Future Boron: Virtual Synthesis is the Next Phase.

What does it look like:

Two views of a possible new high pressure allotrope of Boron- B56

What is it?

It’s an allotrope of Boron. Maybe. In a previous C-365 blog episode, we described an allotrope of Boron that was discovered in 1951. And when we say “discovered” we mean “someone found a crystal, measured it and did the crystallography”.

The cubic B56 structure shown above is a high pressure phase described in a recent open access paper. The catch? No one knows if it actually exists or not. The structure is one of a number “hand created” using some plausible hypotheses and then computer modeled using density functional theory to see if it was “stable”. The “in the box” synthesis of computational chemistry is not new, but in another example of how the world has been changed by Moore’s Law, has become a routine part of the chemist’s toolbox. Still, as the old saying goes, a computer can make more mistakes in a day than you can in a lifetime.

Where does the structure come from?

The paper is: Fan et. al., “Phase transitions, mechanical properties and electronic structures of novel boron phases under high-pressure: A first-principles study.” Scientific Reports 4, 6786 doi:10.1038/srep06786

SmB6: When interesting is skin deep.

What does it look like:

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Samarium hexaboride is a cubic material. In the diagram, the samarium is the big purply atom at the centre and the borons are on the edge of the cube: we’ve highlighted one of the faces to show how the borons are arranged.

This material is currently the subject of a lot of research (for example this open access paper) because it’s a topological insulator. In this context, topological means this is a really cool word that should get us a lot of grant money. I highly recommend this blog post for an explanation.

There’s a saying attributed to Wolfgang Pauli: “God made the bulk; surfaces were invented by the devil.” One of the reasons topological insulators are interesting- and the easiest way of describing them- is that the bulk and surface properties are different. They are electrically insulating in the bulk but electrically conducting on the surface.

Apart from being interesting in their own right- and demonstrating that, once again, condensed matter physicists will find a way to keep busy- it’s strongly suspected that topological insulators could be really useful in fields such as quantum computing.

Where does the structure come from?

This structure is #2104762 in the Open Crystallographic Database. The reference is Funahashi et. al., Acta. Cryst. B., 66 (2010) 292

Typical recent research: W.A. Whelan et. al., Phys. Rev. X. 4 (2014) 031012.

A reference to topological insulators: Hasan and Kane Rev. Mod. Phys. 82 (2010) 3045. Even if you don’t know much about topology you know it’s got to do with holes and donuts and stuff. This paper distinguishes itself by illustrating a torus using a photograph of a genuine bona fide donut. Complete with icing sugar. I wonder how they got that through faculty? “We’d like to stump up the page charge for a colour print so that we can put a picture of a donut in our paper”.

Phase Under Pressure – Cerium gold silicide

What does it look like:

Cerium gold silicide is a 1-2-2 material. The chemical structure was the subject of a previous blog post. The structure is tetragonal. In this image the cerium is shown as light green. The gold is, well, gold. The silicon is shown in blue.

What is it?

Back in the original blog post we pointed out that ternary compounds such as this have a broad range of properties depending on your choice of element, and it’s not uncommon for materials which have been known for ages to reveal fascinating properties when looked at in a new way.

CeAu₂Si₂ is an antiferromagnet at low temperatures (say 5K). The red arrows show the magnetic moments of the cerium atoms that give rise to the antiferromagnet. This magnetic structure was determined by neutron scattering and published in 1984. The structure was done as part of the series CeM₂Si₂ where M = Ag, Au, Pd, Rh so you’d guess their research budget must have been pretty good that year. The work was done at the research reactor at the Brookhaven on Long Island in New York, which was controversially shut down after lobbying by celebrities from the nearby Hamptons. (The story’s a bit more complicated than that, but it’ll do for now).

Anyhoo, a recent open access paper shows that this material is very interesting at high pressure. Between 11.8 and 22.3 GPa, the material is superconducting below about 2K. Then between 2 and 5K (or thereabouts depending on the pressure) it’s magnetically ordered. So the phase diagram has both superconducting and magnetic phases in it.

It used to be thought that magnetic and superconducting phases were “antagonistic” to use a phrase from the paper. In other words a material could be one or the other but not both. But about twenty years ago a material was found which could switch from magnetically ordered to superconducting . In fact this was another 1-2-2 material (CeCu₂Ge₂). What makes CeAu₂Si₂ interesting is that this co-existing range of over 10GPa is much broader than previously observed.

It just goes to show how “old materials” can yield new insights.

Where does the structure come from?

The magnetic structure was published as: B.H. Grier et al, Phys. Rev. B. 29 (1984) 2664. The paper also has a good summary of the overall chemical structure.

The recent high pressure paper is Z. Ren et al., Phys. Rev. X., 4 (2014) 031055

Tetrahedral amorphous carbon: rough in the diamond

What does it look like:

The image shows a computer simulation of tetrahedral amorphous carbon grown on a substrate (at the right). The red atoms are diamond-like, the blue atoms are graphite-like and the green atoms. In this image the carbon atoms would have impacted the surface from the left hand side with an energy of 70eV (a velocity of about 85km/s-1). The green, blue and red atoms have two, three, and four neighbours respectively, where “neighbor” means “within 1.85A”.

What is it?

It’s another allotrope of carbon, this time tetrahedral amorphous carbon (ta-C). Tetrahedral amorphous carbon is not a natural material. It’s formed by firing carbon atoms at high speed onto a substrate using a plasma device such as a vacuum arc.

An amorphous material is one that has no long term regularly repeating structure: it can have local order- order that persists for a couple of unit cells- but no long range order. A good example of an amorphous material is window glass, which is mostly silicon dioxide (SiO₂) with some other stuff thrown in.

Now it may be a bit of a stretch to include ta-C on a blog about crystals, given that “amorphous” means “it’s not a crystal”. However, if we define crystallography in the most general sense as “what gets talked about at crystallography conferences, even if it’s hidden away in the dingy corner of the venue”, then we can use that as an excuse to talk about a fascinating range of materials.

Amorphous carbon (aC) as a structure type is hardly new. It can be found in natural carbon materials such as coal and Rosalind Franklin did pioneering measurements on aC in the 1950’s.

Even though amorphous materials are disordered (by definition!) you can still say things about their bonding and structure. Naturally occurring aC tends to be graphitic in nature: most of the bonds in the material are graphite-like (planar) sp² bonds, and each carbon is more or less surrounded by three other carbon atoms (like graphite).

In tetrahedral amorphous carbon, the majority of bonds are diamond-like (tetrahedral) sp³ bonds and the majority of the carbon atoms have four nearest neighbours (like diamond). Amorphous carbon can be characterized by the proportion of the two bond types.

In a crystal, we can characterise atomic distances to a certain degree of accuracy. In diamond, the distance between adjacent (“nearest-neighbour”) carbon atoms is 1.54A. This is true throughout the crystal.

In an amorphous material, there’s no longer a consistent, exact distance between atoms. Instead you have to ask the question, “for any given carbon atom, what is the probability I’ll find another carbon atom at a particular radius from that one?” This is called the radial distribution function (RDF). Let’s have a look at the following graph:

The important thing here is the black dots which are experimental data (the other lines are various models). At 1.5A is a broad hump which says that, for any individual carbon, its nearest neighbor is about 1.5A, but really it could be anywhere between 1.3 and 1.7A or so. The next nearest neighbor is about 2.5A. In contrast, you won’t find any carbon atoms that are 2A apart from each other. Finally there are no atoms closer to each other than about 1.2A. Otherwise, that would be called “cold fusion”.

So while the ta-C structure is disordered, it’s not entirely random. Each individual atom looks “a bit diamond-y”, and this idea of “order within chaos” is one of the interesting aspects of amorphous materials.

One of the main progenitors of the current interest in ta-C was a seminal 1991 paper, which introduced the idea of producing thin films of the material using vacuum arc. A vacuum arc is a plasma source that produces a stream of energetic, high speed ions. It’s quite different to conventional plasma sources used to make thin films. So the question arose: “what happens if we whack these high speed ions into a surface?” and the answer is “if you do it with carbon, you get a new allotrope”.

The field blossomed, and in the way of so much physics that started as a “what-if” question, is leading to applications such as this one. One of the fascinating aspects of physics (and arguably science in general) is that it doesn’t neatly divide into “pure” and “applied” research. The field began as a “relatively” pure physics project, albeit in an Applied Physics department. A great deal of the follow up modeling (so essential to understanding What Is Going On) could be called “pure” research. You might like to make some sort of inference from this. I couldn’t possibly comment.

Where does the structure come from?

The images are from the 2005 paper by Nigel Marks published in Diamonds and Related Materials, a journal with a surprisingly large research area with many unresolved scientific questions. For example it has not been resolved whether or not Related Materials are also a girl’s best friend.

A good starting point for amorphous materials is the book “Powder Diffraction: Theory and Practice” edited by Dinnebier and Billinge, which has a nice chapter introducing RDFs, G(r) and all the rest.

A. Marks, “Thin film deposition of tetrahedral amorphous carbon: a molecular dynamics study”, Diamond & Related Materials 14 (2005) 1223–1231

D.R. McKenzie et. al., “Compressive-Stress-Induced Formation of Thin-Film Tetrahedral Amorphous Carbon,” Phys. Rev. Lett., 67 (1991) 773

Fanxin Liu et al., “Released Plasmonic Electric Field of Ultrathin Tetrahedral-Amorphous-Carbon Films Coated Ag Nanoparticles for SERS”, Scientific Reports 4, Article number: 4494, doi:10.1038/srep04494. This one’s open access.

Peter J.F. Harris, “Rosalind Franklin’s work on coal, carbon, and graphite”, Interdisciplinary Science Reviews, 26 (2001) 204

Gd5Si2Ge2: Magneto gets cooler

What does it look like:

In the diagram above, the purple atoms are the gadolinium and the grey/ blue atoms are mixed sites: germanium and silicon are randomly mixed 50/50. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

It turns out that the whole “X-Men” cartoon universe doesn’t just suffer from an obviously ludicrous premise. It also suffers from a subtly ludicrous premise: that Magneto needs sidekicks.

Gd₅Si₂Ge₂ is a magneto-caloric material. That means there’s a coupling between the magnetic properties of the material and its thermal properties. In fact, Gd₅Si₂Ge₂ is technically referred to as a giant magneto-caloric material, which means that its magneto-caloric properties are really really big. Scientists can be very specific in their use of language.

Magneto-caloric materials can be used as refrigerators. The idea works like this: if you have a magnetized lump of the material, then it takes energy to demagnetize, and this energy comes out of the thermal motion. So instead of the compression- expansion cycle of an ordinary fridge, you have a magnetization- demagnetization cycle.

Magneto has always been able to bend or move any metal according to his will (even some that would appear to be non magnetic. Apparently no-one uses stainless steel or aluminium in the X-men universe). With magneto-caloric materials, he can make things hot and cold as well. With magneto-electric and magneto-optical materials, he’s pretty much got all the bases covered and he can get rid of the incompetent mutant sidekicks.

Where does the structure come from?

The structure was published by Pecharskii, and Gschneidner in 1997(Journal of Alloys and Compounds 260 (1997) p98-p106. The same authors published the magnetocaloric properties PRL. (Physical Review Letters 78 (1997) 4494). A few years later a different group published an article showing that by adding a bit of iron, the magnetocaloric properties could be improved even more. They published in Nature. There’s a lesson in there somewhere.

Gallium: out of the box

What does it look like:

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

This is gallium, one of the few elements that doesn’t have a cubic or hexagonal structure at room temperature. Gallium is orthorhombic: the cell angles are all 90^o, but the a,b and c axes are different lengths. It turns out that a and c are very close, but this doesn’t make gallium “almost tetragonal”, because it’s missing the required symmetry. Gallium has the space group C m c a, possibly the only one of the 230 space groups to have almost had a Village People song written about it.

All elements are interesting in their own way, and gallium doesn’t disappoint. It has a melting point near 30^oC, so it will sit on the lab bench but melt in your hand. It’s heavily used in the semiconductor industry (mainly in the form of GaAs, gallium arsenide). And it has the bizarre feature of being able to penetrate metals such as steel along grain boundaries, making them brittle. Surely that’s got to be the making of a plot twist in a TV crime show.

Where does the structure come from?

This structure of gallium is #9008085 in the Crystallography Open Database

ThCr2Si2: giving it the old 1-2-2

What does it look like:

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Thorium chromium silicide is the prototypical structure for a whole range of materials of the form RT₂X₂. The structure is tetragonal. R is a Group 1, Group 2 or rare earth element, shown here in green. T is a transition metal (coloured red). X is a group 14 or 15 element (shown in blue).

That gives a lot of combinations.

What’s more, there’s nothing that says each component has to be a single element. For example, the X is commonly Si or Ge, but it could be any mix of the two.

Ternary compounds such as this provide huge scope for exploration. For example, R and T can both have magnetic moments which interact strongly with each other, so the magnetic properties of this class of materials can be highly intricate.

Where does the structure come from?

This structure was first published as: Ban, Z.; and Sikirica, M. Acta Crystallographica 18 (1965) p594-p599

Reference: Andrej Szytula and Janusz Leciejewicz: CRC Handbook of Crystal Structures and Magnetic Properties of Rare Earth Intermetallics

Elements Under Pressure: Barium

What does it look like:

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

This is the high pressure phase Barium IV, published in 1999. It was found using a combination of single crystal and powder x-ray diffraction at 12.1GPa. In other words, about 100,000 times atmospheric pressure.

There may be some deep anthropic reason why most elements are structurally so boring at room temperature and pressure. Then again, it might just be one of those things. But it turns out that in extreme conditions (at least from a human perspective) elements can do bizarre things. This high pressure of structure of barium shows an octagonal cage-like structure (the green atoms) with guest atoms (the dark blue atoms) sitting in the middle. The host structure is tetragonal, and the image shows the ab- plane just slightly offset so you can see the difference between the z=0 and z=1/2 atoms (the z=0 atoms look doubled up).

The first bizarre thing about this structure is that while the structure is commensurate in the a and b directions (meaning the unit cell repeats itself in a regular way), in the c direction- out of the screen- the guest atoms are incommensurate with the host atoms. Turning the structure around, we’ve shown a couple of repetitions along the c-axis of the guest structure:

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

 

So incommensurate means that the cell lengths of the two structures do not divide into each other evenly. So, in the c-axis, while each of the host and guest structures repeats itself, the combined structure never does.

The second bizarre thing is that, while we’ve used different colours to show the host and guest structures, it’s all barium! Both atoms are the same.

Where does the structure come from?

This structure comes from work performed by high pressure specialists from the University of Edinburgh at the synchrotron at Daresbury. The publication is not surprisingly called “Self-hosting incommensurate structure of Barium IV”, Nelmes et al., Physical Review Letters 83 (1999) 4081

Magnetic Thulium: Moments align in seven

What does it look like?

What is it?

Thulium (Tm, Z=69) is a seemingly unremarkable- albeit uncommon- element lurking towards the far end of the rare earth group. At room temperature it has a common hexagonal close packed structure. The chemical unit cell is shown in the bottom of the image.

However, at 4.2K the magnetic moments are aligned in an alternating 3-4-3-4 arrangement along the c- axis (up the screen). This means that the magnetic unit cell in the c- axis is seven times the chemical unit cell. The magnetic structure was determined by single crystal neutron scattering.

This is one of a rich panoply of magnetic structures discovered in the rare earth elements.

Where does it come from?

The chemical structure of Thulium is 9010996 in the Crystallography Open Database. The figure was produced with VESTA.

Reference: Koehler et. al., Phys. Rev. 126 (1962) 1672

 

 

Rare earth magnets: how crystallography can help you stay attractive

What does it look like?

What is it?

If you had to pick the three greatest achievements in modern technology, it’s a no-brainer: the moon landings, ubiquitous digital computing and the spin cycle on your washing machine.

The last in part is due to the material shown above, the “neodymium magnet.” The material has a chemical structure Nd₂Fe₁₄B, so it’s really mostly iron (the Nd are the red atoms in the structure shown above, the B is green). They’re used in a range of applications, from electric motors to hard discs. In the last few decades, a range of powerful new magnetic materials have been discovered which use the rare-earth elements as a constituent; it’s one of the hidden revolutions in material science.

Neodymium magnets have several features that make them industrially useful, one of which is high coercivity. You’d be annoyed if your washing machine stopped working because the magnets died. Coercivity is the ability of a magnet to retain its magnetization in the presence of an externally applied field.

How does this come about? The structure shown above is tetragonal, which means if you rotate the crystal around the ab plane in 90 degree steps, it looks the same. The neodymium atoms would much rather align their magnetic moments along the c axis than in the other directions. This preference for aligning in a particular direction is called the magnetic anisotropy. In a cubic material like iron, you don’t get this distinction between the three axes, and the anisotropy is far lower.

When the magnets are manufactured, they’re processed to induce texture. This means that, instead of being randomly arranged, the individual crystal grains in the magnets are oriented so that their c- axes are more or less pointing in the same direction.

 

These two features together combine to give the high coercivity: the grains are pointing in the same direction, the moments would rather be pointing that way, so it takes a lot of effort to demagnetize the magnet.

Where did the structure come from?

The chemical structure of Nd₂Fe₁₄B is 1511143 in the Crystallography Open Database. The figures were produced with VESTA.

References:

Rivoirard et. al., “Texture investigation of hot-forged Nd-Fe-B magnets”, Philosophical Magazine A, 80 (2000) 1955

R. Skomski and D.J. Sellmyer, “Anisotropy of rare-earth magnets”, Journal of Rare Earth Magnets, 27 (2009) 675