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L Stixrude

Publications and source records attributed to L Stixrude.

5 recordsLinked to original sources
PubMed2005

Bone chemical structure response to mechanical stress studied by high pressure Raman spectroscopy.

While the biomechanical properties of bone are reasonably well understood at many levels of structural hierarchy, surprisingly little is known about the response of bone to loading at the ultrastructural and crystal lattice levels. In this study, our aim was to examine the response (i.e., rate of change of the vibrational frequency of mineral and matrix bands as a function of applied pressure) of murine cortical bone subjected to hydrostatic compression. We determined the relative response during loading and unloading of mineral vs. matrix, and within the mineral, phosphate vs. carbonate, as well as proteinated vs. deproteinated bone. For all mineral species, shifts to higher wave numbers were observed as pressure increased. However, the change in vibrational frequency with pressure for the more rigid carbonate was less than for phosphate, and caused primarily by movement of ions within the unit cell. Deformation of phosphate on the other hand, results from both ionic movement as well as distortion. Changes in vibrational frequencies of organic species with pressure are greater than for mineral species, and are consistent with changes in protein secondary structures such as alterations in interfibril cross-links and helix pitch. Changes in vibrational frequency with pressure are similar between loading and unloading, implying reversibility, as a result of the inability to permanently move water out of the lattice. The use of high pressure Raman microspectroscopy enables a deeper understanding of the response of tissue to mechanical stress and demonstrates that individual mineral and matrix constituents respond differently to pressure.

Animals↗
PubMed2001

Elasticity of iron at the temperature of the Earth's inner core.

Seismological body-wave and free-oscillation studies of the Earth's solid inner core have revealed that compressional waves traverse the inner core faster along near-polar paths than in the equatorial plane. Studies have also documented local deviations from this first-order pattern of anisotropy on length scales ranging from 1 to 1,000 km (refs 3, 4). These observations, together with reports of the differential rotation of the inner core, have generated considerable interest in the physical state and dynamics of the inner core, and in the structure and elasticity of its main constituent, iron, at appropriate conditions of pressure and temperature. Here we report first-principles calculations of the structure and elasticity of dense hexagonal close-packed (h.c.p.) iron at high temperatures. We find that the axial ratio c/a of h.c.p. iron increases substantially with increasing temperature, reaching a value of nearly 1.7 at a temperature of 5,700 K, where aggregate bulk and shear moduli match those of the inner core. As a consequence of the increasing c/a ratio, we have found that the single-crystal longitudinal anisotropy of h.c.p. iron at high temperature has the opposite sense from that at low temperature. By combining our results with a simple model of polycrystalline texture in the inner core, in which basal planes are partially aligned with the rotation axis, we can account for seismological observations of inner-core anisotropy.

Journal Article↗