Only a H2 in A Gilded Cage

“OK, Susan, you’ve led us through doing high-pressure experiments with the Diamond Anvil Cell and you’ve talked about superconductivity and supermagnetism. How do they play together?”

“It’s early days yet, Sy, but Dias and a couple of other research groups may have brought us a new kind of superconductivity.”

“Another? You talked like there’s only one.”

“It’s one of those ‘depends on how you look at it‘ things, Al. We’ve got ‘conventional‘ superconductors and then there are the others. The conventional ones — elements like mercury or lead, alloys like vanadium‑silicon — are the model we’ve had for a century. Their critical temperatures are generally below 30 kelvins, really cold. We have a 60‑year‑old Nobel‑winning theory called ‘BCS‘ that’s so good it essentially defines conventional superconductivity. BCS theory is based on quantum‑entangled valence electrons.”

“So I guess the unconventional ones aren’t like that, huh?”

“Actually, there seem to be several groups of unconventionals, none of which quite fit the BCS theory. Most of the groups have critical temperatures way above what BCS says should be the upper limit. There are iron‑based and heavy‑metals‑based groups that use non‑valence electrons. There are a couple of different carbon‑based preparations that are just mystical. There’s a crazy collection of copper oxide ceramics that can contain five or more elements. Researchers have come up with theories for each of them, but the theories aren’t predictive — they don’t give dependable optimization guidelines.”

“Then how do they know how to make one of these?”

“Old motto — ‘Intuition guided by experience.’ There are so many variables in these complex systems — add how much of each ingredient, cook for how long at what temperature and pressure, chill the mix quickly or anneal it slowly, bathe it in an electrical or magnetic field and if so, how strong and at what point in the process… Other chemists refer to the whole enterprise as witch’s‑brew chemistry. But the researchers do find the occasional acorn in the grass.”

“I guess the high‑pressure ploy is just another variable then?”

“It’s a little less random than that, Sy. If you make two samples of a conventional superconductor, using different isotopes of the same element, the sample with the lighter isotope has the higher critical temperature. That’s part of the evidence for BCS theory, which says that electrons get entangled when they interact with vibrations in a superconductor. At a given temperature light atoms vibrate at higher frequency than heavy ones so there’s more opportunity for entanglement to get started . That set some researchers thinking, ‘We’d get the highest‑frequency vibrations from the lightest atom, hydrogen. Let’s pack hydrogens to high density and see what happens.'”

“Sounds like a great idea, Susan.”

“Indeed, Al, but not an easy one to achieve. Solid metallic hydrogen should be the perfect case. Dias and his group reported on a sample of metallic hydrogen a couple of years ago but they couldn’t tell if it was solid or liquid. This was at 5 megabars pressure and their diamonds broke before they could finish working up the sample. Recent work has aimed at using other elements to produce a ‘hydrogen‑rich’ environment. When Dias tested H2S at 1.5 megabar pressure, they found superconductivity at 203 kelvins. Knocked everyone’s socks off.”

“Gold rush! Just squeeze and chill every hydrogen‑rich compound you can get hold of.”

“It’s a little more complicated than that, Sy. Extreme pressures can force weird chemistry. Dias reported that shining a green laser on a pressurized mix of hydrogen gas with powdered sulfur and carbon gave them a clear crystalline material whose critical temperature was 287 kelvins. Wow! A winner, for sure, but who knows what the stuff is? Another example — the H2S that Dias loaded into the DAC became H3S under pressure.”

“Wait, three hydrogens per sulfur? But the valency rules—”

“I know, Sy, the rules say two per sulfur. Under pressure, though, you get one unattached molecule of H2 crammed into the space inside a cage of H2S molecules. It’s called a clathrate or guest‑host structure. The final formula is H2(H2S)2 or H3S. Weird, huh? Really loads in the hydrogen, though.”

“Jupiter has a humungous magnetic field and deep‑down it’s got high‑density hydrogen, probably metallic. Hmmm….”

~~ Rich Olcott

Fifty Shades of Ice


This week we dip a little deeper into Titan’s weirdness trove.  Check the diagram.  Two kinds of ice??!?  What’s that about?

Carbon allotropes
and water polymorphs

As everyone knows, diamonds and pencil lead (graphite, and I loved learning that graphite is an actual dug-from-the-ground mineral whose name came from the Greek verb “to write”) are both pure carbon, mostly.  Same atoms, just arranged differently.

Graphite‘s carbon atoms are laid out in sheets of hexagons.  Adjacent sheets are bonded together but not as strongly as are the atoms within each sheet.  Sheets can slide past each other, which is why we use graphite as a lubricant and why pencils can write and erasers can unwrite.

Diamond‘s atoms are also laid out in sheets of (rumpled) hexagons, but now the bonds between sheets are identical to the bonds within a sheet.  Turn your head sideways and you’ll see that sheets run vertical, too.  In fact, each carbon atom participates in four sheets, three vertical and one horizontal.  All that symmetrical bonding makes diamond one of the hardest substances we know of.

Neighboring carbon atoms form bonds by sharing electrons between their positive nuclei.  Neighboring water molecules (H2O) don’t share electrons but they do tend to line up with their somewhat positive hydrogen atoms pointing towards nearby somewhat negative oxygens.  That’s a loose rule in liquid water but it dominates when the molecules freeze into ice.

Most of the ice on Earth has an Ice-Ih structure, where the oxygen atoms are arranged in the same pattern as the carbons are in graphite.  Water’s hexagonal sheets aren’t quite flat, but the 6-fold symmetry gives us snowflakes.  There’s a hydrogen atom between each pair of oxygens, but it’s not half-way between.  Instead, each oxygen tightly holds its own two hydrogens while it pulls at further-away hydrogens owned by two neighboring oxygens.

But water molecules have other ways to arrange themselves.  A small fraction of Earth’s ice has a diamond-like Ice-Ic structure with each oxygen participating in four hexagon sheets.  Again, hydrogens are on the lines between them.

Water’s such a versatile molecule that it doesn’t stop with two polymorphs.  Ice scientists recognize seventeen distinct crystalline varieties, plus three where the molecules don’t line up neatly.  (None of them is Vonnegut’s “Ice-nine.”)  Each polymorph exists in a  unique temperature and pressure range; each has its own set of properties.  As you might expect, ices formed at high pressure are denser than liquid water.  Fortunately, Ice-Ih is lighter than water and so ice cubes and icebergs float.

As cold as Titan is and as high as the pressure must be under 180 miles of Ice-Ih and watery ammonia sea (even at 10% of Earth’s gravity), it’s quite likely* that there’s a thick layer of Ice-something around Titan’s rocky core.

clathratesThe primary reason we think Titan is so wet is that Titan’s density is about halfway between rock and water.  We know there are other light molecules on Titan — ammonia, methane, etc.  We don’t know how much of each.  Those compounds don’t have water’s complex phase behavior but many can dissolve in it.  That’s why that hypothetical “Ammonia sea” is in the top diagram.

But wait, there’s more.  Both graphite and water ice are known to form complex polymorphs, clathrates, that host other molecules.  This diagram gives a hint of how that can happen.  Frozen water under pressure forms a large number of more-or-less ordered cage clathrate structures that can host  Titan’s molecular multitude.

At Titan’s temperatures ice rocks would be about as hard as granite.  Undoubtedly they’ll have surprising chemistry and interesting histories.  We can expect clathrate geology on Titan to be as complex as silicate geology is on Earth.

Geochemist heaven, except for the space suits.

~~ Rich Olcott

* – A caveat: we know a great deal about Earth’s structure because we live here and have been studying it scientifically for centuries. On the other hand, most of what we think we know about Titan’s interior comes from mathematical models based on gravitational observations from the Cassini mission, plus 350 photos relayed back from the Huygens lander, plus experiments in Earth-bound chemistry labs. Expect revisions on some of this stuff as we learn more.