Peter Day was a pioneering chemist whose had a deep interest in how atoms transfer energies within materials. He was fascinated by the way atoms can share electrons within a set of materials called mixed valency compounds, what this means for the macroscopic properties of the material. These materials have a long history, dating back to the ancient Greeks who exploited them as glazes for their ceramic vases: mixed valency compounds often have vivid colours.
In 1966 Peter came across a paper published on Prussian blue, the first modern synthetic pigment. He contacted the author, Melvin Robin, and this led to a collaboration which established that more than half the elements in the periodic table could form mixed valency compounds. This understanding underpins a huge range of technological applications, includinh lithium ion batteries and perovskite solar cells.
In his later career he was director of the ILL in Grenoble, France before returning to the UK as director of the Royal Institution. In this post, he was a passionate ambassador for national and international facilities, and led the development of several instruments for the ISIS neutron and muon source.