Study reveals unprecedented control of negative thermal expansion

An international team of scientists led by Oxford University’s Dr Mark Senn have successfully demonstrated that a material can be manipulated to expand or contract at different rates when its temperature is changed. 
 
This new finding, which appears on the cover of the latest issue of the Journal of the American Chemical Society (JACS), has the potential to lead to big benefits in the engineering and electronics sectors where thermal expansion (materials expanding as they get hotter) presents a big challenge. The effects of this range from the design and construction of aeroplanes, buildings, bridges and train tracks to electronic devices such as smartphones and iPods. 
 
Guarding against component and device failure due to the effects of heating caused by sunlight, electric currents, or fuel consumption has troubled humankind through the ages. With the help of the intense X-rays generated at Diamond Light Source, the UK’s synchrotron science facility in Oxfordshire, the team from Oxford, Imperial, Diamond and institutions in Korea and the US, have hit upon a potentially revolutionary finding.
 
   Scientists have been able to precisely control how much    
Perovskite material contracts or expands when is heated up 
  
Their research focuses on a class of materials that have been known about for a long time, those that go against the norm and actually contract when heated. Dr Mark Senn explains, “We have been working with materials that display negative thermal expansion (NTE) to see if we can control this very unusual property. It’s a very exciting area of physics right now and scientists around the world are eager to gain a better understanding of these materials in the hope that by controlling them we can put them to good use”.
 
Mark continues, “We already know that NTE happens when atoms inside a material vibrate in certain ways that actually cause them to move closer together – rather than further apart as you might intuitively expect when you heat a material up.  In this new research we have revealed that it’s possible to manipulate these vibrations in perovskite material by changing the concentration of two key elements: strontium and calcium. This has allowed us to turn on or off the NTE as we choose, and precisely control just how much the material contracts or expands when it is heated up. We now have a “chemical recipe” for doing this, which should prove to have much wider application. ”.
 
Imperial’s Dr Arash Mostofi and Dr Nick Bristowe explained the impact of the work: “Our understanding of the processes underlying the effect means that we can search for it in related materials in the perovskite family or in other classes of materials with wide applications”.
 
This is an early step forwards, but the findings of Senn and his collaborators have opened the door for other researchers looking to control NTE materials. If this method of controlling these “contraction causing” vibrations works in other materials, it opens up very exciting opportunities for furtherfundamental and applied discoveries. 
 
Dr Claire Murray is a support scientist at Diamond on the powder diffraction beamline (I11) where the team carried out their experiments. Her expertise in synchrotron science allowed the group to scrutinise the very small changes that occur in the perovskite on the atomic length scale, as its composition is changed.
 
Dr Claire Murray, Support Scientist at the powder diffraction Beamline I11
 
She said: “Diamond is a very powerful tool for watching chemistry in action at the atomic and molecular level. Researchers are increasingly turning to synchrotrons to deepen their understanding of chemical processes and, in a similar way to chefs adjusting their recipes to get a better texture or taste, scientists are adjusting the elemental composition of materials and thereby controlling their properties and functions in ways that will bring performance and safety benefits in a wide range of areas including transport, construction and new technology.”