Extended Data Fig. 2: Illustration of how the synthetic dataset at 40.4 GPa was generated using the elastic constants reported in our Letter.
From: Kurnosov et al. reply

For every pressure point and platelet, velocities in 18 propagation directions were calculated over an angular range of 360° to simulate in-plane rotations between individual measurements that are representative of our low pressure measurements where no overlap of peaks occurred (solid symbols). We randomly changed the velocity values by up to 0.5% to simulate errors typical for single velocity measurements by Brillouin spectroscopy in the diamond anvil cell13. a, Illustration of a complete velocity dataset, using crystal platelet 1 as an example. b, Illustration of a reduced dataset, using crystal platelet 2 as an example. This assumes limited data availability owing to peak overlap or elasto-optic coupling. The example in b corresponds to a situation where 20 directions have been measured in total (10 on each crystal). Solid and dotted curves are results from the ‘global fit’.