Wataru Utsumi

The Phase Boundary Between α- and β-Mg2SiO4 Determined by in Situ X-ray Observation

The stability of Mg2SiO4, a major constituent in the Earths mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800� to 1600�C. The phase stability of α-Mg2SiO4 and β-Mg2SiO4 was determined by taking account of the kinetic behavior of transition. The phase boundary between α-Mg2SiO4 and β-Mg2SiO4 is approximated by the linear expression P = (9.3 � 0.1) + (0.0036 � 0.0002)T where P is pressure in gigapascals and T is temperature in degrees Celsius.

Post-spinel transition in Mg2SiO4 determined by high P - T in situ X-ray diffractometry

Abstract The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P–T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22xa0GPa in the temperature range from 1550 to 2100xa0K, which is 1–1.5xa0GPa lower than the 660xa0km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from −2 to −0.4xa0MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection.

Unquenchable high-pressure perovskite polymorphs of MnSnO3 and FeTiO3

New high-pressure orthorhombic (GdFeO3-type) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) Å, c=7.690 (8) Å and V= 221.99 (15) Å3. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) Å, b=5.169 (5) Å, c=7.239 (9) Å and V= 187.94 (36) Å3. Based on published data on the quench phases, the FeTiO3 perovskite breaks down to a rocksalt + baddelyite mixture of “FeO” and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.

Thermal expansivity of MgSiO3 perovskite under high pressures up to 20 GPa

Volume measurement of MgSiO3 perovskite was made in the temperature range from 25 to 500°C as a function of pressure up to 20 GPa by in situ x-ray diffraction, using a DIA apparatus combined with synchrotron radiation. The measured thermal expansivity ranges from 1.8 to 2.5*10−5 /K and decreases only slightly with pressure. Our present results are in good agreement with the previous lower pressure (up to 11 GPa) data of Wang et al. [1994] and the higher pressure (36 GPa) data of Funamori and Yagi [1993], but contrast with the results using diamond anvil cell by Mao et al. [1991], particularly in a low pressure range.

High‐pressure generation by a multiple anvil system with sintered diamond anvils

High‐pressure experiments using a multiple anvil high‐pressure system with sintered diamond anvils are presented and discussed. Pressures in excess of 41 GPa were obtained, on the basis of the lattice constants of gold determined by the in situ x‐ray diffraction technique using synchrotron radiation.

A cylindrical furnace with homogeneous temperature distribution for use in a cubic high‐pressure press

A cylindrical furnace with a homogeneous temperature distribution was designed for use in a cubic press by using the difference method. This method consists of three tubes and is applicable to the very small sample chamber of high‐pressure apparatus. The temperature distribution in the furnace thus designed was measured at high pressure and temperature using two methods: (1) by measuring the thermal expansion of NaCl contained in the sample chamber; and (2) by observing the area of molten Pb which was mixed with the NaCl. Both of these measurements were performed by high‐pressure and temperature in situ x‐ray techniques. It was confirmed from these experiments that the furnace has a homogeneous temperature distribution and the difference in temperature is less than 5u2009°C when the center is about 600u2009°C. The homogeneity in temperature is nearly constant, even when the thermal conductivity of the sample is decreased by two orders of magnitude.

Formation and structure of iron hydride under the condition of the Earth’s interior

High pressure and high temperature in situ X‐ray diffraction studies on the systems of Fe‐H and Fe‐MgSiO3‐H2O were carried out to investigate the effect of water on the property of iron under the condition of the Earth’s interior. It was clarified that when water is added to the mixture of Fe and MgSiO3, then both FeHx and FeO are formed. Resulting FeHx has much lower melting temperature and has smaller density compared to those of pure iron. This reaction may affect a lot to the chemical composition and the formation process of the core.

Optically detected magnetic resonance measurements for amorphous silicon treated by hydrostatic pressure at 5 GPa

The origin of photoluminescence quenching due to the application of hydrostatic pressure on a-Si:H has been elucidated by the optically detected magnetic resonance (ODMR) measurements. Hydrostatic pressure of ∼ 5 GPa was applied on a sample by a wedge type cubic anvil apparatus, and after the release of pressure the ODMR measurement was carried out. As a result, it has been found that the quenching signal due to dangling bonds was increased by pressure. This result has also been confirmed by electron spin resonance (ESR) measurement. Photo-induced defect creation was examined for the sample treated by pressure. Further increase of dangling bond density has been observed. These results are discussed on the basis of a model that both of the photo-induced and pressure-induced defect creation are related to void and microvoid in the samples which contain Si-H bond cluster on their internal surface.

Stability field of the orthorhombic perovskite type of MgSiO3

Stability field of the orthorhombic perovskite type of MgSiO3 was studied using an in situ x‐ray observation technique. It was revealed that the orthorhombic perovskite structure is stable from 24 GPa to 36 GPa below 1900 K. This result indicates that the orthorhombic perovskite is the dominant phase of MgSiO3 throughout the lower mantle.

Electrical conductivity and crystal structure of iron hydride under high pressure and high temperature

Electrical resistance and crystal structure of iron hydride were observed simultaneously under high pressure and high temperature using multi‐anvil type of high pressure apparatus. Various new phases of iron hydride were observed up to 1200u2009°C and 6 PGa. It was found that the temperature dependence of electrical resistance differs significantly from that of iron.