In October 2018, a team of scientists from Université Catholique de Louvain (UCLouvain) led by Geoffroy Hautier published the results of a high-throughput computational screening search for electrides, materials in which some of the electrons occupy free space rather than atomic orbitals (Chemistry of Materials). The results predicted the existence of electrides with partially filled d-shells. One such electride has now been experimentally verified, as Hautier and an international team of collaborators report in a recent issue of the Journal of the American Chemical Society.

“Most of the electrons in materials are localized around nuclei. Electrides are [an] exception to this rule,” Hautier explains. In electrides, electrons reside in otherwise empty pockets, channels, or cavities within the material and act as anions. Electrides can display intriguing chemical, transport, optical, and catalytic properties as a result of this unusual chemistry. Very few electrides have been discovered thus far.

The 2018 high-throughput search identified around 60 electrides from its database of 40,000 inorganic materials. Among them, two materials stood out as being especially interesting: Sr₃CrN₃ and Ba₃CrN₃. These materials contain chromium (Cr), a transition metal that by definition has a partially filled d-subshell. This is surprising because it is conventionally assumed that a transition metal in this situation would extract the electrons from free space to fill its d-subshell through a redox reaction.

In a follow-up experiment, the UCLouvain team collaborated with researchers at Cornell University, Chulalongkorn University in Thailand, the Technical University of Münich in Germany (TUM), and Oregon State University (OSU) to experimentally probe the electronic state of Sr₃CrN₃. A conventional oxidation state assignment would be Sr²⁺₃Cr³⁺N^3-₃. However, the computational search predicted Sr²⁺₃Cr⁴⁺N^3-₃:e^-, with the electron residing in a one-dimensional channel in the crystal.

A Cornell team led by Jin Suntivich synthesized Sr₃CrN₃ and characterized its basic properties using x-ray diffraction. Next, a team at OSU probed the electronic structure of the material using x-ray absorption spectroscopy. The data revealed a Cr peak characteristic of an oxidation state higher than +3, a finding that agreed well with computational models. However, hydrogen atoms in a material can sometimes act as anions, also known as hydride ions, and produce the same result.

To look for evidence of hydrogen, the researchers first used x-ray diffraction to determine the lattice parameters of Sr₃CrN₃. Theory predicts that hydrogenated and non-hydrogenated Sr₃CrN₃ have different lattice parameters. The experimental results were in close agreement with the non-hydrogenated structure. Researchers at TU Münich and UCLouvain examined the material with neutron powder diffraction, a technique more sensitive to hydrogen, and the results also indicated that the unexpected electronic state of Sr₃CrN₃ was most likely caused by electrons, not hydrogen atoms. These results suggest that Sr₃CrN₃ is the first known electride with a partially filled d-shell transition metal.

“Electrides with partially filled d-shell exist and we found one: Sr₃CrN₃,” Hautier says. “Although our work is at this stage still fundamental, it offers a perspective on how one can take advantage of the unusual combination of redox and electride behaviors. For instance, one could imagine that the redox activity in an electride could be interesting for catalysis or that partially-filled d-shells could enable strong magnetism in electrides.”

Scott Warren, a chemist at the University of North Carolina at Chapel Hill, calls this research “an important advance toward realizing new kinds of catalysts and, more generally, functional materials.” Warren led a team that synthesized the first electride nanomaterial in 2016 and was not associated with this new discovery. “(T)he partially filled shell could accelerate one step in a catalytic cycle while the electride electrons could help a second step,” he says. “To that end, it would be interesting to see if the distance between the electride electron and the transition metal can be further reduced.”

The researchers are continuing to study the properties of Sr₃CrN₃ and exploring whether similar nitride compounds may also be electrides. “We will keep on working on these avenues but do hope the community will join us,” Hautier says.

Read the abstract in the Journal of the American Chemical Society.