Blogging a crystal structure a day in 2014


Contributed by

William Brant

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 with 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 it 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 with 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.