In the last part, we examined some of the problems with the mainstream model of the Sun. One of the
key concepts underpinning Robitaille's model is that of the entire Sun being composed of Liquid
Metallic Hydrogen. What is this and what state is the research at? Let's dive in and find out more.
Hydrogen is the most abundant atom in the visible universe. In its molecular state, it contains just
one electron and one proton. In 1935 Eugene Wigner, who was one of the founders of modern
solid-state physics, and his colleague Hillard Huntington first tried to predict what would
happen to hydrogen if it were compressed to very high densities. Based on a nearly free electron
picture, they predicted that above 25GPa hydrogen would enter a metallic state. This value turns out
to be quite far off as hydrogen appears to be far more compressible than they first thought.
In modern experiments, they are able to subject hydrogen to a pressure of about 400GPa
which is an almost 16 fold increase compared to what Wigner and Huntington had predicted.
At these pressures, some very exciting phenomena have been observed
but the metallic state remains elusive. As the research continued over the years a better picture
started to emerge about the conditions needed to turn molecular gas into the lightest metal. While
experimentalists are tantalizing close to the pressures required, theory is also moving beyond
current static pressure limits and has predicted that ground-state hydrogen - so at zero kelvin,
would be an entirely new state of matter, which could be superfluid or superconducting.
Currently, five solid phases of hydrogen are known and most importantly to the discussion it
is unique among the stable elements in that full structural information. is absent for all of them,
which prevents modelling and/or predictions of hydrogen behaviour at higher pressures.
Low-temperature solidification of hydrogen was first achieved in 1899 by Dewar. An alternative
approach is through compression at a temperature of 300K. The solid-state under these conditions
is phase I. This phase is characterized by quantum spherically disordered molecules
arranged in a hexagonal close-packed structure.
It is an insulator and shows remarkable pressure stability from 5.5 GPa all the way up to 230 GPa
Phase II is known as the broken symmetry phase and is formed by compressing phase I at pressures
above 60GPa but at temperatures below 100K. This phase is thought to have
ordered molecules, but the nature of their arrangement and their shape is unknown.
Phase III is obtained by compressing phase II above 150 GPa again below 100K.
Nothing so far is known about its structure but it is thought to have
a hexagonal close-packed lattice with unusually intense infrared activity.
It took almost 25 years from the discovery of phase II to the observations of phase IV
of hydrogen. If phase II is compressed at 300K it transforms into phase IV at around 230GPa.
Even though the structure of phase IV is not known on the basis of Raman spectroscopy combined
it is speculated that it is made from alternating layers
consisting of six-atom rings and free-like molecules.
If phase IV is compressed further it gradually transforms into phase V.
This phase has been speculated to be a partially purely atomic state
and a precursor to a fully metallic and atomic state.
In 1989 a group of scientists claimed to have created metallic hydrogen for the first time.
On the basis of the diminishing Raman signed and
increased absorption by the sample, concluded that they had reached the metallic state.
Soon others claimed similar successes. However, with improvements in experimental methods,
it became apparent that the observed effect could be explained by loss of hydrogen at high pressure
and by increased fluorescence of the diamonds being mistaken for closing of the bandgap.
Wind forward 20 years and there was another claim of metallisation
and now based on Raman spectroscopy and direct electrical measurements of resistance.
Here they claimed that they had observed "liquid atomic metallic hydrogen" at above 260GPa. Almost
immediately after publishing their paper, it was shown that hydrogen remains in a mixed molecular
and atomic semiconducting solid phase up to at least 315GPa at 300K. Transforming
back to phase III at lower temperatures. Once more the loss of Raman signal and the drop in
sample resistance were explained by loss of hydrogen and collapse of the sample chamber.
More and more groups started to make similar claims including a recent paper by the Harvard
group who claimed that metallic hydrogen at 500GPa would be a good candidate for rocket fuel.
Their paper title was Observation of Wigner-Huntington transition to metallic hydrogen.
This by itself is a totally misleading title as the paper had not demonstrated either a transition
between molecular state or atomic state which is what the Wigner Huntington transition is.
On top of this 500GPa is currently widely believed to be outside of the range of the
standard diamond anvil cell techniques and it was not accompanied by any scientific evidence!
It is important to understand that at the moment there is no general agreement
in the high-pressure hydrogen research community on the behaviour of hydrogen
above 250GPa. Even the simple phase diagram is hotly debated and there is clear disagreement
as to whether the metallic state was reached and at what pressures.
To date, none of these experiments have produced reproducible results
and this lack of reproducibility also leads to inconsistencies in the literature.
In all these experiments whenever the pressure is removed the material changed its state back
upwards along the phase diagram. One aspect of metallic hydrogen that is also worth considering
is whether or not it is meta-stable. This means that if you can create this shape
and then remove the pressure it would remain in this structure.
This obviously got many people very excited about metallic hydrogen. If it is a superconduct
and meta-stable could it represent the first real room temperature superconductor? It may
also represent a more efficient way of storing hydrogen as a fuel for rockets.
It is important to realise that there are a number of problems that must be answered::
1. What is the minimal energy crystal structure 2. Proof of stability
3. Analysis of the relation between the ground and metastable state structure and the molecular phase
4. Determination of the lifetime of this phase
In a recent paper, they looked to analyse this question through a theoretical approach.
Their conclusions were that at relatively high pressure >300GPa you would indeed see metallic
hydrogen but as the pressure decreased, the structure would switch back to a molecular one.
Below 200GPa metallic hydrogen had no region of stability.
I'm not aware of any research work examining if liquid metallic hydrogen is metastable
but given the theoretical very limited range of metastability in solid metallic hydrogen
it is doubtful that it exists but it is theoretically possible.
In the next part we will dive into the nuts and bolts of the actual model
and then I will discuss my thoughts on the model.