Electron-Deficient-Type Electride Ca₅Pb₃: Extension of Electride Chemical Space

Electrides have been identified so far by two major routes: one is conversion of elemental metals and stoichiometric compounds by high pressure; the other is to search for electron-rich compounds, and this approach is more general. In contrast, few electron-deficient structures in existing databases have been revealed as potential electride candidates. In this work, we found an electron-deficient compound Ca₅Pb₃ could be transformed into electrides upon applying external pressure or strain along the c-axis, which induces the electron immigration from Pb to interstitial sites. Furthermore, the electron doping via Hf substitution of Ca atoms for Ca₅Pb₃ was found to be capable of tuning the interstitial electron density under ambient pressure, resulting in a new stable ternary electride Ca₃Hf₂Pb₃, Hf-substituted Ca₅Pb₃. The electron-deficient electride discovered here is of novel type and can largely expand the research scope of electrides. Considering a recently reported neutral electride Na₃N and the present finding, it is now clarified that electrides can be identified irrespective of stoichiometry (electron-rich, -neutral, or -poor) for compounds.