Fig. 7: The experimental relation pressure-volume dependence in NiO at non-hydrostatic conditions.

The figure combines values of the relative volume change V/V₀ obtained from our nonhydrostatic experiment (run5) and from Eto’s et al work (Ref. ¹⁴). The open triangles correspond to high-pressure phase (HP). For comparison are shown previous experimental data from Eto’s et al. work (Ref. ¹⁴)—circles (open circles—Re calibrated pressure, closed circles—Ruby calibrated pressure). The Birch-Murnaghan equation of state was fitted to the experimental data (run5), and its parameters were found are equal the bulk modulus B₀ = 216 ± 15 GPa and its derivative B’ = 4.3 ± 0.4. The solid line—calculated Birch-Murnaghan equation of state fits very well all set of experimental points including previous one (Ref. ¹⁴). With a pressure increase in the range of 206–224 GPa the onset of the isostructural transition was observed with a volume drop of about 2.7%. In our non-hydrostatic experiment (run5) we observed a coexistence of high- and low-pressure phases in a wide pressure interval from ~230 to ~130 GPa at decompression. For the HP phase, we used fit of non-hydrostatic experimental data for the modified Birch-Murnaghan equation of state (Supplementary Note 5) and its parameters were found are equal to bulk modulus B₁₃₀ = 699 ± 6 GPa its derivative B₁₃₀’ = 4.0 (fixed) with initial pressure parameter of 130 GPa.