| The High-Spin to Low-Spin Electronic Transition in (Mg,Fe)O
at High Pressure
J. Michael Brown
UW Department of Earth and Space Sciences
The
pressure-induced high-spin to low-spin (HSLS) transition in the
transition-metal containing monoxide (Mg1-x,Fex)O has
now been established in a number of studies. Using a transient-grating
laser-induced spectroscopy (Impulsive Stimulated Light Scattering), we
measured all elastic constants to a pressure of 63 GPa in a sample with an
iron concentration of 6%. The transition is non-first-order and all elastic
constants were found to undergo a softening greater than 10% in the
transition region between 40 and 60 GPa. A solution model for the
thermodynamics (in the dilute-iron limit) is reasonably consistent with
experiments (loss of magnetization, volume change, pressure of transition,
and elastic properties). In particular, the decrease of the bulk modulus
results from stress coupling to the volume change of transition. A change in
local site symmetry (Jahn-Teller distortion) between high-spin and low-spin
iron provides a plausible mechanism for coupling to the shear moduli. The
HSLS thermodynamic theory is used to estimate properties of Earth
constituents at high pressures and temperatures. A previously enigmatic
change in seismic properties centered near a depth of 1500 km may be
associated with this transition. |