Sustainable technologies capable of changing the future of electronic devices, batteries and displays can now be created in the laboratory, thanks to the vast variety of mixed anion compounds available. Until the year 2000, many researchers were not paying much attention to mixed anion compounds. This is due to the fact that the Earth’s atmosphere is far more conducive to simpler single-anion compounds such as metal oxides. For example, when elemental iron is heated in the air, iron oxide – or rust – is quickly created. Complex oxides can also be obtained by heating a mixture of binary oxides to high temperatures in a process known as a high-temperature solid-state reaction. The simplicity of this method has allowed a wide range of physicists to work with metal oxides and create a variety of new oxide structures and compositions. ‘On the other hand,’ Kageyama explains, ‘mixed anion compounds cannot generally be prepared in such a facile method as air.’ Inevitably, this makes controlling and creating mixed anion compounds far more difficult. However, the benefits that come from working with mixed anion compounds far outweigh the struggle of creating them. Unlike oxides and nitrides that only contain one type of ‘building block’, that is a singular anion, mixed anion compounds contain a diverse range of anions with different key features, such as valency, ionic radius, electronegativity and polarizability. Like a structure with different toy construction bricks, this variability allows for a wider range of compounds with distinct structures and dimensions. In order to properly work with mixed anion compounds, Kageyama and his team have divided their study into three subgroups: group A01 for the creation of mixed anion compounds; group A02 for the observation of mixed anions and group A03 for the exploration of innovative functionalities. Group A01, led by Dr Hiraku Ogino from the National Institute of Advanced Industrial Science and Technology, is focused on establishing and synthesising mixed anion compounds using techniques such as high-pressure synthesis, topochemical reaction and solvothermal methods. Additionally, this group is also working to define the rules of anion arrangement through both experimental and theoretical analysis. Group A02, led by Professor Katsuro Hayashi of Kyushu University, is focused primarily on assessing light elements such as hydrogen as a means to deepen their understanding of mixed anion compounds. By combining x-rays and neutron diffraction methods, the team can evaluate the chemical state of hydrogen within the mixed anion compounds and, in doing so, paint a picture of the geometry within these complex compounds. Group A03, led by Professor Kazuhiko Maeda from the Tokyo Institute of Technology, is the final step in applying mixed anion compounds in the real world. The team’s primary objective is to generate high-performance energy materials that can both produce and save energy. With these new developments, group A03 hopes to establish usable technologies that are far more diverse than single anion compounds and can sustain our future society. So far in this five-year study, the group has discovered many fascinating characteristics about mixed anion compounds. The laboratory has been working particularly with a well-established functional oxide known as barium titanate (BaTiO3). In order to induce catalytic activity in BaTiO3, the group added a hydride (H-) ion – or hydrogen anion – and discovered that this addition can break a triple bond in an N2 molecule. They discovered that the H- ion defies chemistry’s ‘scaling rule’, which refers to the chemical process required to cause a specific chemical reaction.