Electronic state and cage distortion in the room-temperature stable electride [Ca₂₄Al₂₈O₆₄]⁴⁺(O²⁻)_2−x(e⁻)_2x as probed by ²⁷Al NMR

We report a microscopic study of the electronic states of the inorganic electride [Ca₂₄Al₂₈O₆₄]⁴⁺(O²⁻)_2−x(e⁻)_2x (x∼0, 1, and 2) by the ²⁷Al NMR technique. The ²⁷Al Knight shift as well as the Korringa behavior of the spin-lattice relaxation rate, T₁⁻¹, reveal a weak metallic character at one of two crystallographically inequivalent Al sites while insulating properties are exhibited at the other Al site on carrier doping, x∼1 and 2. A detailed analysis of the Knight shift and Korringa values indicates that the Fermi-level density of states comes predominantly from the cage conduction-band electrons, proving the electronic states as electride for this material from the microscopic viewpoint. In addition, we find a decrease in an inverse of a product of T₁ and temperature below T^∗∼40 and 15 K for the electron-doped samples with x∼1 and 2, respectively. This is explained by electron localization around the deformed cages due to the strong electron-lattice interaction at low temperatures.