High-to-low spin transition in magnesiumwustite under pressure*

Mg-Fe substitution is most commonly seen in natural solid solution minerals. High resolution X-ray spectroscopy has recently demonstrated that the major lower mantle (LM) minerals undergo a high-to-low spin transition at LM pressures (23-135 GPa). Previous failures of standard DFT and ``LDA+U'' approaches to describe this phenomenon have hindered its investigation and consequences of fundamental importance to geophysics, such as heat transport in Earth?fs mantle. Here, using a rotationally invariant first principles formulation of LDA+U, where the Hubbard U parameter is computed in a internally consistent way, we report the first successful study of this transition in low solute concentration (Mg$_{1-x}$Fe$_x$)O, magnesiumwustite. This is believed to be the second most abundant phase of Earth's LM. This encouraging result appears to open for exploration a new class of problems of enormous significance to deep Earth geophysics.