High-density electron anions in a nanoporous single crystal: [Ca₂₄Al₂₈O₆₄]⁴⁺ (4e^-)

We removed ∼100% of clathrated oxygen ions from the crystallographic cages in a single crystal of 12CaO·7Al₂O₃, leading to the formation of high-density (∼2 x 10²¹ cm^-3) electrons highly localized in the cages. The resulting electron forms a structure that we interpret as an F⁺ center and migrates throughout the crystal by hopping to a neighboring cage with conductivity ∼100 siemens per centimeter, demonstrating that the encaged electron behaves as an anion. The electron anions couple antiferromagnetically with each other, forming a diamagnetic pair or singlet bipolaron. The resulting [Ca₂₄Al₂₈O₆₄]⁴⁺(4e^-) may be regarded as a thermally and chemically stable single crystalline "electride".