Fig. 2.

Mössbauer absorption spectra as a function of pressure for (Mg_xFe_1–x)O magnesiowüstites having Mg content x = 0.20 at 300 K (a) and 6 K (b) and for x = 0.50 (c) and x = 0.80 (d) at 10 K. Antiferromagnetic ordering (hyperfine splitting) appears by 50 GPa in a and is already present at zero pressure in b, c, and d, whereas the unsplit absorption feature characteristic of the nonmagnetic (low-spin) state emerges at pressures of 40–50 GPa in a and b, 60 GPa in c, and 80 GPa in d. This is but an illustrative selection of spectra that lead to the transition pressures and temperatures (and uncertainties) quoted in Table 1. Our current sampling of pressures and temperatures (both increasing and decreasing) nevertheless does not allow us to reliably determine the slope (dP/dT) of the high-spin to low-spin Fe transition boundary.