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

Will Brant tells us about growing Sr_1-xTi_1-2xNb_2xO₃ 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, Sr_0.8Ti_0.6Nb_0.4O₃ and Sr₃TiNb₄O₁₅ (or Sr_0.6Ti_0.2Nb_0.8O₃).

The first of the two crystals I attempted to grow, Sr_0.8Ti_0.6Nb_0.4O₃, is a defect perovskite structure derived from the cubic perovskite SrTiO₃. To get to Sr_0.8Ti_0.6Nb_0.4O₃ 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).

Sr_0.8Ti_0.6Nb_0.4O₃ was originally created in an attempt to produce new kinds of A-site deficient perovskites for lithium ion conduction applications. While Sr_0.8Ti_0.6Nb_0.4O₃ 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 process began by preparing a pure Sr_0.8Ti_0.6Nb_0.4O₃ 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!

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 Sr_0.8Ti_0.6Nb_0.4O₃ crystals because another material was proving to be much more successful, Sr₃TiNb₄O_15.

To be continued….

Tags: floating zone   crystal growing   perovskite