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Figure 2.2 shows a summary of periclase (Per) and Fp stress measurements for studies that exceed ~20 GPa. All of these measurements have been made in the DAC with the exception of the measurements of Girard et al. (2016). Of these, the measurements of Girard et al. (2016) and those of Miyagi & Wenk (2016) were made on a two‐phase mixture of Brg and Fp. A few trends are clear in the data. At lower pressures and room temperature pure Per (Figure 2.2, black symbols) appears to support higher differential stresses than Fp compositions (Merkel et al., 2002; Singh et al., 2004). In all Fp measurements except those of J.‐F. Lin et al. (2009) Fp exhibits a steep rise in differential stress with pressure as noted by Marquardt & Miyagi (2015). The reason for the discrepancy between the data of J.‐F. Lin et al. (2009) and the other measurements is not clear. There is one very low data point (brown cross) from Miyagi & Wenk (2016). This data point was collected immediately after conversion to Brg + Fp, and as the sample has not yet been deformed, this point is not reflective of a flow stress. By ~60 GPa differential stress values in Fp achieves similar values to those of Per. It is possible that differences between Per and Fp may be due to composition, but the effects of grain size on the yield strength should be explored. Singh et al. (2004) suggest a significant effect of grain size on the strength of Per but the difference in grain size – for example between Singh et al. (2004) and Marquardt & Miyagi (2015) – is not known, as recovered samples were not studied for microstructure (in the latter case, due to anvil failure).

Figure 2.2 Summary of high‐pressure differential stress measurements on periclase and ferropericlase. Measurements in periclase are in black and ferropericlase are in colored symbols.

During higher‐temperature deformation at 1400 K (Figure 2.2, red open squares), the results of Immoor et al. (2018) show a similar steep trend with pressure, but the magnitude is lower than the room temperature experiments of Marquardt & Miyagi (2015). The results of Immoor et al. (2018) are also generally consistent with the results from Miyagi et al. (2013) (Figure 2.2, open circles), which used resistive heating and laser + resistive heating to achieve high temperatures in the DAC. Notably, all three of these studies used the same Fp sample. Data points from Miyagi et al. (2013) at ~775 K (Figure 2.2, yellow open circles) and ~1000 K (Figure 2.2, orange open circles) using resistive heating are very close to the values of Immoor et al. (2018). The ~2300 K data point using resistive + laser heating (Figure 2.2, red open circle) is slightly below the data of Immoor et al. (2018), but with overlapping error bars. One should note that the error bars on these data sets represent the range of lattice strains measured on various lattice planes, and large error bars are associated with the large plastic anisotropy of this phase.

The data points of Girard et al. (2016) at ~ 2000 K (Figure 2.2, blue open diamonds) represent the only large volume experiment at these pressures. Only data collected after ~20 % strain was used for these data points, as this is where steady state stress levels are achieved. These data points lie far below the DAC measurements, and also have a greater separation from the measurements of Marquardt & Miyagi (2015) than the ~2300 K measurement of Miyagi et al. (2013). This difference may be due to lower strain rates in RDA, but may also be due to grain size and the fact that this sample was deformed as part of a two‐phase assemblage. There is additionally another low data point from Miyagi & Wenk (2016) that was annealed at ~1600 K for ~30 minutes (Figure 2.2, yellow cross). It is unsurprising that this point is low, as this is essentially a stress relaxation experiment.

When comparing DAC experiments to DAC experiments, the results on Per (Merkel et al., 2002; Singh et al., 2004) are quite consistent. Aside from the data of J.‐F. Lin et al. (2009), room‐temperature results of Marquardt & Miyagi (2015), Miyagi et al. (2013) Miyagi & Wenk (2016), and Tommaseo et al. (2006) are all quite similar, particularly when considering compositional variations between these experiments. There is not an obvious trend for strength and composition. However, a low‐pressure deformation study by Long et al. (2006) found that Fp samples with high Fe contents (>50%) develop texture faster than samples with low Fe contents, indicating a weakening with high Fe content. The effects of Fe content, grain size, strain rate, and temperature still need to be systematically explored for Fp at mantle conditions.