Biomedical subjects

Ho-kwang Mao

Publications and source records attributed to Ho-kwang Mao.

8 recordsLinked to original sources

PubMed2004

In situ high P-T Raman spectroscopy and laser heating of carbon dioxide.

In situ high P-T Raman spectra of solid CO(2) up to 67 GPa and 1,660 K have been measured, using a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system in diamond anvil cells. A metallic foil was employed to efficiently absorb the incoming Nd:YLF laser and heat the sample. The average sample temperature was accurately determined by detailed balance from the anti-Stokes/Stokes ratio, and was compared to the temperature of the absorber determined by fitting the thermal radiation spectrum to the Planck radiation law. The transformation temperature threshold and the transformation dynamics from the molecular phases III and II to the polymeric phase V, previously investigated only by means of temperature quench experiments, was determined at different pressures. The P-T range of the transformation, between 640 and 1,100 K in the 33-65 GPa pressure interval, was assessed to be a kinetic barrier rather than a phase boundary. These findings lead to a new interpretation of the high P-T phase diagram of carbon dioxide. Furthermore, our approach opens a new way to perform quantitative in situ Raman measurements under extremely high pressures and temperatures, providing unique information about phase relations and structural and thermodynamic properties of materials under these conditions.

Journal Article↗

PubMed2003

Natural occurrence and synthesis of two new postspinel polymorphs of chromite.

A high-pressure polymorph of chromite, the first natural sample with the calcium ferrite structure, has been discovered in the shock veins of the Suizhou meteorite. Synchrotron x-ray diffraction analyses reveal an orthorhombic CaFe2O4-type (CF) structure. The unit-cell parameters are a = 8.954(7) A, b = 2.986(2) A, c = 9.891(7) A, V = 264.5(4) A3 (Z = 4) with space group Pnma. The new phase has a density of 5.62 g/cm3, which is 9.4% denser than chromite-spinel. We performed laser-heated diamond anvil cell experiments to establish that chromite-spinel transforms to CF at 12.5 GPa and then to the recently discovered CaTi2O4-type (CT) structure above 20 GPa. With the ubiquitous presence of chromite, the CF and CT phases may be among the important index minerals for natural transition sequence and pressure and temperature conditions in mantle rocks, shock-metamorphosed terrestrial rocks, and meteorites.

Biophysical Phenomena↗

PubMed2003

Bonding changes in compressed superhard graphite.

Compressed under ambient temperature, graphite undergoes a transition at approximately 17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the pi-bonds between graphite layers convert to sigma-bonds, whereas the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, whereas the nonbridging carbon atoms remain unpaired with pi-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

Journal Article↗

PubMed2003

Raman spectroscopy of hot dense hydrogen.

High P-T Raman measurements of solid and fluid hydrogen to above 1100 K at 70 GPa and to above 650 K in 150 GPa range, conditions previously inaccessible by static compression experiments, provide new insight into the behavior of the material under extreme conditions. The data give a direct measure of the melting curve that extends previous optical investigations by up to a factor of 4 in pressure. The magnitude of the vibron frequency temperature derivative (dnu/dT)(P) increases by a factor of approximately 30 over the measured pressure range, indicating an increase in intrinsic anharmonicity and weakening of the molecular bond.

Journal Article↗

PubMed2003

Stability of magnesiowustite in Earth's lower mantle.

Magnesiowüstite [(Mg,Fe)O] is the second most abundant mineral of Earth's lower mantle. Understanding its stability under lower mantle conditions is crucial for interpreting the physical and chemical properties of the whole Earth. Previous studies in an externally heated diamond anvil cell suggested that magnesiowüstites decompose into two components, Fe-rich and Mg-rich magnesiowüstites at 86 GPa and 1,000 K. Here we report an in situ study of two magnesiowüstites [(Mg(0.39),Fe(0.61))O and (Mg(0.25),Fe(0.75))O] at pressures and temperatures that overlap with mantle conditions, using a laser-heated diamond anvil cell combined with synchrotron x-ray diffraction. Our results show that addition of Mg in wüstite (FeO) can stabilize the rock-salt structure to much higher pressures and temperatures. In contrast to the previous studies, our results indicate that Mg-rich magnesiowüstite is stable in the rock-salt structure in the lower mantle. The physical and chemical properties of magnesiowüstite should change gradually and continuously in the lower mantle, suggesting that it does not make a significant contribution to seismic-wave heterogeneity of the lower mantle. Stable Mg-rich magnesiowüstite in lowermost mantle can destabilize FeO in the core-mantle boundary region and remove FeO from the outer core.

Journal Article↗

PubMed2002

Nature of the high-pressure transition in Fe2O3 hematite.

We present a new method to separate the crystallographic and electronic phase transitions in hematite using x-ray emission spectroscopy and x-ray diffraction. Our observations, based on the behavior of a metastable high-pressure phase in the stability domain of the low-pressure phase, show that the electronic transition is preempted by the crystallographic transition. The former occurs only afterwards in the high-pressure phase, possibly as a result of a Mott transition. The idea that the electronic transition drives the transition in hematite is therefore invalidated. Such methods should help elucidate the mechanics and the driving forces behind a number of first-order high-pressure phase transitions.

Journal Article↗

PubMed2002

Superconductivity in dense lithium.

Superconductivity in compressed lithium is observed by magnetic susceptibility and electrical resistivity measurements. A superconducting critical temperature (Tc) is found ranging from 9 to 16 kelvin at 23 to 80 gigapascals. The pressure dependence of Tc suggests multiple phase transitions, consistent with theoretical predictions and reported x-ray diffraction results. The observed values for Tc are much lower than those theoretically predicted, indicating that more sophisticated theoretical treatments similar to those proposed for metallic hydrogen may be required to understand superconductivity in dense phases of lithium.

Journal Article↗

PubMed2002

Redefinition of the mode Grüneisen parameter for polyatomic substances and thermodynamic implications.

Although the value of the thermal Grüneisen parameter (gamma(th)) should be obtained by averaging spectroscopic measurements of mode Grüneisen parameters [gamma(i) identical with (K(T)/nu(i)) delta nu(i)/ delta P, where K(T) is isothermal bulk modulus, nu is frequency, and P is pressure], in practice, the average is up to 25% lower than gamma(th). This discrepancy limits the accuracy of inferring physical properties from spectroscopic data and their application to geophysics. The problem arises because the above formula is physically meaningful only for monatomic or diatomic solids. We redefine gamma(i) to allow for the presence of functional groups in polyatomic crystal structures, and test the formula against spinel- and olivine-group minerals that have well-constrained spectra at pressure, band assignments, thermodynamic properties, and elastic moduli, and represent two types of functional groups. Our revised formula [gamma(i) identical with (K(X)/nu(i)) delta nu(i)/ delta P] uses polyhedral bulk moduli (K(X)) appropriate to the particular atomic motion associated with each vibrational mode, which results in equal values for , gamma(th), and gamma(LA) (the Grüneisen parameter of the longitudinal acoustic mode). Similar revisions lead to the pressure derivatives of these parameters being equal. Accounting for differential compression intrinsic to structures with functional groups improves the accuracy with which spectroscopic models predict thermodynamic properties and link to elastic properties.

Journal Article↗