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Although the theoretical and experimental results both indicate similar aggregate P-wave and S-wave velocities as a function of pressure5, there is an inconsistency in the elastic constants that Mao et al.5 have attempted to resolve by multiplying the P-wave velocities at 16 and 211 GPa by 1.08. However, this treatment results in a 15-40% change in elastic parameters at 211 GPa and is accompanied by a 7-30% decrease in the S-wave velocity (Fig. 1). The authors assume that the values at 39 GPa are correct, but even at that pressure some of the elastic parameters have changed by up to 15% (Fig. 1). Furthermore, in their correction, Mao et al.5 obtain a value for the elastic constant C 33of 491 GPa, which is 63% less than the value of 802 GPa obtained at 39 GPa pressure4. Such a low value for C 33yields a very low Poisson's ratio (0.04) along the symmetry axes. Mao et al. 's5 value for C 44is 38% higher than that obtained at 211 GPa pressure4.

Figure 1: P-wave and S-wave velocities (VP and VS, respectively) as a function of the propagation direction relative to the c -axis of the hexagonal close-packed iron crystal for the original and corrected tables of Mao et al.5.
figure 1

The subscript ‘o’ denotes values derived from the original table and ‘n’ denotes those from their amended table. The percentage increase or decrease in velocity from the original values is indicated. The difference between the two results is significant: the P-wave anisotropy has decreased from 24% to 8%.

Our analysis suggests that the uncertainty in the individual elastic constants is much greater than in parameters based on a combination of them, such as the P-wave velocity. Given the large uncertainties and the physical implications of the elastic parameters, the results of Mao et al.5 must be interpreted with caution.