We have synthesized a single crystalline YC electride of centimeter-scale by floating-zone method and successfully characterized its anisotropic electrical and magnetic properties. In-plane resistivity upturn at low temperature together with anisotropic behavior of negative magnetoresistance is ascribed to the stronger suppression of spin fluctuation along in-plane than that along the c-axis, verifying the existence of magnetic moments preferred for the c-axis. A superior magnetic moment along the c-axis to that along the in-plane direction strongly demonstrates the anisotropic magnetism of YC electride containing a magnetically easy axis. It is clarified from the theoretical calculations that the anisotropic nature of the YC electride originates from strongly localized anionic electrons with an inherent magnetic anisotropy in the interlayer spaces.

An electride, a generalized form of cavity-trapped interstitial anionic electrons (IAEs) in a positively charged lattice framework, shows exotic properties according to the size and geometry of the cavities. Here, we report that the IAEs in layer structured [Gd 2 C] 2+ •2e − electride behave as ferromagnetic elements in two-dimensional interlayer space and possess their own magnetic moments of~0.52 μ B per quasi-atomic IAE, which facilitate the exchange interactions between interlayer gadolinium atoms across IAEs, inducing the ferromagnetism in [Gd 2 C] 2+ •2e − electride. The substitution of paramagnetic chlorine atoms for IAEs proves the magnetic nature of quasi-atomic IAEs through a transition from ferromagnetic [Gd 2 C] 2+ •2e − to antiferromagnetic Gd 2 CCl caused by attenuating interatomic exchange interactions, consistent with theoretical calculations. These results confirm that quasi-atomic IAEs act as ferromagnetic elements and trigger ferromagnetic spin alignments within the antiferromagnetic [Gd 2 C] 2+ lattice framework. These results present a broad opportunity to tailor intriguing ferromagnetism originating from quasi-atomic interstitial electrons in low-dimensional materials.

Electrides have emerged as promising materials with exotic properties, such as extraordinary electron-donating ability. However, the inevitable instability of electrides, which is caused by inherent excess electrons, has hampered their widespread applications. We report that a self-passivated dihafnium sulfide electride ([Hf2S]2+∙2e−) by double amorphous layers exhibits a strong oxidation resistance in water and acid solutions, enabling a persistent electrocatalytic hydrogen evolution reaction. The naturally formed amorphous Hf2S layer on the cleaved [Hf2S]2+∙2e− surface reacts with oxygen to form an outermost amorphous HfO2 layer with ~10-nm thickness, passivating the [Hf2S]2+∙2e− electride. The excess electrons in the [Hf2S]2+∙2e− electride are transferred through the thin HfO2 passivation layer to water molecules under applied electric fields, demonstrating the first electrocatalytic reaction with excellent long-term sustainability and no degradation in performance. This self-passivation mechanism in reactive conditions can advance the development of stable electrides for energy-efficient applications.