We report a microscopic study of the electronic states of the inorganic electride ${\left[{\text{Ca}}_{24}{\text{Al}}_{28}{\text{O}}_{64}\right]}^{4+}{\left({\text{O}}^{2-}\right)}_{2-x}{\left(e^{-}\right)}_{2x}$ ($x\sim 0$, 1, and 2) by the ${}^{27}\text{A}\text{l}$ NMR technique. The ${}^{27}\text{A}\text{l}$ Knight shift as well as the Korringa behavior of the spin-lattice relaxation rate, $T_1^{-1}$, 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, $x\sim 1$ 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 $T_1$ and temperature below $T^{\ast }\sim 40$ and 15 K for the electron-doped samples with $x\sim 1$ and 2, respectively. This is explained by electron localization around the deformed cages due to the strong electron-lattice interaction at low temperatures.

• Received 7 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.245103