Hirotake Shigematsu

New Ferroelectric BaTi2O5

A needle-shaped BaTi2O5 crystal was grown from a Ti-rich solution. The chemical composition of the crystal was analyzed by an electron probe micro-analyzer (EPMA). X-ray Weissenberg photographs were taken at room temperature (RT): the crystal is monoclinic with a=16.91 A, b=3.94 A, c=9.49 A, β=103.0° and Z=6. A broad and large dielectric anomaly reaching 30,000 was observed at 430°C for the dielectric constant along the b-axis. Also, spontaneous polarization was estimated to be approximately 7 µC/cm2 from the D-E loops at RT. Thus the crystal is a new ferroelectric substance with a high Curie temperature of 430°C and the space group C2 at RT.

High temperature phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 by X-ray diffractometry and differential thermal analysis

Abstract The two phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 were studied by high temperature X-ray diffraction and differential thermal analysis in the temperature range 300–1723 K. The presence of the two kinds of phase transitions in CaTiO3 and (Ca0.85Nd0.15)TiO3 at about 1390 K and about 1530 K recently found by the heat capacity measurement in our laboratory was confirmed. From the X-ray diffraction patterns of CaTiO3 and (Ca 0.85Nd0.15)TiO3, the first phase transition at lower temperature was due to the change from the orthorhombic Pbnm to the orthorhombic Cmcm structure, and the second one at higher temperature was due to change from Cmcm to the cubic Pm3m structure, originating from the sequential rotations of the TiO6 octahedra about one of the axes. Since no clear peak was seen at the phase transition around 1390 K in the DTA curve in contrast with the presence of the endothermic peak at the transition around 1530 K, it is thermodynamically considered that the former and the latter transition were second- and first-order phase transition, respectively.

Order-disorder nature of ferroelectric BaTi2O5

Ferroelectric single crystals of BaTi2O5 have been studied in detail by Raman and high-resolution micro-Brillouin scattering with a focus on the central component of the relaxation mode over a wide temperature range (20–600°C). A relaxation mode has been clearly observed in these single crystals. The temperature dependences of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model. The ferroelectric phase transition shows, at least near the vicinity of Tc, an order-disorder nature. Our results indicate that the spontaneous polarization mainly resulted from the dynamic disorder of the Ti ions in the octahedra along the polar b axis of BaTi2O5.

EXAFS analyses of CaTiO3 doped with Ce, Nd and U

Abstract The EXAFS (extended X-ray absorption fine structure) analyses were carried out for Ti–K, Ce–L3, Nd–L3 and U–L3 edges to make clear the local structures in (Ca 1− x Ce x )TiO 3 ( x =0.10, 0.20), (Ca 1− x Nd x )TiO 3 ( x =0.05, 0.10, 0.15, 0.20) and (Ca 1− x U x )(Ti 1−2 x Al 2 x )O 3 ( x =0, 0.05). With increasing neodymium content in (Ca 1− x Nd x )TiO 3 the first nearest titanium–oxygen distances decreased. On the other hand, with increasing cerium content in (Ca 1− x Ce x )TiO 3 , the first nearest titanium–oxygen distances increased. The first nearest titanium–oxygen distance in (Ca 1− x U x )(Ti 1−2 x Al 2 x )O 3 was larger than that in undoped CaTiO 3 . With increasing dopant content, the first nearest dopant (cerium or neodymium)–oxygen distances increased. From the result of the compositional dependencies of bonding length (cation–oxygen distance), which is inversely proportional to the bonding energy, it is estimated that magnitude of the leaching rate of Ce-doped CaTiO 3 is larger than those of Nd-doped and undoped CaTiO 3 . The experimental results on the leaching rates of doped and undoped CaTiO 3 were consistent with this estimation. The structural change of (Ca 0.80 Ce 0.20 )TiO 3 after a leaching test in HCl at 363 K for 140 days was also measured by X-ray diffraction and XANES analysis.

Synthesis of BaTi2O5 Nanopowders by Sol–Gel Method and the Dielectric Properties of the Ceramics

A sol–gel process using barium diethoxide and titanium tetraisopropoxide as the sources for Ba and Ti, respectively, was employed to synthesize BaTi2O5 nanopowders and ceramics. BaTi2O5 nanopowders in the range of 20–50 nm in size were obtained by calcination of the gel precursors at 650 °C. By a further firing at 1000 °C, some BaTi2O5 nanopowders were grown to 200 nm in size. Conventional sintering at 1150 °C using these BaTi2O5 nanopowders yields BaTi2O5 ceramics of 80–85% relative density. A dense BaTi2O5 ceramic with a relative density of above 95% was fabricated by spark plasma sintering (SPS) at 1000 °C. Furthermore, by SPS combined with the hot-forging technique, we obtained a certain degree of grain orientation along the b-axis in the dense BaTi2O5 ceramic and observed an increase in the dielectric constant reaching 580.

Observation of Relaxation Mode in Ferroelectric Barium Dititanate by Micro-Brillouin Scattering

New ferroelectric single crystals of BaTi2O5 have been studied in detail by high-resolution micro-Brillouin scattering with focus on the central component of the relaxation mode over a wide temperature range (20–600°C). A relaxation mode has been clearly observed in these single crystals for the first time. In addition, the usual phonon features are also observed in the Brillouin shifts determined by selection rules for a monoclinic phase. The ferroelectric phase transition at Tc~470°C, sharp anomaly has been observed from both the frequency shift and hypersonic damping. The relaxation time slows down and intensity of central peak increases markedly when temperature approaches Tc from above and below. Furthermore, the relaxation mode has an A(y) symmetry which implies that the relaxation mode involves interwell motion of Ti ions parallel to the polar axis. The temperature dependence of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model in which the off-center ion in the octahedra tunnels between two sites of a double-well potential.

Phase equilibria of CaTi03 doped with Ce, Nd and U

Abstract The solubility limits of Ce, Nd and U in CaTi0 3 , one of the promising host matrix for fixation of lanthanide and actinide elements, were determined by X-ray diffraction to be between 20 and 30 mol%, between 20 and 30 mol% and between 0 and 3 mol%, respectively. By substituting Al to Ti in CaTiO 3 , the solubility limit of U in CaTiO 3 increased to be between 5 and 7.5 mol%. The thermogravimetric analyses of (Ca 0.80 Ce 0.20 )TiO 3±y was carried out at 1273 K and 1373 K in the oxygen partial pressure range of 10 −14 - 10 −6 Pa. In the higher oxygen partial pressure region, the sample of single CaTiO 3 phase decomposed into three phases consisting of perovskite phase, CeO 2 and TiO 2 .

Neutron Powder Diffraction Study of the Phase Transition in BaTi2O5

The structural phase transition and crystal structure of BaTi 2 O 5 sol-gel powder have been studied by neutron powder diffraction. The crystal structure of BaTi 2 O 5 at high-temperature phase has symmetry with the space group of C2/m from an extinction rule and a disappearance of spontaneous polarization. Lattice parameters are a = 17.0040(4) Å, b = 3.9455(1) Å, c = 9.5236(2) Å, β = 102.941(2)° and V = 662.706(23) Å 3 at 900 K. There are two kinds of Ba (Ba1 and Ba2) and three kinds of TiO 6 octahedra (Ti1O 6 , Ti2O 6 and Ti3O 6 ), and the unit cell has six BaTi 2 O 5 molecules, Z = 6. All atoms are situated on the mirror planes with y coordinates of 0 of 1/2. The high-to-low temperature phase transition will be characterized by the atomic displacement of Ti and other oxygen atoms like a Slater mode, in Ti1O 6 octahedron.

Reinvestigation of Low-Temperature Phase Transitions in Rb2ZnBr4

Low-temperature phase transitions in Rb 2 ZnBr 4 single crystal were reinvestigated by means of neutron and X-ray diffractions. Below 116 K some superlattice reflections appeared at the same position (h/2, k/2, 0) as in other members of Rb 2 ZnCl 4 group crystals, where h and k denote the Miller indices in the normal phase (space group P m c n ). The temperature dependence of the integrated intensity of the superlattice reflection changed below 78 K. However, the anomaly around 50 K reported previously was not observed. Below 116 K an incommensurate phase as reported for K 2 ZnCl 4 was not detected. The space group of phase between 78 and 116 K was directly confirmed to be C 1 c 1 by the verification of extinction rules of X-ray diffraction.

Thermal Properties on Single Crystals of Ferroelectric Barium Dititanate

Single crystals of ferroelectric BaTi2O5 were grown by a floating zone method. Heat capacities of the crystals were precisely measured over a wide temperature range of 2–800 K. The crystals show successive heat-anomalies consisting of a λ -type anomaly and a broad hump. For the λ -type anomaly, the excess specific heat Δ C p reaches 2.4 Jmol− 1K− 1 at the ferroelectric Curie temperature of T C = 726 K.; the transition enthalpy Δ H and transition entropy Δ S were estimated to be 80 Jmol−1 and 0.11 Jmol− 1K−1, respectively. The broad hump shows a maximum at approximately T g = 540 K and a symmetrical shape spreading over a wide temperature range of 450–650 K. Moreover, a sharp hump was observed in the C p /T 3 vs. T curve at 23 K. Some discussions are presented about these anomalies and causes of the ferroelectric phase transition.