Takehiko Yagi

New phases of C60 synthesized at high pressure

The fullerene C60 can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C60 on reheating to 300�C at ambient pressure. For synthesis temperatures between 300� and 400�C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter ao = 13.6 angstroms. When treated at 500� to 800�C at the same pressure, C60 transforms into a rhombohedral structure with hexagonal lattice parameters of ao = 9.22 angstroms and co = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C60 molecules are linked.

Thermoelastic properties of MgSiO3 perovskite determined by in situ X ray observations up to 30 GPa and 2000 K

In situ X ray experiments on MgSiO3 perovskite at pressures of 21–29 GPa and temperatures of 300–2000 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. The thermal expansion at 25 GPa up to 2000 K was determined from interpolation of the P-V-T data obtained in the present study. The 95% confidence level was estimated by taking all possible errors into account, including statistical error and systematic error caused by uncertainty of pressure scales, etc. The thermal expansivity at 25 GPa is expressed as αT,25 = a25 + b25T − c25T2 with the best-fit parameters of a25 = 2.11 × 10−5 K−1, b25 = 1.80 × 10−9 K−2, and c25 = 1.93 K. The equation of state of MgSiO3 perovskite has been determined using our new data combined with the lower-pressure data of Wang et al. [1994] and Utsumi et al. [1995]. The optimal set of parameters of the third-order Birch-Murnaghan equation of state, which is expressed as P = (3/2)KT,0[(VT,0/V)7/3 − (VT,0/V)5/3]{1 − (3/4)(4 − KT,0′)[(VT,0/V)2/3 − 1]}, where KT,0 = K300,0 + (∂KT,0/∂T)P(T − 300), KT,0′ = K300,0′, VT,0 = V0exp ∫300TαT,0dT, and αT,0 = a0 + b0T − c0T−2, is K300,0 = 261 GPa, K300,0′ = 4, a0 = 1.982 × 10−5 K−1, b0 = 0.818 × 10−8 K−2, c0 = 0.474 K, and (∂KT,0/∂T)P = −0.0280 GPa/K. The reliability of the result is discussed in detail.

Structure and crystal chemistry of perovskite-type MgSiO3

Synthetic clinoenstatite (MgSiO3) has been converted to a single phase with the perovskite structure in complete reactions at approx. 300 kbar in experiments that utilize the laser-heated diamond-anvil pressure apparatus. The structure of this phase after quenching was determined by powder X-ray diffraction intensity measurement to be similar to that of the distorted rare-earth, orthoferrite-type, orthorhombic perovskites, but it is suggested that such distortion from ideal cubic perovskite would diminish at high pressure.The unit cell dimensions and density of perovskite-type MgSiO3 at ambient conditions (1 bar, 25°C) are a=4.780(1) Å, b=4.933(1) Å, c=6.902(1) Å, V=162.75 Å3, and ρ=4.098(1) g/cm3. This phase is 3.1% denser than the isochemical oxide mixture [periclase (MgO)+stishovite (SiO2)]. The small crystal-field stabilization energy of the cation site in the perovskite structure may play an important role in limiting the high-pressure stability field of perovskites that contain transition metal cations. Approximate calculations of the crystal-field effects indicate that perovskite of pure FeSiO3 composition may become stable at 400–600 kbar; pressures greater than 800 kbar would be required to stabilize CoSiO3 or NiSiO3 perovskite.

Direct determination of coesite- stishovite transition by in-situ X-ray measurements

The coesite—stishovite transition was determined over the temperature range 500–1100° C by means of in-situ X-ray measurements with NaCl as an internal pressure standard. A cubic-anvil type of high-pressure apparatus and a high-power X-ray diffraction system were used for this study. Based on Deckers new pressure scale (1971), the coesite—stishovite transformation curve was represented by the linear equation P(kbar) = (80 ± 2) + (0.011 ± 0.003)(° C). Both the transition pressure at 1000° C and the slope dP/dT obtained in the present investigation are a little smaller than the previous determinations by the quenching method. Comparison with the thermodynamic data suggests that some uncertainty is still involved in the determination of the entropy of stishovite.

Experimental determination of thermal expansivity of several alkali halides at high pressures

Abstract Unit-cell volumes of four alkali halides, LiF, NaF, KF and CSCl, were measured to 90 kbar and 800°C using X-ray powder diffraction techniques. NaCl was used as an internal pressure standard. Experimental results were analyzed based on Deckers equation of state for NaCl and thermal expansivities of these four materials were determined as a function of pressure. Volume dependence of thermal expansivity is different for the NaCl and CsCl structures. Comparisons of the present results with theoretical calculations by Birch and Anderson are presented.

In situ measurements of the phase transition boundary in Mg3Al2Si3O12: implications for the nature of the seismic discontinuities in the Earth’s mantle

Abstract Here we report the phase boundary of pyrope garnet (Mg3Al2Si3O12) to Al-bearing silicate perovskite plus corundum, with the highest transition pressure determined by in situ measurements in a multi-anvil apparatus at high temperature. The consistency of the pressure scales by different standards of Au, NaCl, Pt, W, and Mo at high temperature was also evaluated by in situ X-ray measurements. Our results, together with recent in situ measurements of the post-spinel transition in Mg2SiO4 [Irifune et al., Science 279 (1998) 1698–1700] and the ilmenite–perovskite transition in MgSiO3 [Ono et al., Geophys. Res. Lett. (2000) submitted], show that pressures determined in conventional quench experiments [Ito and Takahashi, J. Geophys. Res. 94 (1989) 10637–10646] could have been overestimated by more than 2 GPa at pressures corresponding to the bottom of the transition zone. On the basis of the in situ measurements, the post-spinel transition occurs at a depth (∼600 km) that is too shallow to match with the 660-km seismic discontinuity in the Earth’s mantle. Therefore, an olivine dominant mantle compositional model may be inconsistent with the seismic observations. Alternatively, we propose a pyroxene–garnet dominant transition zone with an appropriate Al2O3 content (ca. 6–8 mol%), in which majorite garnet transforms to perovskite at the depth of the 660-km discontinuity. Any alternative models would have to consider chemical stratification in the mantle.

High-pressure and high-temperature phase relations in CaSiO3 and CaMgSi2O6 and elasticity of perovskite-type CaSiO3

Abstract Phase transformations in CaSiO 3 and CaMgSi 2 O 6 were studied up to ∼ 50 GPa using both high-pressure in situ X-ray diffraction study and quench experiments. A new intermediate high-pressure and high-temperature phase named CaSiO 3 -III was found in the pressure range from 10 to 13.8 GPa, above which a perovskite-type CaSiO 3 was stable. Both CaSiO 3 -III and perovskite-type CaSiO 3 are unquenchable. The compression curve of perovskite-type CaSiO 3 was determined up to 31.5 GPa at room temperature. Least-squares fitting of the Birch-Murnaghan equation of state gave the following values for the bulk modulations and a zero-pressure volume: K 0 = 325 ± 10 GPa and V 0 = 45.58 ± 0.07 A 3 where d K 0 d P was assumed to be 4. This bulk modulus, however, is very sensitive to the choice of the zero-pressure volume and has large uncertainty. CaMgSi 2 O 6 was found to break down into CaSiO 3 and MgSiO 3 at 17 GPa. CaSiO 3 crystallizes into cubic perovskite-type structure while MgSiO 3 breaks down further into Mg 2 SiO 4 plus SiO 2 . With increasing pressure, the last transforms into ilmenite, and then into perovskite structure, while the coexisting perovskite-type CaSiO 3 remains unchanged. Solubilities of both Ca and Mg in MgSiO 3 and CaSiO 3 perovskites are found to be very low.

High pressure and high temperature in situ X‐ray observation of MgSiO3 Perovskite under lower mantle conditions

High pressure and high temperature in situ X-ray observations of MgSiO3 perovskite were carried out using a newly developed Drickamer-type apparatus combined with synchrotron radiation. Variations of the unit cell parameters of the perovskite were observed as a function of temperature up to about 1900 K at 36 GPa. The orthorhombic distortion changed very little and no indication of transformations to higher symmetry phases was observed up to 1900 K. This result indicates that the orthorhombic perovskite is the stable phase of MgSiO3 in the most part of the lower mantle. The thermal expansivity of the perovskite is determined to be larger than 1.7(2) × 10−5 /K at 36 GPa and room temperature. This value will give an important constraint to the chemical composition of the lower mantle.

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.

Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy study

Natural pyrope garnets with three different chemical compositions have been transformed at 30–60 GPa in the laser-heated diamond-anvil cell (DAC). Recovered samples were examined by analytical transmission electron microscopy (ATEM). For all three pyropes, the dominant post-garnet phase was Mg-rich orthorhombic silicate perovskite. The Al content of Mg-perovskite increased significantly with increasing pressure and temperature, and its chemical composition became close to that of the starting material. Mg-perovskite with Al2O3 content less than 25 mol% was quenched as a single phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al2O3 content of 25–28 mol% transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al2O3 content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on {1012} at ambient conditions. The high Al content may also induce the stabilization of Fe3+ in the perovskite structure accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase (the NAL phase) were recognized with Mg-perovskite mostly at lower pressures. The NAL phase is close to M3Al4Si1.5O12 (M=Mg, Fe, Ca, Na, K), i.e., the middle of A2O3 and AB2O4 type compounds, and is accompanied by stishovite. The electron diffraction patterns are consistent with the space group P63/m or P63 with a=8.85 (2) A, c=2.78 (2) A, V=188 (1) A3. The existence of large cations such as Ca, K, and Na seems to stabilize the NAL phase relative to the corundum phase, but excess Ca and Na contents are likely to induce the formations of Ca-rich cubic perovskite and the calcium ferrite phase, respectively.