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

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Growing crystals for your PhD – The trials and tribulations Part 2

In yesterday’s post I reminisced about a not-so-successful crystal growth of the defect perovskite material Sr0.8Ti0.6Nb0.4O3. However, my crystal growing adventures did not end there! I had begun a new crystal growth which would prove to be much more successful. In fact, it was to be my first crystal. The material in question was Sr3TiNb4O15.

Sr3TiNb4O15 and Sr0.8Ti0.6Nb0.4O3 are actually very closely related. In fact they are a part of the same series derived from SrTiO3. If substitution of niobium for titanium is continued beyond the Sr0.8Ti0.6Nb0.4O3 composition, the defect perovskite structure is no longer stable and a tungsten bronze type structure is formed instead. The exact composition at which a pure tungsten bronze type structure resulted was Sr0.6Ti0.2Nb0.8O3, which corresponds to Sr3TiNb4O15.[1]

Figure 1: Crystal structure of Sr3TiNb4O15 generated in VESTA. Structural information can be found in reference 1.

Figure 1: Crystal structure of Sr3TiNb4O15 generated in VESTA. Structural information can be found in reference 1.

Initially, Sr3TiNb4O15 was an impurity to me. It was something I was trying to avoid in order to synthesise a highly defect perovskite structure. However, Sr3TiNb4O15 itself is interesting as a relaxor ferroelectric material. Traditional ferroelectric materials exhibit a permanent reversible dipole moment which makes them extremely useful in capacitors, non-volatile data storage and other applications. Relaxor ferroelectrics differ slightly compared to traditional ferroelectric materials due to their complex compositions. Rather than exhibiting a strong ferroelectric response at a specific temperature (like traditional ferroelectrics), the response is spread out over a range of temperatures. This means that the dielectric constant of the material will be very close to its maximum for a wide range of temperatures, making them much more useful in applications.

Much like traditional ferroelectrics, the crystal symmetry plays a large role in determining the properties of the material. Specifically, the crystal symmetry can tell us whether a permanent dipole moment is possible within a structure. The best way to determine the structure of a crystal is diffraction from a single crystal! So once more I set out to grow crystals using the floating zone furnace.

Contrary to my rather lofty plans for Sr0.8Ti0.6Nb0.4O3, I did not require an especially huge crystal. Anything sub-millimetre scale would have been great. However, in comparison to the defect perovskite, a crystal of Sr3TiNb4O15 was really quite easy to grow. The main difficulty encountered was that the crystal growth was a little bit temperamental and unless it was “baby-sitted” it would tend to destabilise after about an hour. So I spent a nice quiet afternoon in the basement of the chemistry building slowly growing my crystal. Every now and then I would adjust some growth parameters to ensure that the melt wouldn’t destabilise. Eventually I obtained a lovely transparent yellow crystal.

Figure 2: Close up of the tungsten bronze crystal.

Figure 2: Close up of the tungsten bronze crystal.

Although the resulting column was comprised of multiple crystals, the crystal fragments were big enough for both X-ray and neutron diffraction experiments (however I was very hesitant to break up my crystal to do these!). While the work to determine the exact symmetry of the structure has now been passed down to another student, I will always be proud of my first ever successful crystal growth.

[1]                  T. A. Whittle, W. R. Brant & S. Schmid (2013). S. Schmid, R. L. Withers, R. Lifshitz (Eds.), Aperiodic Crystals, Proceedings of the 7th Conference on Aperiodic Crystals. Dordrecht: Springer. pp. 179 – 186

Growing crystals for your PhD, – The trials and tribulations Part 1

Will Brant tells us about growing Sr1-xTi1-2xNb2xO3 crystals for his PhD

For this post (and the next) I decided to do something a little different and talk about stories from my PhD which are in line with a crystal growing theme. Growing nice large crystals is not easy; there are so many factors which can influence the outcome of the growth of one specific type of crystal that some may consider it an art form. I found this out the hard way through my forays into trying to grow two seemingly similar, but ultimately very different, oxide materials. That is, Sr0.8Ti0.6Nb0.4O3 and Sr3TiNb4O15 (or Sr0.6Ti0.2Nb0.8O3).

The first of the two crystals I attempted to grow, Sr0.8Ti0.6Nb0.4O3, is a defect perovskite structure derived from the cubic perovskite SrTiO3. To get to Sr0.8Ti0.6Nb0.4O3 from SrTiO3 the TiO+4 cations are incrementally substituted with Nb+5 cations. In order to balance the difference in charge between titanium and niobium, vacancies are produced on the strontium position. (see structure below)

Figure 1

Sr0.8Ti0.6Nb0.4O3 was originally created in an attempt to produce new kinds of A-site deficient perovskites for lithium ion conduction applications. While Sr0.8Ti0.6Nb0.4O3 itself is not a very good lithium ion conductor it did exhibit some interesting short range structural distortions.[1] By growing a large single crystal, these distortions could be studied in greater detail.

My crystal growing method of choice was the floating zone furnace. To grow a crystal using this method, the ends of two sintered powder rods of a near-pure target material are melted using the focused heat from high power lamps. These molten regions are brought into contact with one another to produce a molten zone suspended between the two powdered rods. As the rods are moved vertically the molten zone travels through the polycrystalline powder. As the melt leaves the hot region crystals begin to form as the material cools. Over time, if all goes well, the growth of one of these crystals will predominate and a large, high quality single crystal will result.

The floating zone furnace.

The floating zone furnace.

The process began by preparing a pure Sr0.8Ti0.6Nb0.4O3 powder. This took around two to three weeks of repeated grinding and sintering. Once a rod of the polycrystalline material was prepared the growth could begin. By begin in this sense I mean about four trial growths were attempted to establish the best conditions. The hard work and long hours finally paid off however, as by the fifth attempt a stable growth had begun! However, this came to a grinding halt as a bubble began to form inside the melt. The bubble become larger and larger as the growth continued before finally bursting sending molten material flying out from the melt zone.

So what went wrong? Ironically, the presence of a few vacancies, while not in sufficient quantity to allow for lithium diffusion, encouraged entirely different, unintentional ion diffusion. That is, of oxygen ions! At high temperatures and slightly reducing conditions, oxygen is removed from the material. This gas became trapped in the melt gradually accumulating to the point where eventually a bubble of O2 burst and ruined everything.

It was time for round 6: remove as much oxygen as possible before attempting crystal growth. Given that oxygen rapidly left the material at high temperatures, this involved heating both the powder and then the sintered rod up to 1350 °C for extended periods of time. This time around, no bubbles formed and it appeared as though the melt and crystal growth were actually stable.

Lump!

Lump!

What did I get for my effort? A large lump of super dense highly fused polycrystalline ceramic. So, not a crystal. The lump was so large and dense that I couldn’t actually break it into smaller pieces despite my enthusiastic efforts to do so (RIP mortar and pestle). This lump of perovskite now sits in my desk. By this point I had decided to abandon trying to grow Sr0.8Ti0.6Nb0.4O3 crystals because another material was proving to be much more successful, Sr3TiNb4O15

To be continued…..

Cadmium tricyanomethanide tetramethoxyborate (try saying that three times!)

More tales from the Batten lab.

What does it look like?

image

 

What is it?

The best way to grow nice crystals is for the crystals to form slowly and undisturbed. This allows the molecules to assemble in the most regular fashion they can, with time to correct mistakes in the packing as the crystal grows.

Sometimes, however, crystals take much longer to grow than expected, and are found in crystal growing reactions that normally should have been abandoned and thrown away months beforehand. One such example is the rather complicated structure shown above.

The crystals of Cd(tcm)B(OMe)4.xMeOH took six months to grow, and seemed to appear almost overnight as beautiful and very large elongated octahedral. Note that they didn’t grow slowly and evenly over those six months, but rather the solution was completely devoid of crystals for almost all the time, and then suddenly surprise! Huge Crystals!

So what happened? It turns out that the chemistry is probably the key. The structure contains tetramethoxyborate anions, yet no such anions were added to the reaction. Rather, the boron in the anions comes from another anion that was added – tetraphenylborate. It seems that this anion slowly reacted with the methanol solvent to produce the new anion. The concentration new anion presumably built up over time until there was enough to start growing the crystals. And once it did start growing the crystals, well, it didn’t do it in half measures! They’re still some of the most spectacular crystals I’ve ever grown.

Where did the structure come from?

As for the structure itself, it contains a complicated coordination polymer that has small chiral pores running through it. Those pores contain helices of hydrogen bonded methanol molecules. The chiral nature of the coordination polymer, at that time, was very unusual, so the crystals were certainly worth the wait. Sometimes putting off cleaning up your lab bench of ‘failed’ crystallisation attempts can pay off!

Source: “Solvolysis of [B(C6H5)4] in methanol to give the chiral coordination polymer Cd(tcm)[B(OMe)4].xMeOH, x ≈ 1.6”, S.R. Batten, B.F. Hoskins and R. Robson, Angew. Chem. Int. Ed. Engl., 1997, 36, 636. DOI: 10.1002/anie.199706361

CCDC Refcode: RIVDEF

Do try this in your own home! Copper sulfate pentahydrate

What is it?

Image of a copper sulfate pentahydrate crystal, by Stephanb from http://en.wikipedia.org/wiki/File:Copper_sulfate.jpg

Image of a copper sulfate pentahydrate crystal, by Stephanb from http://en.wikipedia.org/wiki/File:Copper_sulfate.jpg

Who amongst us has not, as a kid, grown single crystals of copper sulfate from a Home Science Kit, or even by pilfering the ingredients from your parents’ garden shed? After a few trials you learn the knack of controlling the concentration and the location (i.e. temperature) of the solution of blue powder in water that will produce a large multifaceted single crystal with few fellow travellers. The surface of a dry crystal may begin to decompose within a few days, but you know you can always regenerate the lustre with further immersion in the solution.

 

Where did the structure come from?

We do not know if any of Walter Friedrich’s, Paul Knipping’s, or Max von Laue’s, parents showed them how to produce single crystals of copper sulfate this way, but we are very pleased that the young scientists had at least one good crystal for the historic recording of the first diffraction by X-rays in 1912 [1].

The first diffraction pattern, from copper sulfate pentahydrate, taken by Laue's group

The first diffraction pattern, from copper sulfate pentahydrate, taken by Laue’s group

The first X-ray Laue pattern had just four obvious Laue spots. Improvement to the optics increased this to 20 or so within a short time, but still hardly enough unique observations to yield a definitive crystal structure. Fast forward some 20 years and monochromatic X-ray techniques had improved to the point where such ‘complicated’ structures could be solved, often by recourse to elegant chemical and physical arguments that nowadays are usually neglected in favour of brute-force numerical computation. Charles Beevers and Henry Lipson’s description of the solution of the crystal structure of CuSO4.5H2O is a joy to read [2].

That was not the full story though, copper sulfate hydrate, or to use its mineral name, chalcanthrite, crystallises as a pentahydrate, and the hydrogen atoms of the five water molecules were not visible in the X-ray studies of the 1930’s. Another three decades were to pass before Bacon and Curry (we kid you not, that was their names!) located the hydrogen atoms by neutron diffraction [3].

The crystal structure of copper sulfate pentahydrate, without the hydrogen positions as determined by Beevers.  The blue atoms are copper, the red oxygen and the yellow are sulfur.  Image generated by the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

The crystal structure of copper sulfate pentahydrate, without the hydrogen positions as determined by Beevers and Lipson, crystallographic open database #1010527. The blue atoms are copper, the red oxygen and the yellow are sulfur. Image generated by the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

You might think that interest in copper sulfate should have waned by now, but no! Just last year Martin Mourigal and coworkers explored the low-dimensional magnetism in a visually elegant experiment that is the envy of many physicists [4].

[1] Interferenzerscheinungen bei Röntgenstrahlen, W. Friedrich, P. Knipping, & M. Laue, Ann. Physik, 346 (1913) 971.

[2] The crystal structure of copper sulphate pentahydrate, CuSO4.5H2O, C.A. Beevers & H. Lipson, Proc. Roy. Soc. Lond. A 146 (1934) 570.

[3] The water molecules in CuSO4.5H2O, G.E. Bacon & N.A. Curry, Proc. R. Soc. London Ser. A, 266, (1962) 95.

[4] Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain, M. Mourigal, M. Enderle, A. Klöpperpieper, J.-S. Caux, A. Stunault & H. M. Rønnow, Nat Phys. 9 (2013) 435.