Han purple – colour of the terracotta warriors.

In 1974, local farmers digging a well in China’s Shaanxi province uncovered the tomb of the first Qin Emperor, who died in the 3rd century BC. Buried with the emperor were thousands of life-size terracotta figures, now famously known as the Terracotta Army. The warriors were once brightly painted, and although they have faded over the last 2000 years, the colours can still be seen. One of the prominent pigments used is known as Han purple, which is chemically a barium copper silicate, BaCuSi2O6.

Terracotta warriors, coloured with now-faded Han purple pigment. Source: q-files.com

Terracotta warriors, coloured with now-faded Han purple pigment. Source: q-files.com

Han purple was the first synthetic purple pigment, and was used from at least the 8th century BC until the end of the Han dynasty in the 3rd century AD, when the secrets of its production were lost. It appears to have been made from a mixture of barium and copper minerals, quartz, and a lead salt as a special extra ingredient that acts as a catalyst and flux. The mixture needed to be heated to between 900 and 1000 °C – any hotter and you’ll get Han blue (BaCuSi4O10), which is closely related to Egyptian blue (CaCuSi4O10), the oldest synthetic pigment.

The structure of Han purple, BaCuSi2O6, viewed along two different directions, with barium atoms in green, copper in blue, silicon in yellow, and oxygen in red. It can be found in entry 9001237 of the Crystallography Open Database.

The structure of Han purple, BaCuSi2O6, viewed along two different directions, with barium atoms in green, copper in blue, silicon in yellow, and oxygen in red. It can be found in entry 9001237 of the Crystallography Open Database.

Modern interest in Han purple goes beyond its colour. Crystallography has revealed that it has a layered structure with layers of barium atoms sandwiched between copper silicate layers. In 2006 it was discovered that at temperatures very close to absolute zero, and in the presence of a strong magnetic field, the material effectively loses a dimension, transforming from a 3D material to a 2D Bose-Einstein condensate as it crosses a quantum critical point.

Getting your cakes to rise – bicarbonate of soda

What does it look like?

Yellow atoms are sodium, red are oxygen and the brown are carbon atoms.  As this structure was found by x-ray diffraction no-one is quite sure where the hydrogen atoms are. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Yellow atoms are sodium, red are oxygen and the brown are carbon atoms. As this structure was found by x-ray diffraction no-one is quite sure where the hydrogen atoms are. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Raising agents (yeast, bicarbonate of soda and baking powder) work by adding carbon dioxide (CO2) gas to your mixture. This is produced as bubbles, which rise and add volume to your baking. Yeast works by a fermentation process; they eat the sugars in your bread and ‘breath’ out carbon dioxide. But this is quite a slow process, which is why to leave bread to rise, and not always suitable for cakes and lighter baking. Bicarbonate of soda, sodium hydrogen carbonate (NaHCO3) is the quicker alternative for cakes. This releases carbon dioxide when reacted with something acidic (like fruit, buttermilk or honey). This acts straight away, that is why you can’t leave your baking batter for long before you put it in the oven.

But, if you’re baking something without the acidicness (i.e. chocolate chip cookies) you’ll need to use baking powder. In your baking powder is an ‘in built’ acid – usually cream of tatar (potassium hydrogen tartrate KC4H5O6) and when this released with moisture it reacts with the bicarbonate of soda to release the carbon dioxide.

Where did structure come from?

The structure was first determined in 1933 by Zachariasen, but later modified (with better data) by Sass and Scheuerman in 1962. We’ve features Sass and Scheuerman’s structure of sodium bicarbonate but both can be found in the American Mineral Database under the mineral name of sodium bicarbonate – Nahcolite

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/

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”.

Local Order Hidden Inside the Average Order in PZN

Short Range Order, what is it and why is it tricky?

Modelling short-range order (SRO) is tricky. Since in a system with SRO you cannot assume that all unit cells are the same, conventional crystallographic ideas — like Rietveld refinement or conventional single crystal refinement of diffraction data — just don’t work. These tools are useful for giving you the global average structure, but they don’t tell you about the local configurations that make up that average.

It is a bit like looking at the average of a room full of people. The average person might be 45% male and 55% female, 1.7m tall, 68kg in weight… but there is a lot more information to be had. What is the average weight of the males? Average height of the females? And, of course, the average person does not really exist, so knowing this average might not be very useful.

Similarly with materials, if we want to know how the useful properties arise from the structure and we only have the average structure, then if the system shows disorder, then this average might not ever actually occur, so doing crystal chemical calculations based on it will lead us astray.
A good example of this is the family of relaxor ferroelectrics, one of the best known being PZN, PbZn1/3Nb2/3O3. It is a perovskite, which we’ve seen plenty of this year. In this case, the PZN unit cell can be modelled by considering the Pb sites to really consist of 12 Pb sites, one along each of the [110] directions.

What does it look like?

A unit cell of PZN, with Pb (green), Zn.Nb (blue) and O (red).  The Pb are modelled as 12 split sites, on each of the [110] directions.

A unit cell of PZN, with Pb (green), Zn.Nb (blue) and O (red). The Pb are modelled as 12 split sites, on each of the [110] directions.

But what does the diffuse scattering look like?

Evidence for SRO shows up in the diffuse scattering, the scattering between the Bragg reflections. Some diffuse scattering for PZN looks like this:

Diffuse scattering from PZN, (a) in the hk0 layer and (b) in the hk1. Streaks, diamonds and other shapes are all clearly visible, all containing information about the SRO in PZN.

Diffuse scattering from PZN, (a) in the hk0 layer and (b) in the hk1. Streaks, diamonds and other shapes are all clearly visible, all containing information about the SRO in PZN.

Now, the diffuse scattering in PZN can be modelled quite well by assuming that the Pb displace along [110] and these displacements are correlated in certain ways, and that the other atoms then relax around this Pb configuration. The problem is that these models are largely descriptive and artificial. They are based on human interpretation of the data, and do not necessarily relate directly to the underlying chemistry.

Modelling the Diffuse Scattering

In the simulation, atomic positions were swapped around and configurations kept or rejected based on whether they reduced the global instability index or not. This was after initial random distributions had been established based on the average structure. This means that the histograms of atomic separations always give the correct average and overall atomic displacement parameters, so all this SRO can be ‘hidden’ inside a conventional average structure.

Calculated hk0 diffuse from PZN.

Calculated hk0 diffuse from PZN.

And this approach really can give diffuse scattering which looks a lot like the observed and which reveals what is ‘inside’ the average and see how the Nb behaves compared to the Zn, and so on…so below we can see that the O-Nb distances change compared to the O-Zn, even though there is no long-range order in the distribution of Nb and Zn.

plot_dist_size_bvs_o_b

By modelling the SRO, it is possible to see how the environment around Nb is different to that around Zn, while retaining the overall average shown by conventional studies.

The highly structured diffuse scattering, and the local ordering that gives rise to it, can exist in the system without influencing the outcomes of conventional structural studies. This indicates that short-range order may be present, and crucial, and unsuspected, in many systems whose structures are thought to be thoroughly determined.

Where did this all come from?

This is all from work described in a paper by Whitfield et al.

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.

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

Absorbing and beautiful, zinc nitrate

What does it look like?

Zinc atoms are grey, nitrogen light blue and the red atoms represent oxygen (some of which will be the centre of water moleucles.)

Zinc atoms are grey, nitrogen light blue and the red atoms represent oxygen (some of which will be the centre of water moleucles.)

What is it?

Zinc nitrate is one of those materials that every chemist has in their cupboard. It’s very deliquescent, meaning that it will absorb water from the atmosphere. The reason we chose to feature it today is that we stumbled upon this beautiful video, on the RI Channel.

Watch the full video with related content here: http://richannel.org/beautiful-chemical-reactions

This shows, among other things, beautiful zinc nitrate ‘trees’ growing – silver atoms are being substituted for zinc as the material is re-crystallising.

Where did the structure come from?

Given it’s affinity to water it’s really very rare to crystallise pure zinc nitrate with no water molecules in the crystal structure.  So we’ve featured zinc nitrate dihydrate (two waters for every zinc nitrate unit), a structure determined by D. Petrovic and B. Ribár in 1975

Neo-mag: The strongest permanent magnet of them all!

What does it look like?

Left: High-resolution transmission electron microscopy image of Nd2Fe14B; right: crystal structure with unit cell marked.  Nd in blue, Fe in red and B in yellow (source: http://en.wikipedia.org/wiki/Neodymium_magnet)

Left: High-resolution transmission electron microscopy image of Nd2Fe14B; right: crystal structure with unit cell marked. Nd in blue, Fe in red and B in yellow (source: http://en.wikipedia.org/wiki/Neodymium_magnet)

What is it?

Many of today’s technological devices that we often take for granted (cars, mobile phones, laptops, memory devices) rely heavily on the magnetic properties of materials. One such property of a magnet that scientists are trying to enhance is its “hardness” – or strength as a permanent magnet. This can be seen in the rare-earth magnetic material Nd2Fe2B, which is considered to be the strongest permanent magnet around and is ferromagnetic (with all magnetic moments aligned in the same direction) for all temperatures up to 400°C. It has an exceptionally high anisotropy, indicating that the magnetic moments have a strong preference to align along one crystallographic direction. When we apply a magnetic field in the opposite direction to the moments, Nd2Fe2B also shows a strong resistance to flip the spins indicating a high coercivity (750–2000 kA/m). It is these properties that make Nd2Fe2B such a useful magnetic material. Another hard magnet with similar properties to Nd2Fe2B is SmCo5.

Where did the structure come from?

Since the development of this material by General Motors and Sumitomo Special Metals in 1982 Nd2Fe2B has found its way into a myriad of modern devices such as computer hard disks, magnetic resonance imaging (MRI) scanners, door locks, electric motors (in particular for cordless devices), hybrid and electric cars, and electric generators. There are, however, caveats to consider when using this material – it is highly corrosive necessitating Ni plating of all parts, and the rare-earth component makes them relatively expensive to produce.