Dicalcium nitride as a two-dimensional electride with an anionic electron layer

The ionic crystal Ca2N is shown to be an electride in terms of [Ca2N]+·e−, with diffusive two-dimensional transport in dense electron layers. The physical properties of electrides — ionic crystals in which electrons behave as anions — significantly depend on the topology of the confining cavity for anionic electrons. Thus, an essential step towards practical electride applications is to discover new confinement spaces with unique topologies. Confined two-dimensional electron layers have previously been achieved by artificially fabricating hetero-interface structures usually of semiconducting materials. Here the authors extend the range of materials demonstrating such behaviour to an electride, dicalcium nitride (Ca2N). This compound has ideal properties for electron confinement: a layered structure with appropriate interlayer spacing and a chemistry that allows for loosely bound electron layers without electron trapping. By providing a new material image for electrides, this work should lead to a series of two-dimensional electrides with unique physical properties. Recent studies suggest that electrides—ionic crystals in which electrons serve as anions—are not exceptional materials but rather a generalized form, particularly under high pressure1,2,3. The topology of the cavities confining anionic electrons determines their physical properties4. At present, reported confining sites consist only of zero-dimensional cavities or weakly linked channels4. Here we report a layered-structure electride of dicalcium nitride, Ca2N, which possesses two-dimensionally confined anionic electrons whose concentration agrees well with that for the chemical formula of [Ca2N]+·e−. Two-dimensional transport characteristics are demonstrated by a high electron mobility (520 cm2 V−1 s−1) and long mean scattering time (0.6 picoseconds) with a mean free path of 0.12 micrometres. The quadratic temperature dependence of the resistivity up to 120 Kelvin indicates the presence of an electron–electron interaction. A striking anisotropic magnetoresistance behaviour with respect to the direction of magnetic field (negative for the field perpendicular to the conducting plane and positive for the field parallel to it) is observed, confirming diffusive two-dimensional transport in dense electron layers. Additionally, band calculations support confinement of anionic electrons within the interlayer space, and photoemission measurements confirm anisotropic low work functions of 3.5 and 2.6 electronvolts, revealing the loosely bound nature of the anionic electrons. We conclude that Ca2N is a two-dimensional electride in terms of [Ca2N]+·e−.