Ho-kwang Mao

Specific volume measurements of Cu, Mo, Pd, and Ag and calibration of the ruby R1 fluorescence pressure gauge from 0.06 to 1 Mbar

The wavelength shift with pressure of the ruby R1 fluorescence line (Δλ) has been calibrated in the diamond‐window pressure cell from 0.06 to 1 Mbar. This was done by simultaneously making specific volume measurements of four metals (Cu, Mo, Ag, and Pd) and referring these results to isothermal equations of state derived from shock‐wave experiments. The result is P (Mbar) = (19.04/5) {[(λ0+Δλ)/λ0]5−1}, where λ0 is the wavelength measured at 1 bar.

Coesite-bearing eclogite from the Dabie Mountains in central China

Coesite and Coesite pseudomorphs are recognized in eclogite from the Dabie Mountains, a collision zone between the Sino-Korean and Yangtze cratons in central China. This third known occurrence of Coesite in deep crustal rocks is within an Archean gneiss terrane and is the first locality where Coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location. Coesite-bearing eclogite is estimated to form at pressures of >28 kbar and temperatures from 600 to 710 °C. The Dabie Mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation. The preservation of Coesite in eclogite is found to be strongly controlled by the pressure-temperature-time path that coesite- bearing eclogite followed during its uplift and retrograde metamorphism.

Origin of morphotropic phase boundaries in ferroelectrics.

A piezoelectric material is one that generates a voltage in response to a mechanical strain (and vice versa). The most useful piezoelectric materials display a transition region in their composition phase diagrams, known as a morphotropic phase boundary, where the crystal structure changes abruptly and the electromechanical properties are maximal. As a result, modern piezoelectric materials for technological applications are usually complex, engineered, solid solutions, which complicates their manufacture as well as introducing complexity in the study of the microscopic origins of their properties. Here we show that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions, but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. Our results show that the high electromechanical coupling in solid solutions with lead titanate is due to tuning of the high-pressure morphotropic phase boundary in pure lead titanate to ambient pressure. We also find that complex microstructures or compositions are not necessary to obtain strong piezoelectricity. This opens the door to the possible discovery of high-performance, pure-compound electromechanical materials, which could greatly decrease costs and expand the utility of piezoelectric materials.

The fate of subducted basaltic crust in the Earth's lower mantle

The subduction of oceanic lithosphere into the Earths deep interior is thought to drive convection and create chemical heterogeneity in the mantle. The oceanic lithosphere as a whole, however, might not subduct uniformly: the fate of basaltic crust may differ from that of the underlying peridotite layer because of differences in chemistry, density and melting temperature. It has been suggested that subducted basaltic crust may in fact become buoyant at the mantles 660-km discontinuity, remaining buoyant to depths of at least 800 km, and therefore might be gravitationally trapped at this boundary to form a garnetite layer, . Here we report the phase relations and melting temperatures of natural mid-ocean ridge basalt at pressures up to 64 GPa (corresponding to ∼1,500 km depth). We find that the former basaltic crust is no longer buoyant when it transforms to a perovskitite lithology at about 720 km depth, and that this transition boundary has a positive pressure–temperature slope, in contrast to the negative slope of the transition boundary in peridotite. We therefore predict that basaltic crust with perovskitite lithology would gravitationally sink into the deep mantle. Our melting data suggest that, at the base of the lower mantle, the former basaltic crust would be partially molten if temperatures there were to exceed 4,000 K.

Quasi-hydrostatic compression of magnesium oxide to 52 GPa: Implications for the pressure-volume-temperature equation of state

Room temperature static compression of MgO (periclase) was performed under nearly hydrostatic conditions using energy dispersive synchrotron X-ray diffraction in a diamond anvil cell with methanol-ethanol (to 10 GPa) or helium (to 52 GPa) as a pressure-transmitting medium. Highly precise cell parameters were determined with an average relative standard deviation 〈Δa/a〉 = 0.0003 over all the experimental pressure range. Fixing the bulk modulus K0T = 160.2 GPa, a fit of the data to the third-order Birch-Murnaghan equation of state yields: V0 = 74.71±0.01 A3, (∂K0T/∂P)T = 3.99±0.01. A fit of different P-V-T datasets, ranging to 53 GPa and 2500 K, to a Birch-Murnaghan-Debye thermal equation of state constrained the Gruneisen parameter γ0 = 1.49±0.03, but not its volume dependence q, which was constrained to 1.65±0.4 by thermodynamic theory. A model based on a constant value of q cannot explain the ultrahigh pressure (P = 174–203 GPa) shock compression data. We developed a model in which q decreases with compression from 1.65 at 0.1 MPa to 0.01 at 200 GPa. This model, within the framework of the Mie-Gruneisen-Debye assumptions, satisfactorily describes the low-pressure static data (〈ΔV/V〉 = 0.4% to 53 GPa) and the high-pressure Hugoniot data (〈ΔV/V〉 <1% to 203 GPa). Average values of the thermal expansion coefficient α range between 14.1±2.8 and 16.3 ± 2.7 × 10−6 K−1 at P = 174–203 GPa. The pressure dependence of the melting temperature yields an initial pressure derivative ∂Tm/∂P = 98 K/GPa. Our analysis shows that it is possible to develop a simple model of the volume dependence of the Gruneisen parameter that can successfully describe the P-V-T equation of state of MgO from ambient conditions to 203 GPa and 3663 K.

Effect of pressure, temperature, and composition on lattice parameters and density of (Fe,Mg)SiO3‐perovskites to 30 GPa

High-pressure, high-temperature properties of MgSiO3, (Fe0.1Mg0.9)SiO3, and (Fe0.2Mg0.8)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation. The first direct measurements of unit cell distortions and equation-of-state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained. The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite. The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c. Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions. The 298-K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment. The zero-pressure bulk modulus KT0 = 261 (±4) GPa with its pressure derivative KT0′ = 4 is close to that determined in previous static high pressure measurements. The thermal expansion obtained from the high P - T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence. The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is −6.3(±0.5)×10−2 GPa/K. Analyses of the high-temperature data give a value for the Anderson-Gruneisen parameter δT of 6.5–7.5, which is significantly higher than that used in recent lower mantle models.

Spin transition of iron in magnesiowüstite in the Earth's lower mantle.

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earths interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by ∼35 per cent and bulk sound velocity increases by ∼15 per cent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.

Melting and crystal structure of iron at high pressures and temperatures

High-pressure melting, phase transitions and structures of iron have been studied to 84 GPa and 3500 K with an improved laser heated diamond anvil cell technique and in situ high P-T x-ray diffraction. At pressures below 60 GPa, the lower bound on the melting curve is close to those measured by Boehler [1993] and Saxena et al. [1993]; however, at pressures above 60 GPa our data indicate melting at higher temperatures than these studies, but still lower than the melting curve of Williams et al [1990]. The e-γ-1 triple point is 60(±5) GPa and 2800(±200) K, based on our data of the e-γ phase transition and the observation of melting by in situ x-ray diffraction. No solid phases other than e-Fe and γ-Fe were observed in situ at high temperatures (>1000 K) and pressures to 84 GPa. However, the diffraction patterns of temperature quenched products at high pressure can be fit to other structures such as dhcp.

Analysis of lattice strains measured under nonhydrostatic pressure

The equations for the lattice strains produced by nonhydrostatic compression are presented for all seven crystal systems in a form convenient for analyzing x-ray diffraction data obtained by newly developed methods. These equations have been used to analyze the data on cubic (bcc α-Fe) and hexagonal (hcp e-Fe) systems. The analysis gives information on the strain produced by the hydrostatic stress component. A new method of estimating the uniaxial stress component from diffraction data is presented. Most importantly, the present analysis provides a general method of determining single crystal elastic constants to ultrahigh pressures.

Elasticity and rheology of iron above 220 GPa and the nature of the Earth's inner core

Recent numerical-modelling and seismological results have raised new questions about the dynamics, and magnetism, of the Earths core. Knowledge of the elasticity and texture of iron, at core pressures is crucial for understanding the seismological observations, such as the low attenuation of seismic waves, thelow shear-wave velocity, and the anisotropy of compressional-wave velocity. The density and bulk modulus of hexagonal-close-packed iron have been previously measured to core pressures by static and dynamic, methods. Here we study,using radial X-ray diffraction and ultrasonic techniques, the shear modulus, single-crystal elasticity tensor, aggregate compressional- and shear-wave velocities, and orientation dependence of these velocities in iron. The inner core shear-wave velocity is lower than the aggregate shear-wave velocity of iron, suggesting the presence of low-velocity components or anelastic effects in the core. Observation of a strong lattice strain anisotropy in iron samples indicates a large (∼24%) compressional-wave anisotropy under the isostress assumption, and therefore a perfect alignment of crystals would not be needed to explain the seismic observations. Alternatively the strain anisotropy may indicate stress variation due to preferred slip systems.