hand, the LN-type ZnSnO3 is composed of two cations with the electronic configuration of (n 1)dns. Thus, the previous discovery of LN-type ZnSnO3 by the HP technique suggested a new strategy to explore new NCS crystals with R3c symmetry. Nevertheless, searching for new compounds using HP techniques could demand numerous experiments on a general trial-and-error basis. Knowledge of the pressure-dependent phase stability and the relationship between physical properties and crystal/electronic structures may offer the way to systematically search for new polar oxides. In this study, we demonstrate a first-principles approach to predict the pressure dependence of the phase stability for Zn2þ Sn4þ O ternary systems. In addition, Born effective charge tensors and spontaneous polarization of LN-type ZnSnO3 were calculated and compared with those of LiNbO3 reported in the literature. [24]

An approach to control of band gap energy and photoluminescence upon band gap excitation in Pr(3+)-doped perovskites La(1/3)MO3 (M=Nb, Ta):Pr3+.

We synthesized polycrystalline pristine and Pr(3+)-doped perovskites La(1/3)MO(3) (M = Nb, Ta):Pr(3+) and investigated their crystal structure, optical absorption, and luminescence properties. The optical band gap of La(1/3)NbO(3) (3.2 eV) is smaller than that of La(1/3)TaO(3) (3.9 eV), which is primarily due to the difference in electronegativity between Nb and Ta. In La(1/3)NbO(3):Pr(3+), the red emission assigned to the f-f transition of Pr(3+) from the excited (1)D(2) level to the ground (3)H(4) state upon band gap photoexcitation (near-UV) was observed, whereas the f-f transition of Pr(3+) with blue-green emission from the excited (3)P(0) level to the ground (3)H(4) state was quenched. On the other hand, in La(1/3)TaO(3):Pr(3+), the blue-green emission upon band gap photoexcitation was observed. Their differences in emission behavior are attributed to the energy level of the ground and excited states of 4f(2) for Pr(3+), relative to the energy levels of the conduction and valence bands, and the trapped electron state, which mediates the relaxation of electron from the conduction band to the excited state of Pr(3+). La(1/3)NbO(3):Pr(3+) is a candidate red phosphor utilizing near-UV LED chips (e.g., λ = 375 nm) as an excitation source.

Direct observations of La ordering and domain structures in La0.61Li0.17TiO3 by high resolution electron microscopy

La0.61Li0.17TiO3 microstructures have been studied by high resolution electron microscopy. A local lattice distortion occurs in the vicinity of the domain boundary region due to the twin with an angle of 89°. The average domain size of La0.61Li0.17TiO3 is greater than 20 nm. The domain size and structures of La0.61Li0.17TiO3 differ greatly from those of La-poor compounds, such as La0.55Li0.35TiO3. At a nanoscopic level, microdomains of 20–100 nm in size construct a two-dimensional structure in La-rich compounds, while microdomains of 5–10 nm in size construct a three-dimensional structure in La-poor compounds. In addition, the Li-ion conduction mechanisms for La-rich and La-poor compounds are two- and three-dimensional, respectively.

High-pressure synthesis, structure, and characterization of a post-perovskite CaPtO3 with CaIrO3-type structure.

A new ternary platinum oxide, CaPtO3 was synthesized under a pressure of 7 GPa and a temperature of 1000 degrees C. The crystal structure of CaPtO3 was determined by Rietveld analysis of the X-ray powder diffraction data. CaPtO3 has a layered CaIrO3-type structure (orthorhombic, space group: Cmcm), which is the same as that of a post-perovskite MgSiO3 in the Earths lower mantle. The magnetic susceptibility data indicate that the Pt ion in CaPtO3 is tetravalent in the low spin state with an electron configuration of t2g(6)eg(0)(S = 0). This finding is consistent with the insulating behavior.

High Pressure Synthesis, Lattice Distortion, and Dielectric Properties of a Perovskite Bi(Ni 1/2 Ti 1/2 )O 3

A perovskite Bi(Ni 1/2 Ti 1/2 )O 3 was successfully synthesized under high pressure 6 GPa. This compound has orthorhombic cell with the parameters, a = 2 a m sin( g /2) = 0.5626(2) nm, b = 4 b m = 1.5681(6) nm, and c = 2 c m cos( g /2) = 0.5548(2) nm containing eight monoclinic primitive perovskite unit with the parameters, a m = c m = 0.39507(7) nm, b m = 0.39203(15), and g = 90.80(2), and the lattice distortion is expected to be antiferroelectric. In addition, the sudden increase in the dielectric constant accompanied by the anomaly of dielectric loss was observed at 490 K as the temperature increases. This is though to correspond to the transition from antiferroelectric phase to another one.

Synthesis and magnetic and charge-transport properties of the correlated 4d post-perovskite CaRhO3.

A high-quality polycrystalline sample of the correlated 4d post-perovskite CaRhO(3) (Rh(4+): 4d(5), S(el) = 1/2) was attained under a moderate pressure of 6 GPa. Since the post-perovskite is quenchable at ambient pressure/temperature, it can be a valuable analogue of the post-perovskite MgSiO(3) (stable higher than 120 GPa and unstable at ambient pressure), which is a significant key material in earth science. The sample was subjected for measurements of charge-transport and magnetic properties. The data clearly indicate it goes into an antiferromagnetically ordered state below approximately 90 K in an unusual way, in striking contrast to what was observed for the perovskite phase. The post-perovskite CaRhO(3) offers future opportunities for correlated electrons science as well as earth science.