Using Topology to Predict Electrides in the Solid State

Electrides are characterized
by electron density highly localized
in interstitial sites, which do not coincide with direct interatomic
contacts. The rigorous quantum mechanical definition of electrides
is based upon topological criteria derived from the electron density,
and in particular the presence of non-nuclear attractors (NNAs). We
employ these topological criteria in combination with crystal structure
prediction methods (the XtalOpt evolutionary algorithm), to accelerate
the discovery of crystalline electrides at ambient and nonambient
pressures. The localization and quantification of NNAs is used as
the primary discriminator for the electride character of a solid within
a multiobjective evolutionary structure search. We demonstrate the
reliability of this approach through a comprehensive crystal structure
prediction study of Ca5Pb3 at 20 GPa, a system
previously theorized to exhibit electride character under compression.
Our strategy could predict, and sort on-the-fly, several unknown low-enthalpy
phases that possess NNAs in interstitial loci, such as the newly discovered P4/mmm structure. These results demonstrate
how evolutionary algorithms, guided by rigorous topological descriptors,
can be relied upon to effectively survey complex phases to find new
electride candidates.