Electride Formation of (Ca_{1- x}Sr_x)₃CrN₃ Induced by Negative Chemical Pressure

Electrides are materials in which electrons occupy interstitial sites and act as anions. Conventional strategies for stabilizing electrides have relied on either anion removal or the application of high pressure. Here, we show that in (Ca_{1- x}Sr_x)₃CrN₃, negative chemical pressure induces electride formation together with a reconstruction of the crystal framework. Structural analyses reveal that overbonded Cr undergoes oxidation, releasing electrons while driving a redistribution of nitrogen within the framework, which reconstructs the Ca sublattice into one-dimensional (1D) octahedral chains that host interstitial electrons. High-pressure experiments and first-principles calculations further demonstrate that external pressure destabilizes the electride phase, whereas negative chemical pressure stabilizes it. Moreover, (Ca_0.35Sr_0.65)₃CrN₃ exhibits metallic-like conduction, providing the first evidence for itinerant anionic electrons in a 1D electride. The enhanced itinerancy is attributed to the short interstitial-electron separation and the resulting increase in electride-band bandwidth. These findings establish negative chemical pressure as a new design principle for electrides and identify interstitial-electron separation as a key parameter governing electron itinerancy in low-dimensional electrides.