What would superionic water ice look like?

Question

Phys.org's Giant lasers crystallize water with shockwaves, revealing the atomic structure of superionic ice links to

• Nature Physics: Experimental evidence for superionic water ice using shock compression
• Nature: Nanosecond X-ray diffraction of shock-compressed superionic water ice

Question: What would superionic water ice look like if it could be maintained at ultra-high pressure long enough? Would it be fairly transparent and have a low index of refraction like glass or "normal" water ice? Or would it be opaque or shiny, or perhaps transparent but high index of refraction like diamond?

With a description like "solid lattice of oxygen and liquid-like hydrogen superionic ice" it certainly sounds "shiny/metallic" to me!

---

The Phys.org article says:

In 1988, scientists first predicted that water would transition to an exotic state of matter characterized by the coexistence of a solid lattice of oxygen and liquid-like hydrogen—superionic ice—when subjected to the extreme pressures and temperatures that exist in the interior of water-rich giant planets like Uranus and Neptune. These predictions remained in place until 2018, when a team led by scientists from LLNL presented the first experimental evidence for this strange state of water. (emphasis added)

[...]

The researchers performed a series of experiments at the Omega Laser Facility at the University of Rochester's Laboratory for Laser Energetics (LLE). They used six giant laser beams to generate a sequence of shockwaves of progressively increasing intensity to compress a thin layer of initially liquid water to extreme pressures (100-400 gigapascals (GPa), or 1-4 million times Earth's atmospheric pressure) and temperatures (3,000-5,000 degrees Fahrenheit).

"We designed the experiments to compress the water so that it would freeze into solid ice, but it was not certain that the ice crystals would actually form and grow in the few billionths of a second that we can hold the pressure-temperature conditions," said LLNL physicist and co-lead author Marius Millot.

[...]

"Water is known to have many different crystalline structures known as ice Ih, II, III, up to XVII," Coppari said. "So, we propose to call the new f.c.c. solid form 'ice XVIII.' Computer simulations have proposed a number of different possible crystalline structures for superionic ice. Our study provides a critical test to numerical methods."

Answer 1

As suggested in the comments, "what does superionic ice look like" in terms of its "natural" (reflected/transmitted) color is difficult to access because the phase seems to exist only at high temperatures where its radiant "heat" would glow yellow/white. While the "natural" color can still be measured with the proper experimental setup (eg. using a tunable chopped laser with a photodiode and lock-in amplifier (samples amenable to such a setup are not readily available. If we were to compress water at a "non-glowing" temperature, even several hundred degrees Celsius, then at about 60 GPa or more we would see not superionic ice but Ice X. Ice X is a macromolecular phase in which the hydrogen and oxygen atoms are held by delocalized, but fixed bonds.

Ice X might thus be considered as (fully) frozen superionic ice. It has cubic crystals, but information related to color/absorption spectra seems limited to infrared wavelengths. Chemistry LibreTexts reports:

Ice-ten looks identical as seen from the x-, y- or z-direction. Using a first-principles DFT code, ideal crystals of ice X have been shown to have a Raman peak at 998 cm$^{−1}$ and two infrared absorption peaks at 450 cm$^{−1}$ and 1507 cm$^{−1}$ [1].

(Visible light would be 12500-25000 cm$^{−1}$.)

Cited Reference

1. L. Jiang, S.-K. Yao, K. Zhang, Z.-R. Wang, H.-W. Luo, X.-L. Zhu , Y. Gu and P. Zhang, "Exotic spectra and lattice vibrations of ice X using the DFT method", Molecules, 23 (2018) 2780.