Comment on "Observation of the Wigner-Huntington transition to metallic hydrogen"

TL;DR

This paper critically examines claims of observing metallic hydrogen at 495 GPa, arguing that the evidence does not support the record pressure or phase transition, and contradicts previous unconfirmed results.

Contribution

It provides a critical analysis challenging the validity of the original experimental claims regarding metallic hydrogen.

Findings

No supporting data for record pressure claim

Contradicts previous unconfirmed results

Questions the evidence for metallic transition

Abstract

Dias and Silvera (Letters, p. 715, 2017) claim the observation of the Wigner-Huntington transition to metallic hydrogen at 495 GPa. We show that neither the claims of the record pressure or the phase transition to a metallic state are supported by any data and contradict the authors' own unconfirmed previous results.

Full text

Comment on ”Observation of the Wigner-Huntington transition to metallic hydrogen”

Xiao-Di Liu1, Philip Dalladay-Simpson2, Ross T. Howie2, Bing Li2,3 and Eugene Gregoryanz1,2,4

1Key Laboratory of Materials Physics, Institute of Solid State Physics, CAS, Hefei, China

2Center for High Pressure Science and Technology Advanced Research, Shanghai, China

3Center for the Study of Matter at Extreme Conditions, Department of Mechanical and Materials Engineering, Florida International University, Miami, USA

4Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, Edinburgh, U.K.

Dias and Silvera (Letters, p. 715, 2017) claim the observation of the Wigner-Huntington transition to metallic hydrogen at 495 GPa. We show that neither the claims of the record pressure or the phase transition to a metallic state are supported by any data and contradict the authors’ own unconfirmed previous results.

Ref. (1) presents only one experimental run claiming the record pressure of 495 GPa. The paper presents 2 figures of the phase diagram, iPhone photos of the sample, and the deduced reflectivity of 4 wavelengths only at the highest pressure point. The supplementary materials provide the processed and fitted infra-red (IR) absorption spectra from 135 to 335 GPa, the pressure versus force curve assuming the linear dependence, and the Raman spectra of the stressed diamond from 1200 to 2200 cm*-1*. The absence of data combined with the uncritical claims have led to the unprecedented 3 Comments (2-4) written within a month of publication of Ref. (1).

In the past 5 years we have conducted $\sim$120 experiments on hydrogen reaching above 200 GPa (5-10). In $\sim$30 runs out of 120 the pressure exceeded 300 GPa and in only 5 out of 120 the pressure exceeded 350 GPa. The extensive statistics show that the diamond culet sizes of 30 $\mu$m diameter (used in Ref. (1)) could be used to reach maximum pressures of $\sim$315$\pm$10 GPa with the probability of 20$\%$. In order to reach pressures close to 400 GPa, with lower probability of 10$\%$, the culet sizes of 15 $\mu$m must be used with the sample contracting to 2-3 $\mu$m at the highest pressure. Our statistics are in an excellent agreement with other groups working on hydrogen at high pressure (11-13). The authors of Ref. (1) had 2 experimental runs in the past year using culets of 30 $\mu$m diameter (14,1) claiming the unsubstantiated pressures of 420 and 495 GPa.

Ref. (1) claims that a combination of annealing, fine polishing and coating the culet with Al2O3 has led to the increase in maximum pressure with larger culets (and samples) compared with previous studies (5-13). Those techniques would likely decrease the probability of the premature failure due to hydrogen diffusion, but there is no supportive evidence that these techniques would improve the mechanical stability of diamond. The record pressure of 600 GPa was recently shown on metallic samples i.e. Re and/or Pt using novel double stage approach with the pressure generating area having the linear size of 3 $\mu$m only (15) (100 times smaller working area than in (1)).

Fig. 1 shows the pressure versus load curve from (1) and our own data. Ref. (1) uses 3 different, non-overlapping calibration methods at $\sim$100 and $\sim$300 GPa. Measuring pressure by estimating the load is not a direct method, as it does not probe the sample or/and calibrant in situ. The loading curve is unique for each experimental run and depends only on the sizes of the culets, angles of the bevels, compressibility of the gasket and sample. The dependency cannot be linear as shown in (1) but always consists of 3 distinct regimes, each of which is sub-linear with differing gradients: plastic deformation of the gasket, fast rise of pressure beyond the plastic deformation, followed by the much slower pressure increase (saturation) due to the bending (”cupping”) of the diamonds.

In order to make the dependence linear, Ref. (1) rescales the point from 420 to 400 GPa taken from a different experiment (14) and uses 4 meaningless points of ”visual observations”. The last point of 495 GPa (as well as 420 GPa point from (14)) is deduced from the single Raman spectrum which does not cover the wide energy range showing the signal from hydrogen and/or pressure induced fluorescence. This rules out any critical assessment that the sample did not diffuse out and that the assigned peak of the stressed diamond is not due to any other factors.

The IR absorption data are consistent with the loss of the sample and contradict the authors’ own previous claims (14). In Fig. 1 we plot the raw data provided by but not plotted in (1) together with the normalised spectra. The sample was clearly diffusing out between the claimed pressures of 314 and 338 GPa and completely lost above this. The lack of IR transmission above 338 GPa and that the ”sample” appears dark at 415 GPa correlates with the gasket flow scenario. In (14) the authors present the same IR data up to 420 GPa, but do not provide any explanation why their results are so different in two consecutive experiments. Also, no explanation is given as to why there was no IR transmission/absorption between 335 and 495 GPa, the pressure range in which hydrogen is semiconducting.

The analysis of the photos provided is also very consistent with the loss of the sample at an earlier stage in the experiment. The sample at 415 GPa is expanded under pressure and is slightly bigger than at 205 GPa, while it is the same size at 495 GPa. In (8) we demonstrate the photos of the sample up to 280 GPa which show the flow of metal in the sample chamber and diffusion of H2 out of the chamber. As the chamber closes, the thin layer of hydrogen between the collapsed gasket and the diamond gives the dark appearance (compare Fig. 2B in (1) and Fig. 3 in (8)). With the increasing pressure all hydrogen diffuses out of the chamber and the dark area becomes shiny as it bridges the anvils but would have different reflectivity from the rest of the gasket as in Fig. 2C in (1) (see also photo of ”metallic hydrogen” in Fig. 3D in (11)).

The presented phase diagram (Figs. 1 and 4) show an unconfirmed H2-PRE phase for which no data are presented in the paper. The existence of this phase was surmised in (14) and even though no supporting data are given, Ref. (1) states ”after the 335 GPa pressure point resulted in our sample starting to turn black (Fig. 2B) as it transitioned into the H2-PRE phase” which the authors claimed to be transparent (see Fig. 3S in (14)). Furthermore, Ref. (1) cites one of the authors’ own publication on melting but, without any explanation, plots the smooth melting curve (see Fig. 1) contradictory to the authors’ cited paper which claims the existence of the sharp peak at 60 GPa.

References and Notes

R. Dias and I. Silvera, Science 355, 715 (2017). 2. 2.

A. Goncharov and V. Struzhkin, arXiv:1702.04246 (2017) 3. 3.

M. Eremets and I. Frozdov, arXiv:1702.05125 (2017) 4. 4.

P. Loubeyere et al., arXiv:1702.07192 (2017) 5. 5.

R. Howie et al., Phys. Rev. Lett., 108, 125501 (2012) 6. 6.

R. Howie et al., Phys. Rev. B, 86, 214104 (2012) 7. 7.

R. Howie et al., J. Appl. Phys, **114 **, 073505 (2013) 8. 8.

R. Howie et al., Phys. Rev. Lett., 113, 175501 (2014) 9. 9.

R. Howie et al., Nat. Mat, 14, 495 (2015) 10. 10.

P. Dalladay-Simpson et al. Nature, 529, 63 (2016) 11. 11.

M. Eremets and I. Troyan Nat. Mat., 10, 927 (2011) 12. 12.

P. Loubeyre et al., Phys. Rev. B, 87, 134101 (2013) 13. 13.

C. Zha et al., Phys. Rev. Lett., 110, 217402 (2013) 14. 14.

R. Dias et al., arXiv:1603.02162 (2016) 15. 15.

L. Dubrovinsky Nat. Comm. 3, 1163 (2012)