A crystal in which some electrons belong to no atom at all. In table salt, each sodium atom hands an electron to a chlorine atom; in an electride the metal atoms give up electrons too, but there is no atom to take them, so they sit on their own in the empty space between the layers, playing the chlorine’s part. Electrons held that loosely leave easily, which makes such crystals unusually good at giving electrons to whatever touches them.
Going deeper
In an electride the negative ions are electrons themselves, held in the empty space of the crystal. In a layered electride that space is the interlayer gallery, so the anion is a two-dimensional electron gas built into the structure.
A salt whose anion is an electron
Ordinary ionic crystals balance their positive ions with negative ones. An electride balances them with electrons that belong to no atom: they occupy interstitial space, in cavities in the three-dimensional case and in the gallery between layers in the two-dimensional one. The electrons are localised enough to count as ions in the formula and delocalised enough to carry current, which is an unusual combination.
Ca2N is the canonical layered example, written [Ca2N]+·e−. Its calcium–nitrogen slabs carry a positive charge, and the electrons between them form a sheet roughly where an anion layer would be in a normal nitride. That distinguishes an electride from an compound, where the guest species is added between layers, and from a doped , where carriers come from impurity atoms: here the electron sheet is part of the .
What loosely bound electrons are good for
Electrons held in empty space rather than by a nucleus are easy to remove, which is the same as saying the is low. That makes electrides attractive as electron-injection layers in devices, as cold-emission sources, and as reducing agents in chemistry – the loosely bound electrons that make ammonia synthesis easier on electride-supported are the same ones that make the material a metal.
The layered members can also be exfoliated. Liquid of Ca2N gives that keep the structure, the stoichiometry and the metallic character of the bulk, with optical properties matching density functional calculations – evidence that the interlayer electron sheet is still there in a flake a thick.
The handling problem
Everything useful about an electride is also what destroys it. A strong reducing agent reacts with water and with oxygen, so crystals are grown, handled, exfoliated and measured under inert gas, and flakes survive on the order of a month in nitrogen or in carefully chosen solvents rather than indefinitely. Any device work has to solve before it solves anything else.
The open questions follow from the structure. Whether the electron sheet survives in a true monolayer, where there is only one gallery and it faces the outside world on both sides, is not settled. Neither is the intrinsic of those electrons, since every measured value so far includes damage from exfoliation. And the family is small: which other layered electrides can be thinned the same way is still being worked through.
For specialists
An ionic crystal whose anions are electrons confined in interstitial space – in the layered case a two-dimensional electron gas in the interlayer gallery. The anionic layer gives a low work function, a high and metallic conduction, and it survives exfoliation. Ca2N was the first layered example; the same loosely bound electrons drive catalytic ammonia synthesis and electron-injection layers.