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.