Keiji Kusaba

Baddeleyite-Type High-Pressure Phase of TiO2

A high-pressure phase of TiO2, which had been observed by shock-wave experiments and remained unresolved, has been studied by in situ x-ray diffraction. The single phase was formed at 20 gigapascals and 770�C with the use of sintered-diamond multianvils; it has the same structure as baddeleyite, the stable phase of ZrO2 at ambient conditions. The coordination number of Ti increases from six to seven across the rutile to baddeleyite transition, and the volume is reduced by approximately 9 percent.

Shock-induced phase transition of M2O3 (M = Sc, Y, Sm, Gd, and In)-type compounds

Abstract Sintered specimens of cubic Sc2O3-type structure (C-type, Ia 3 ), Sc2O3, Y2O3, Sm2O3, and In2O3, and monoclinic Sm2O3-type structure (B-type, C2 m ) Sm2O3, and powdered specimens of C-type Gd2O3 were shock-loaded to 2–50 GPa using flyer plates accelerated by a propellant gun. Recovered specimens were studied by X-ray powder diffraction analysis at room and high temperature. Y2O3 began to transform to the B-type structure above 12 GPa, and the transformation was completed at 20 GPa. Gd2O3 also transformed to the B-type structure, but residual temperature effects were observed in the yield and crystallinity of the high pressure phase. In2O3 transformed to the corundum structure ( R 3 c ) in the pressure range of 15–25 GPa, although the yield was very small. Shock-induced phase transition from the C-type to the B-type structure was inferred to proceed via hexagonal La2O3-type structure (A-type, P 3 m1 ), in comparison with static high pressure experiments.

Shock behavior of zircon: phase transition to scheelite structure and decomposition

Abstract Both single-crystal and powdered specimens of zircon (ZrSiO4) were shocked to peak pressures between 30 and 94 GPa using the gun method, and specimens recovered were studied by means of X-ray diffraction analysis, transmission electron microscopy and infrared spectroscopy. Transformation to the scheelite structure started above 30 GPa, and was completed above 53 GPa in the case of single crystal specimens. Tetragonal unit cell parameters of the scheelite type ZrSiO4 at room condition are measured to bea = 4.7341(1)A, c = 10.51(1)A, c/a = 2.219(2) andV = 235.5(2)A3, which is smaller than that of the zircon type by 9.9%. The recovered scheelite-type ZrSiO4 reverts to the zircon type after rapid heating to 1200°C at room pressure. This transformation from the zircon type to the scheelite type is unique in that it is fast, displacive-like, but does not reverse. Tetragonal ZrO2 was detected as decomposition product in the single-crystal specimen shocked to 94 GPa, and further confirmed in a powdered specimen shocked to 53 GPa where enhancement of temperature is expected because of high porosity. Decomposition behavior of zircon observed in natural shock events is discussed on the basis of present experimental results.

Ferromagnetism and large negative magnetoresistance in Bi1−xCaxMnO3 (x ≥ 0.8) perovskite

Abstract Polycrystalline perovskite manganites Bi 1−x Ca x MnO 3 (x ≥ 0.8) have been prepared by standard ceramic technique, and their magnetic and electronic property are examined down to 5 K. Ferromagnetic moment and reduced resistivity are observed for Bi 1−x Ca x MnO 3 (x ≥ 0.875), and also large negative magnetoresistances appear below T ′ c .

Crystal Structures of YBa2Cu3-δAδO9-γ (A=Ni, Zn and Co)

A structural study of orthorhombic YBa2Cu3O6.94, YBa2Cu2.8Ni0.2O6.85 and YBa2Cu2.7Zn0.3O6.75, and tetragonal YBa2Cu2CoO7.16 by means of X-ray and time-of-flight type (TOF) neutron diffractometers was carried out to determine the locations of the dopant atoms, Ni, Zn and Co, in the 1-2-3 (Y/Ba/Cu) oxide. The nickel atoms occupy only the Cu2 site 2q(00z) with z=0.36. The zinc atoms occupy the Cu1 site 1a(000) and the Cu2 site with occupancies of 0.20(5) and 0.05(2). In the tetragonal YBa2Cu2CoO7.16, cobalt atoms were found at both the Cu1 and Cu2 sites with occupancies of 0.83(2) and 0.08(1).

Structural considerations on the mechanism of the shock-induced zircon-scheelite transition in ZrSiO4

Abstract The crystal structure of the scheelite-type ZrSiO 4 , which was transformed from the zircon-type under shock compression, was studied using an X-ray powder diffraction analysis. Positional parameters of O 2- in the 16 f site are determined to be u = 0.28, v = 0.14 and w = 0.07 (space group, I4 1 /a). The X-ray powder diffraction lines were broad, indicating that the scheelite-type ZrSiO 4 has considerable residual strain in the crystal. The mechanism of this shock-induced phase transition is discussed in terms of the displacive mechanism, where the [110] direction of the zircon-type is converted to the [001] direction of the scheelite-type. This model can explain why this zircon-scheelite transition occurs so fast under shock compression.

Structural study on high-Tc superconductor Bi2−x(Ca, Sr)3Cu2+xO9−y

The crystal structure of the high-Tc Bi2-x(Ca, Sr)3Cu2+xO9-y with x=0.2 and y=0.78 was studied by means of neutron and X-ray powder diffraction. Since the unit cell parameters were relatively large for powder diffraction analysis, an averaged structure was refined, assuming an orthorhombic sub-cell (a=5.39 A, b=5.39 A and c=30.37 A) which corresponds to 1/5 of the unit cell: This is a derivative of Bi4Ti3O12. The double perovskite unit interleaved by the double bismuth layers is B-type, i.e. a CuO6 octahedron is surrounded by eight alkaline earth metal ions. The pyramidal CuO5 configuration is found in this oxide. The copper valency is estimated at about 2.3 on the basis of the determined chemical composition.

Anisotropic phase transition of rutile under shock compression

Shock recovery experiments for single crystal and powdered specimens of TiO2 with the rutile structure were performed in the pressure range up to 72 GPa. Single crystal specimens were shocked parallel to [100], [110] and [001] directions. X-ray powder diffraction analysis showed that the amount of α-PbO2 type TiO2 produced by shock-loading depended strongly on the shock propagation direction. The maximum yield (about 70%) was observed for shock loading to 36 GPa parallel to the [100] direction. In the [001] shock direction, the yield is much smaller than that of the [100] direction. This anisotropic yield was consistent with the observed anisotropy of the phase transition pressure in shock compression measurements. However, transformation to the α-PbO2 type cannot explain the large volume change observed above about 20 GPa. On the basis of the high pressure behavior of MnF2, we assumed that the high pressure phase was either fluorite or distorted fluorite type and that the phase conversion to the α-PbO2 type was induced spontaneously in the pressure reduction process.We present a displacive mechanism of phase transition under shock compression from the rutile structure to the fluorite structure, in which the rutile [100] is shown to correspond to the fluorite [001] or [110] and the rutile [001] to the fluorite [110]. Direct evidence is obtained by examining the [100] shocked specimen by high resolution electron microscopy.

Yielding and phase transition under shock compression of yttria-doped cubic zirconia single crystal and polycrystal

Inclined‐mirror Hugoniot measurements of yttria (Y2O3) ‐doped (9.6 and 8.0 mol %) cubic zirconia single crystal and polycrystal were performed in the pressure range up to 120 GPa to study yielding and phase transition. The Hugoniot‐elastic limit (HEL) stresses parallel to the 〈100〉 and 〈110〉 axes were approximately 14 and 25 GPa, respectively, while that of the polycrystal was approximately 13 GPa. Above the HELs the Hugoniot data parallel toward the 〈100〉 and 〈110〉 axes converged on each other, and showed large relief to an isotropic compression state, while those of the polycrystal preserved a considerably larger shear strength. A phase transformation took place at approximately 53 GPa (both 〈100〉 and 〈110〉 axis directions), and was completed by about 70 GPa. The phase transition pressure was much higher than those of the monoclinic‐ or tetragonal‐orthorhombic II phase transitions under static compression. The shock velocity Us versus particle velocity Up relation of the final phase of the single crysta...

STRUCTURE OF FeS UNDER HIGH PRESSURE

Abstract Compression behavior of FeS at room temperature up to 15 GPa is investigated using a combination of a synchrotron white X-ray source and a large volume press (MAX80). Two high pressure phase transitions at 3 and 7 GPa are reconfirmed. The high pressure phase between 3 and 7 GPa can be explained by a MnP type related structure. From 26 diffraction lines at 10.30 GPa, unit cell dimensions of the higher pressure phase, of which the crystal structure has been unknown, are determined to be a = 8.044(3) A, b = 5.611(2) A, c = 6.433(4) A, β = 93.11(4)° and Z = 12 for a monoclinic system. Possible space group for the cell is P2 1 or P2 1 /m, and there are multi-sites for iron atoms in the cell. The cell dimensions show that the higher pressure phase still has a NiAs type framework in the structure. The high pressure phase is about 6% denser than the MnP type phase at 7 GPa, and is estimated to be 12% denser than the troilite phase at ambient condition.