Monday, 26 November 2012

Chemical of the Week: Solid Oxygen

Last week, we had a look at dicyanoacetylene, a highly reactive chemical found in our very own solar system, in the atmosphere of Saturn's moon Titan. This week, we will look at the curious case of a synthetic chemical formed under crushing pressures right here on Earth.

Oxygen. We all know it, and for better or worse, love it. After all, oxygen makes up approximately 21% of our atmosphere. Aside from being a rather useful chemical, it gives us something to breathe other than that oh-so-common nitrogen that those nitrogen-fixing microorganisms and symbiotic plants allegedly rave about.

Clover. Thanks to the diazotrophic rhizobia in its root nodules, it now has a thing for nitrogen. 

In our common experience, oxygen is either a simple diatomic gas, or found cloistered away in minerals, oxides and the like. In terms of oxygen in its gaseous state, what happens when it is subjected to freezing temperatures? As with many other gases, oxygen will condense to form a liquid, and when cooled further, form a solid. In the case of oxygen, it has a melting point of -218.79°C, and a boiling point is at -182.95°C, so we rarely encounter diatomic oxygen in anything but its gaseous state.

Now, as I mentioned above oxygen freezes at -218.79°C to form solid oxygen, though this is only the beginning of our story. By changing the temperature of solid oxygen, and by subjecting it to different pressures, it undergoes phase transitions to form different phases of solid oxygen. That is to say, changing the temperatures and pressures solid oxygen to will generally still leave us with solid oxygen, but these disparate phases will have different physical properties.

In the article "Solid Oxygen", by Freiman and Jodi (2004), it is noted that "the existence of six solid-state phases is established unambiguously". Accordingly, these six different phases of solid oxygen are the:

  • α phase
  • β phase
  • γ phase
  • δ phase
  • ε phase: "red oxygen", the phase we are interested in at the moment.
  • ζ phase: a truly fascinating phase, where solid oxygen becomes metallic

Of these different phases, today we shall first focus on the ε phase, also known as "red oxygen". Why is it called "red oxygen"? Well, liquid oxygen and three of the phases of solid oxygen (α, β and γ phases) are various shades of blue in colour. In contrast, the ε (epsilon) phase of solid oxygen is dark red in colour, hence the name "red oxygen".

A sample of cryogenic liquid oxygen displays its charming blue colouration.
Maybe it's cold?

Unlike the α, β and γ phases of solid oxygen, the ε phase does not require freezing temperatures to form. Instead, it simply requires the application of pressure. Lots of pressure. In fact, to form the ε phase, you need to subject liquid or solid oxygen to more than 10 gigapascals (GPa) of pressure. In this phase, "red oxygen" is made up of four pairs of diatomic oxygen molecules in rhomboid cluster, forming an Ocrystal structure. Previous theoretical work expected some manner of O4 molecule to form, with no-one predicting the formation of an O8 rhombohedral unit.

The mysterious crystal structure of solid oxygen in the ε phase.
Note the four O2 molecules connected by the shorter bonds.

Depending on your point of reference and your knowledge of air pressure, and pressure in general, it may be hard to fathom the unbelievably crushing pressures needed to form the ε phase of solid oxygen. 

To put this into perspective, we need to define what a pascal is. In relation to atmospheric pressure, a pascal is a unit of measurement derived by the Système international d'unités, and one atmosphere is equivalent to  1.01325 ×105 pascals. 

Taking this one step further, the pressure required to form the ε phase of solid oxygen is equivalent to a wee bit over 98,692 atmospheres.


Going back to the list of the phases of solid oxygen, the final, ζ (zeta) phase was given a very brief description of "where solid oxygen becomes metallic". I cannot simply leave it at that. The ζ phase forms when solid oxygen is subjected to more than 96 GPa of pressure whilst at room temperature. Once again putting this pressure into perspective, 96 GPa is equivalent to 947,446.34 atmospheres. We may well have thought that the ε phase exists under incredible conditions, but the ζ phase truly takes the cake (if one assumes solid oxygen is partial to cake).

To understand how the metallic phase of solid oxygen is formed, we need to look at the work of Akahama et. al. (1995). Here, liquid oxygen was placed in a diamond anvil cell, and whilst at room temperature, the sample of liquid oxygen was placed under increasing pressure, up to 116 GPa. The different phases of solid oxygen encountered were analysed by x-ray diffraction and synchrotron radiation. It was found that visual observation of the ζ phase yielded something rather fascinating:

"From visual observation under a metallurgical microscope, we saw that the appearance of the oxygen sample became as shiny as the metal gasket after the structural transition at 96 GPa"

In other words, not only can solid oxygen adopt a variety of colours, including blue, pink, orange and red, the ζ phase has a metallic lustre.

Pictured: The metal ζ phase of solid oxygen

The amazing properties of the ζ phase doesn't end here. In Letters to NatureShimizu et. al. (1998) report another remarkable piece of information with regards to metallic oxygen. When the metallic ζ phase is cooled to below 0.6 K (-272.55°C), the electrical resistance of solid oxygen rapidly drops, and in turn, the ζ phase exhibits superconductivity!

Superconductivity of the metallic ζ phase was confirmed at the above reported temperature, by subjecting the sample to a 0.0225 tesla magnetic field. The sample of metallic oxygen expelled the applied magnetic field, or alternately, the sample exhibited the Meissner effect, and thus its status as a superconducting material was proved.

In closing, it is amazing to see how a fairly common chemical can have so many amazing properties and phases.

Now off to contemplate how much a diamond anvil cell costs,


  1. can someone plz tell me the density of oxygen when it is a solid

  2. AMAZING!!! It is simply fascinating that the element Oxygen can go through so many distinct phases! Oxygen is an amazing element, and you starred it in this post. Thank you!

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