Thermodynamic cycles of the alkali metal-ligand complexes central to electride formation.

Alkali metal-ligand complexes are the building blocks of the exotic organic alkalide and electride materials. In this work, density-functional theory is used to construct thermodynamic cycles for the alkali metal-ligand complexes, highlighting the energy changes that enable alkalide and electride formation. Strong alkali metal- and cation-to-ligand binding energies are predicted and Rydberg-like ground states of the alkali metal-ligand complexes are identified, consistent with previous work. Calculations on molecular electride species do not reveal consistency with the identified trends, suggesting that the molecular electrides are a class of material unto themselves. The ionisation potentials of the alkali metal-ligand complexes are calculated to be consistently between 1 and 2 eV, suggesting that a specific ionisation potential (IP) is central to electride formation. Further, the thermodynamic cycle for the simplest electride, Cs+ (15C5)2 e- , shows stabilisation of the solid crystal due to electride formation that is consistent in magnitude with the IP of the equivalent alkali metal-ligand complex. In light of this, computational screening of the alkali metal-ligand complexes' IP presents a new design criterion for alkalide and electride materials.