Solid solutions

Some of the most important materials in our everyday life are made by mixing more than one material together. The stainless steel that comprise our knives and forks is a mixture of chromium and iron, designed in such a way as to be strong but not rust. Combining materials in smart ways to get a specific set of properties is central in engineering sciences. Knowing how to do this comes with challenges, especially when the materials are more complicated.

Rare earth nickelates can be immensely useful as materials with applications ranging from catalysis to sensing. We often want to fine-tune particular properties and this can be easily done by mixing different rare earth nickelates together in a so-called solid solution.

In our 2021 APL Materials paper, we showed that solid solutions of lanthanum nickelate and neodymium nickelate have a complex structural evolution from one material to the other. It turns out that the two materials have similar structures but not quite similar enough and a continuous transition between them is not allowed - yes really, by the laws of the Universe! We were able to study what's going on using advanced scanning transmission electron microscopy, a type of microscope that sends electrons instead of light at the sample and allows us to look at individual atoms.What our solid solution samples seem to do is adopt a mixed structure over a certain range of lanthanum:neodymium ratio. This makes sense and reaffirms out faith in physics, specifically group theory! We were a featured article on APL Materials and a Scilight article  was written about our work.

Follow-up work looked at the physics of these solid solutions. Apart from a structural change, the electronic and magnetic properties should change as you mix the neodymium and lanthanum together. Lanthanum nickelate is a paramagnetic metal and neodymium nickelate is a antiferromagnetic insulator at low temperature. We combine theoretical and experimental approaches to try to understand how that transition takes place. With increasing the content of neodymium, it appears that the metal/insulator transition occurs extremely sharply, but the paramagnetic/antiferromagnetic transition happens more gradually. Our theory suggests that over a very very narrow range of La/Nd ratio there is probably a paramagnetic metal/antiferromagnetic insulator phase coexistence with fluctuating magnetic moments. We published these results in Journal of Physics: Condensed Matter in 2023.