Spin-crossover in geophysics

Most of my posts on spin-crossover materials have been concerned with organometallic compounds. However, this phenomena can also occur in inorganic materials. Furthermore, it may be particularly relevant in geophysics. A previous post discussed how strong electron correlations may play a role in geomagnetism and DMFT calculations have given some insight.

A nice short overview and introduction is
Electronic spin transition of iron in the Earth's deep mantle 
Jung‐Fu Lin Steven D. Jacobsen Renata M. Wentzcovitch

[It contains the figure below]
The main material of interest is magnesiowüstite, an alloy of magnesium and iron oxide,
(Mg1−xFex)O



Experimental studies and DFT calculations suggest that as the pressure increases the iron ions undergo a transition from high spin to low spin. The basic physics is that the pressure reduces the Fe-O bond lengths which increases the crystal field splitting.
In geophysics, the pressure increases as one goes further underground.

DFT+U calculations are reported in
Spin Transition in Magnesiowüstite in Earth’s Lower Mantle 
Taku Tsuchiya, Renata M. Wentzcovitch, Cesar R. S. da Silva, and Stefano de Gironcoli

The main result is summarised in the figure below.

There is a smooth crossover from high spin to slow spin, as is observed experimentally. However, it should be pointed out that this smoothness (versus a first-order phase transition with hysteresis) is built into the calculation (i.e. assumed) since the low spin fraction n is calculated using a single site model.  On the other hand, the interaction between spins may be weak because this is a relatively dilute alloy of iron (x=0.1875).
Also, the vibrational entropy change associated with the transition is not included. In organometallics, this can have a significant quantitative effect on the transition.

The elastic constants undergo a significant change with the transition. This is important for geophysics because these changes affect phenomena such as the transmission of earthquakes.

Abnormal Elasticity of Single-Crystal Magnesiosiderite across the Spin Transition in Earth’s Lower Mantle 
Suyu Fu, Jing Yang, and Jung-Fu Lin

A previous post considered changes in the elasticity and phonons in organometallic spin-crossover. Unfortunately, that work did not have the ability to resolve different elastic constants.