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

• Published in: Nature (London) Vol. 494; no. 7437; pp. 336 - 340
• Main Authors: Lee, Kimoon, Kim, Sung Wng, Toda, Yoshitake, Matsuishi, Satoru, Hosono, Hideo
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 21.02.2013; Nature Publishing Group
• Subjects: 639/301/119/995; 639/638/298/917; Anions; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Crystal structure; Crystals; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport in condensed matter; Exact sciences and technology; Humanities and Social Sciences; Ionic crystals; letter; Magnetic fields; Mixed conductivity and conductivity transitions; multidisciplinary; Nitrides; Optical constants: refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of bulk materials and thin films; Physical properties; Physics; Science; Single crystals; Surface double layers, schottky barriers, and work functions; Topology; Ionic conduction; Work functions; Mean free path; Layered crystals; Magnetic field effects; Monocrystals; Reflection spectrum; Field orientation; Anisotropy; Electron mobility; Magnetoresistance; Carrier density; Ionic crystals
• DOI: 10.1038/nature11812
• ISSN: 0028-0836

The ionic crystal Ca 2 N is shown to be an electride in terms of [Ca 2 N] + ·e − , with diffusive two-dimensional transport in dense electron layers. A new format for electrides 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 (Ca 2 N). 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 pressure 1 , 2 , 3 . The topology of the cavities confining anionic electrons determines their physical properties 4 . At present, reported confining sites consist only of zero-dimensional cavities or weakly linked channels 4 . Here we report a layered-structure electride of dicalcium nitride, Ca 2 N, which possesses two-dimensionally confined anionic electrons whose concentration agrees well with that for the chemical formula of [Ca 2 N] + ·e − . Two-dimensional transport characteristics are demonstrated by a high electron mobility (520 cm 2  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 Ca 2 N is a two-dimensional electride in terms of [Ca 2 N] + ·e − .