Thermal equation of state and stability of (Mg0.06Fe0.94)O

• Addition of Mg into FeO expands the stability field of the B1 structure with respect to B8.
• Experiments with and without an in situ fugacity buffer are indistinguishable.
• The measured equation of state of (Mg0.06Fe0.94)O is distinct from that of FeO.

We present the pressure–volume–temperature (P–V–T) equation of state of polycrystalline (Mg0.06Fe0.94)O (Mw94) determined from laser-heated X-ray diffraction experiments up to 122 GPa and 2100 K, conditions approaching those of the deep mantle. We conducted two sets of experiments, one with an in situ   Fe metal oxygen fugacity buffer and one without such a buffer. The internal pressure markers used in these experiments were B2B2-NaCl and hcp  -Fe in the buffered experiment and B2B2-NaCl in the unbuffered experiment. In the sampled P–T   range of the high temperature part of this study, only the B1B1 structure of Mw94 was observed, indicating that the addition of Mg to FeO stabilizes the B1B1 phase with respect to the B8B8 phase at these conditions. Both datasets were fit to a Birch–Murnaghan and Mie–Grüneisen–Debye thermal equation of state using a new open-source fitting routine, also presented here. Analysis of these data sets using the same internal pressure marker shows that the P–V–T data of Mw94 obtained in the unbuffered experiment are well explained by the equation of state parameters determined from the buffered data set. We have also compared the thermal equation of state of Mw94 to that of wüstite and conclude that Mw94 has measurably distinct thermoelastic properties compared with those of wüstite. We use the results obtained in the buffered experiment to determine the density and bulk sound velocity of Mw94 at the base of the mantle and compare these values to geophysical observations of ultralow-velocity zones.