Akihito Ohtani

High-pressure X-ray diffraction studies on β- and γ-Mg2SiO4

Abstract Pressure effects on the lattice parameters of β- and γ-Mg 2 SiO 4 have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes ( ΔV / V 0 ) at 90 kbar are 5.4 · 10 −2 and 4.2 · 10 −2 for β- and γ-Mg 2 SiO 4 , respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg 2 SiO 4 , respectively. The α → γ transition obeys Wangs linear V φ − ρ relation but the α → β transition does not.

Fixed points for pressure calibration above 100 kbars related to semiconductor‐metal transitions

Pressures assigned to the semiconductor‐metal transitions in ZnTe, ZnS, GaAs, and GaP have been determined by detecting the electrical resistance change of the semiconductors while the lattice parameter of standard material NaCl was simultaneously measured with x‐ray diffraction techniques. A pressure vessel of the split‐octahedron type and various pressure‐transmitting media have been employed. Pressures were estimated according to the equation of state for NaCl proposed by Decker. The transition pressures, 1296 kbars for ZnTe, 1557 kbars for ZnS, 1888 kbars for GaAs, and 25310 kbars for GaP, constitute fixed points for pressure calibration above 100 kbars at room temperature.

Polymorphism of ZnTe at elevated pressure

Abstract Simultaneous measurements of the electrical resistance and X-ray diffraction under pressure have revealed that ZnTe exhibits two subsequent structural transitions at approximately 85 and 130 kbar, accompanied by a conspicuous rise and drop in the resistance, respectively.

Multi-Anvil Apparatus for High Pressure X-Ray Diffraction

An octahedral multi-anvil apparatus has been developed for the study of X-ray diffraction under high pressure. High pressures up to 150 kbar can be generated in the centre of split cones when they move together as external load is applied upon the outer surfaces. The X-ray beam is focused to the specimen through a horizontal plane between the two cones. Standard Debye-Scherrer camera method is utilized. The pressures required for the transitions of bismuth I-II, bismuth V-VI, tin I-II and lead I-II are determined from the lattice parameter of sodium chloride.

The electrical properties of HgTe and HgSe under very high pressure

Abstract The electrical properties of HgTe and HgSe have been investigated at pressures up to 200 kbar in an octahedral apparatus. Measurements of the electrical resistivity at room temperature showed that, beyond the well-known transition from the semimetallic to semiconductive state, both become metallic, at 84 kbar and 155 kbar, respectively. The energy gap at various fixed pressures was obtained from the resistance-temperature relationships. The energy gap of semiconducting HgTe decreases monotonically with pressure, the coefficient being −l.53 × 10−5 eV bar . The energy gap of HgSe is rather insensitive to pressures up to 75 kbar, above which it decreases continuously ( dE dP = −1.59 × 10−5 eV bar ) before vanishing around 150 kbar. At high pressures the temperature coefficient of the resistance in the metallic state is 3.25 ~ 4.70 × 10 −3 deg for HgTe, and 5.7 ~ 5.9 × 10 −3 deg for HgSe.

Pressure apparatus of split-octahedron type for x-ray diffraction studies.

An apparatus of split-octahedron type has been developed for the study of x-ray diffraction at high pressure. The apparatus consists of a cylinder, a sphere, and an octahedron, all split up and assembled in a multistaged arrangement. Three ways of splitting the innermost octahedron are utilized in matching the cell geometry to the standard camera method. High pressures in excess of 220 kilobars can be generated in the center of the split octahedron by using a composite mixture of diamond powder and epoxy resin as a pressure-transmitting medium. The pressures required for the semiconductor-to-metal transitions of ZnTe, ZnS, GaAs, and GaP are determined by simultaneously monitoring the electrical resistance of the semiconductors and the lattice parameter of sodium chloride.

Reversibility of Phase Transition of Antiferromagnetic CoCl2·6H2O under Hydrostatic Pressure

The hydrostatic pressure dependence of the Neel temperature, magnetic phase diagram and crystal structure of the antiferromagnetic CoCl 2 ·6H 2 O ( T N =2.29 K) have been studied by measurements of magnetic heat capacity, susceptibility and X-ray analysis. The anisotropy field H A ( p ) and the exchange constant J ( p ) in the two-dimensional a - b plane have been found to depend linearly on pressure p as H A ( p )= H A ( p 0 ) (1+0.24 p ) and J ( p )= J ( p 0 ) (1+0.031 p ) in the range p 0 (∼0.001 kbar)< p <6 kbar. The Neel temperature T N ( p ) has been also revealed to depend linearly on p and to be reversible in the process of \(p_{0}{\rightleftarrows}p\). The appearance of rounding of heat capacity peak at T N ( p ), however, has been found irreversible in the process of compression in the critical temperature region e (=| T - T N | T N )<2×10 -2 . The effect of variation of magnetic dimensionality on phase transition has been discussed considering these magnetic data and pressure dependence of atom...

Effects of hydrostatic pressure on phase transition of some low-dimensional systems

Abstract The possibility of controlling the dimensionality of magnetic interactions and spin symmetry has been undertaken by applying the hydrostatic pressure to magnetic compounds. the Neel temperature, magnetic phase diagram, exchange constant and anisotropy field have been obtained as a function of the applied hydrostatic pressure in CoCl2·6H2O.