Shrinking in the heat – lanthanoid hexacyanidocobaltates, a.k.a. LnCo(CN)6

While most materials expand when heated, a few show the opposite behaviour, known as negative thermal expansion (NTE). The record holder, which has the greatest rate of volume contraction upon warming over a wide temperature range, is single-network cadmium cyanide (we previously blogged about the double-network version). This unusual phenomenon is useful – if you combine an NTE material with a normal material in just the right ratio, you can make a mixture with zero thermal expansion. Such a composite is immune to the undesirable effects of thermal expansion, from the buckling of railway tracks in the heat, to quartz-crystal clocks gaining or losing time when it’s too hot or cold.

A related group of frameworks which also show NTE are the lanthanoid hexacyanidocobaltates, with chemical formula LnCo(CN)6, where Ln can be any element from the lanthanoid row of the periodic table from lanthanum (La) to lutetium (Lu), Co is cobalt, and CN is cyanide. The cyanide is tightly bound to the cobalt, so these materials aren’t particularly toxic, unlike potassium cyanide.  They can be easily crystallised from a solution as shown in this time-lapse video, recorded over the course of a few hours.

LaCo(CN)6·5H2O is easy to grow, and forms nice hexagonal crystals. By the end of the video, they are about 2 mm in size and have started to merge together. Once the water is removed from the crystal structure by heating, the NTE properties are activated.

The key to the NTE is the networked structure of the materials. The lanthanoid and cobalt atoms are ‘bridged’ together by the cyanide (CN) ions. As the temperature rises, these linkers start to vibrate more and more in skipping-rope-like transverse (sideways) vibrations. Just like the ends of a skipping rope get closer together when you vibrate it, these transverse motions cause the average metal-metal distances to decrease, thereby causing contraction of the crystal.

The LnCo(CN)6 frameworks’ lanthanoid and cobalt metal atoms are linked together by cyanide bridges. ‘Skipping rope’ transverse thermal vibrations of these bridges bring the metal atoms closer together and cause the contraction of the whole material as it heats up.

The LnCo(CN)6 frameworks’ lanthanoid and cobalt metal atoms are linked together by cyanide bridges. ‘Skipping rope’ transverse thermal vibrations of these bridges bring the metal atoms closer together and cause the contraction of the whole material as it heats up.

Crystallography (powder diffraction in this case) allows the dimensions of the unit cell to be precisely measured, which lets us monitor the thermal expansion of materials by taking diffraction measurements at a number of different temperatures. The results for a series of LnCo(CN)6 compounds are shown in the plot below, and demonstrate that we can tune the NTE properties just by making the material with different lanthanoid metals. Swapping Lu for La doubles the steepness of the plot. This is because La bonds more loosely to the cyanide linkers than Lu does, making a more flexible framework with bigger transverse vibrations, and therefore bigger NTE.

NTE_2

The crystal structure of LaCo(CN)6 is made up of trigonal prismatic lanthanum (green) and octahedral cobalt (blue) metal atoms linked together by cyanide bridges (grey/blue). The plot shows the negative thermal expansion (NTE) behaviour of several different LnCo(CN)6 frameworks (with Ln = La, Sm, Y, Ho and Lu) measured using X-ray powder diffraction at the Advanced Photon Source. Larger lanthanoid metals, such as La, give a more flexible framework and a stronger NTE effect. The neutron diffraction measurements (black) allow us to go to lower temperatures, revealing how the NTE behaviour dies out, deviating from the nice straight line as the transverse vibrational modes “freeze out”. These results were published in S.G. Duyker et al. (2013), Angew. Chem. Int. Ed., 52: 5266–5270.

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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.

It Breathes but isn’t Alive – MIL-53, a Flexible Framework

What does it look like?

The open wine-rack structure of MIL-53(Cr), with chromium atoms in blue, and benzenedicarboxylate linkers in brown and red (hydrogen atoms not shown). The crystal structure data can be found in the original paper describing the structure. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software.

The open wine-rack structure of MIL-53(Cr), with chromium atoms in blue, and benzenedicarboxylate linkers in brown and red (hydrogen atoms not shown). The crystal structure data can be found in the original paper describing the structure. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software.

What is it? Where did the structure come from?

MIL-53(Cr) is a metal-organic framework made up of chromium atoms linked together by benzenedicarboxylate molecules, forming a three dimensional wine-rack-like structure. MIL stands for Materials Institut Lavoisier, after the place where the material was first made by C. Serre et al. in 2002.

MIL-53 exhibits a ‘breathing’ effect, where the structure opens and closes depending upon what’s inside the pores. When hydrated, rather than causing swelling of the framework, the water molecules inside have an attractive effect on the walls and actually hold the wine rack closed. Once the water is removed by heating, the flexible wine rack opens and the volume of the structure expands by an impressive 50%.

The ‘breathing’ effect in MIL-53 (from C. Serre et al., JACS 2002, image © American Chemical Society)

The ‘breathing’ effect in MIL-53 (from C. Serre et al., JACS 2002, image © American Chemical Society)

Similar breathing phenomena have since been found in other frameworks, and can be used for selectively separating different gases, when the structure only opens up for a certain type of molecule.

Porosity on your Plate – An Edible Metal-Organic Framework

What is it?

Metal-organic frameworks (MOFs), which are porous crystals constructed from metal atoms linked together by organic molecules, have a wide range of interesting properties, including magnetic switching, catalytic activity, and the ability to store gases such as hydrogen or methane. Until a few years ago, nobody had thought to add edibility to this list.

MOFs are often synthesised from toxic metals such as cobalt or cadmium, and unpalatable linkers like terephthalate or pyrazine. In contrast, this cyclodextrin-based MOF (CD-MOF-1) has been made using food-grade ingredients. Mixing γ-cyclodextrin and potassium benzoate (food additive E212) in water, followed by the addition of Everclear grain alcohol, gives a framework that should be safe to eat. We’re not sure what it tastes like, but you could potentially put all kinds of tasty molecules into the three-dimensional network of pores running through the material.

What does it look like?

An edible cyclodextrin-based metal-organic framework (CD-MOF-1). The purple polyhedra represent the potassium ions which are surrounded by eight oxygen atoms. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

An edible cyclodextrin-based metal-organic framework (CD-MOF-1). The purple polyhedra represent the potassium ions which are surrounded by eight oxygen atoms. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

Where did the structure come from?

This edible MOF was created by R. Smaldone et al. in 2010. The structure can be found in entry 773709 of the Cambridge Crystallographic Data Centre (CCDC).

X is for xenotime

What is it?

So far we’ve had a crystal structure starting with every letter of the alphabet, except one. Xenotime is not some kind of bizarre extra-terrestrial timekeeping system, but is in fact a mineral. Ironically, it was supposed to be spelled with a “k”, after the ancient greek kenós (κενός) which means “vain”, but an early misprint replaced the “k” with an “x”, which stuck. The main component of xenotime is yttrium orthophosphate. It also contains relatively high levels of uranium and thorium impurities, which make it very useful for U-Th-Pb isotopic dating of sedimentary rocks.

What does it look like?

A xenotime crystal. Source: www.mindat.org

A xenotime crystal. Source: http://www.mindat.org

 

 

Yttrium orthophosphate, the main component of xenotime. Yttrium atoms are shown in green, phosphorus in purple, and oxygen in red. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

Yttrium orthophosphate, the main component of xenotime. Yttrium atoms are shown in green, phosphorus in purple, and oxygen in red. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

Where did the structure come from?

The structure was determined by M. Strada and G. Schwendimann in 1934, and can be found in the Open Crystallography database (#1011143).

Keggin your pardon, ma’m – the structure of a heteropoly acid

What does it look like?

 Dodecatungstophosphoric acid hexahydrate. Phosphorus atoms are shown in green, tungsten in purple, oxygen in red, and hydrogen in pink. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

Dodecatungstophosphoric acid hexahydrate. Phosphorus atoms are shown in green, tungsten in purple, oxygen in red, and hydrogen in pink. Image generated using the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

What is it?

A heteropoly acid is an acid whose anion (negatively charged part) is made up of multiple metal oxide units built around a ‘heteroatom’, such as phosphorus. This one is called dodecatungstophosphoric acid hexahydrate, with chemical formula (H5O2+)3(PW12O403-). As you can see, this is a bit more complicated than your average acid, and the structure of the anion was not known until the 1930s, when  J.F. Keggin worked it out using X-Ray diffraction. This class of anions has since been known as Keggin ions.

The anions self-assemble in an acidic solution into the structure shown above. The phosphorous (P) heteroatom is tetrahedrally bonded to four oxygen atoms, which are enclosed by a tungsten (W) oxide cage. The water molecules and acidic protons fill up the spaces between the anions. Various other elements can be substituted for P and W, giving ions with different structures and properties. They can be used as catalysts for processes such as the hydration of hydrocarbons to alcohols.

Where did the structure come from?

This udpate on Keggin’s original 1933 crystal structure was determined in 1977 by G.M Brown et al. and can be found in the Open Crystallography database (#1008010). They used a combination of X-ray and neutron diffraction, which is why the hydrogen atoms are so well resolved – neutrons are much better than X-rays at seeing hydrogen!

Not another ice structure… Methamphetamine

What does it look like?

Dextromethamphetamine hydrochloride. Image generated using CrystalMaker: http://www.crystalmaker.com

Dextromethamphetamine hydrochloride. Image generated using CrystalMaker: http://www.crystalmaker.com

What is it?

Let’s face it – it was only a matter of time until ‘crystal’ meth showed up on this blog.

Methamphetamine was first synthesised in 1893 by Japanese chemist Nagai Nagayoshi, with the crystalline form shown here developed in 1919. The substance was found to elevate mood and counteract fatigue, and was given to Allied bomber pilots during World War II. Unfortunately, it is also extremely addictive, and post-war Japan experienced the first meth epidemic. In higher doses, it can cause psychosis, among other nasty effects.

More specifically, this is the crystal structure of dextromethamphetamine hydrochloride (the green atoms in the structure are chlorines). It is common for drugs (both licit and illicit) to be made as hydrochloride salts because it increases their water solubility and stability. The non-salt ‘free base’ of methamphetamine is not even a crystal at room temperature, but a colourless liquid. The dextro prefix means that this is the more potent right-handed form of the methamphetamine molecule. The left handed mirror image, levomethamphetamine, has somewhat different properties and is actually sold as an over-the-counter nasal decongestant in some countries.

Where did the structure come from?

The crystal structure was re-determined in 2008 by P. Hakey et al. It can be found in the Open Crystallography database (#2218183).