Understanding Metals

All chemical elements have specific properties, but just exactly how these properties are acquired is a challenging scientific question at the heart of Nicola Gaston’s research.

A theoretical chemist at Industrial Research Ltd, Gaston was appointed as a Principal Investigator with the MacDiarmid Institute at the start of 2010, following two years of close collaborations with other institute members. “I’ve always wanted to do something that combined physics and chemistry,” she says. “I am particularly interested in understanding the complex changes in electronic structure that occur when you go from a simple pair of atoms, to larger clusters, and then finally all the way to the bulk.”

Take metals for example. Most of us think of metals as solid and shiny materials that conduct electricity and heat, but Gaston has examined the behaviour of tiny metal clusters of just a few atoms. “A metal is something that we think of as infinite in principle, if you think of the electronic wave functions through a metal system, so when you have a small number of metal atoms it’s a very good question whether the material is a metal or not, whether it’s more of a metal or more of a molecule.”

With most metals, the transition from molecule to metal happens as soon as there are more than a handful of atoms, but some metals are more complicated than others. “Typically with metals, once you have of the order of seven atoms, you have quite a delocalised electronic density and you can say it’s somehow metallic, but not all metals are so well behaved. With mercury, for example, which I worked on for my PhD, you might need hundreds of atoms before it becomes metallic and it’s very weakly bound when you only have a couple of atoms.”

Gaston worked on her PhD at the University of Auckland and subsequently at Massey University, trying to understand the bulk behaviour of mercury. “The way we were doing the calculations was looking at the local environment of each atom and taking that into account in a very accurate way, so even though we were predicting the properties of the bulk metal, a lot of the insights came about through thinking about what a metal is in a smaller piece. The calculations were done with small numbers of atoms and then we tried to say something about the bulk metal.”

Her focus is on finding alternative ways of calculating properties of bulk materials. “I’m interested in developing methods for calculation that don’t rely on experimental data and parameterisation to the same extent as typical methods. They are methods that require no input and from purely quantum mechanical principles can predict structural information about metallic systems.”

Gaston says she finds the work very rewarding, partly because few people are working in the area, but mostly because it connects physics and chemistry. “The electronic structure of metals is really physics from 50 years ago. The basic understanding from then in some ways hasn’t moved on that much, even though we’ve got used to using quantum mechanics to describe the electronic structure very accurately in small molecules and being very precise in making predictions about the structure. My project really took the best of both, the physical understanding of what a metal is and these very sophisticated techniques that are used in chemistry for very small molecules. I try to make a bit of progress in the understanding of why different metals can be quite different structurally, even though in physics they are considered to be quite similar.”

Gaston left New Zealand in 2005 to join the Max Planck Institute for the Physics of Complex Systems in Dresden to pursue her research as a post-doctoral fellow. At the time she didn’t think she would return. “It was a hard decision to come back. I had assumed that I wasn’t coming back because I thought that I’d never find work here. People with experimental skills seem to be a bit more flexible but theoretical chemistry seemed a bit of a stretch for New Zealand and I thought I’d just continue in Europe.”

Although the main reasons for her return were personal, with her mathematician partner working on his PhD at Massey University, she says the MacDiarmid Institute played a significant role in the process. “My partner heard a talk by (MacDiarmid Institute Deputy Diretor and IRL colleague) Shaun Hendy at a mathematics conference and so I looked him up … and felt that there might actually be a future in New Zealand.”

Gaston’s collaboration with Hendy meant a slight shift in her research towards investigating surfaces of functional materials, for example electro-catalysts, but it fitted well with other research at IRL and led to other projects, including a Marsden grant to study gallium. “Gallium has a very strange structure in the bulk, which I knew about from my work at the Max Planck where my colleagues were interested in it and had wanted me to do some calculations on it. The melting properties of gallium are also unique and there are some interesting things that happen when you get small pieces of gallium. Small clusters melt at a very different temperature to the bulk material, they superheat.”

The first experimental evidence for superheating was observed in tin clusters, which melt at significantly higher temperatures than the bulk metal. Experiments on gallium and aluminium subsequently confirmed that superheating in these metal clusters was not an isolated phenomenon, but it contradicts the accepted theory, which predicts that melting temperatures should decrease with the size of the sample.

Gaston says gallium is an unusual element, sitting in the periodic table beneath aluminium, a classic metal, and boron, which isn’t metallic at all. “Gallium is really in between the two of them in many ways, in terms of its chemistry and its structure. It’s well behaved in very small molecules and it seems that it’s reasonably metallic in larger clusters. But when you go to the bulk it becomes somehow molecular and you have these pairs of atoms that are bonded covalently to each other, so you have two different kinds of bonds coexisting, with some electrons being in these covalent bonds and other electrons making metallic bonds.”

For Gaston, the superheating of small clusters is an important phenomenon to study. “The fundamental importance is that once you start making devices that are this small and only have tens of atoms, you want to know if it’s going to be stable and if it’s going to melt at a temperature that is very different from the bulk melting point. Trying to understand how these properties change when you go to very small materials is very important for a lot of the technologies that we think could be coming along in the next decades.”

Apart from continuing collaboration with Shaun Hendy, Gaston has already set up links with other MacDiarmid Institute colleagues, including Simon Brown and Richard Tilley and she looks forward to more in the future. “At IRL we’re focused on well defined projects in smaller teams. Having the larger network at the MacDiarmid Institute will be very complementary.”

She feels that it might also expand her horizon towards applied research initiatives. “It’s a privilege to be here at IRL doing fundamental research that’s a long way from applications, and to be doing that in a place where the focus is on working for and with New Zealand industry to increase the technology focus. It makes me aware of benefits that I might not see if I weren’t working in this incredibly broad scientific environment.”